Physics In the Playground [Archive] - Giant in the Playground Forums

hiryuu

2012-02-28, 05:24 PM

Jacob Barnett recently disproved at least part of the big bang theory

Reh-he-heally? That sounds like it would show up in all the astronomy and physics blogs I follow. I wonder why it hasn't?

So. Some google searching turns up what this kid has to say about the big bang.

There are two different types of when stars end. When the little stars die, it’s just like a small poof. They just turn into a planetary nebula. But the big ones, above 1.4 solar masses, blow up in one giant explosion, a supernova. What it does, is, in larger stars there is a larger mass, and it can fuse higher elements because it’s more dense.

So you get all the elements, all the different materials, from those bigger stars. The little stars, they just make hydrogen and helium, and when they blow up, all the carbon that remains in them is just in the white dwarf; it never really comes off.

So, um, in the big-bang theory, what they do is, there is this big explosion and there is all this temperature going off and the temperature decreases really rapidly because it’s really big. The other day I calculated, they have this period where they suppose the hydrogen and helium were created, and, um, I don’t care about the hydrogen and helium, but I thought, wouldn’t there have to be some sort of carbon?

Otherwise, the carbon would have to be coming out of the stars and hence the Earth, made mostly of carbon, we wouldn’t be here.

I have to stop here. WAIT, WHAT? This is WRONG. First, about 98.5% of Earth's crust isn't Carbon (those elements happen to be Oxygen, Silicon, aluminium, iron, calcium, sodium, potassium, and magnesium; for the record, the core's pretty much all nickel and iron, all molten and chewy). And second, yes, Carbon DOES come out of dying stars. In fact, the fusing of heavier elements in the core of dying stars (like iron and carbon!) is one of the symptoms of preparation for going nova.

But what's this about Carbon being created during the big bang? Sure, there's probably lots of high energy Carbon-13 going around being formed really rapidly, I mean, we see that in dying stars all the time, why wouldn't it be made during the big b-

So I calculated, the time it would take to create 2 percent of the carbon in the universe, it would actually have to be several micro-seconds. Or a couple of nano-seconds, or something like that. An extremely small period of time. Like faster than a snap. That isn’t gonna happen.

Oh, I see. ...wat?

Based on what figures? He doesn't say, and I can't find anywhere that he does say. He just asserts this. That the carbon couldn't have formed in a high energy state. BUT IT DOES. ALL THE TIME. WE CALL IT THE HOYLE STATE. ARGH.

Because of that, that means that the world would have never been created because none of the carbon would have been given 7 billion years to fuse together. We’d have to be 21 billion years old ... and that would just screw everything up.

Kid basically assumes that every element as we see it comes from the big bang, and that the universe is essentially steady state, so therefore, big bang wrong. You heard it here first, folks! All our photos of deep space objects that require our knowledge of the big bang to be right to even calibrate for are basically just wrong. Uh, yeah, so, no.

Icewalker

2012-02-28, 06:56 PM

Added bonus: sounds to me like he's assuming that all of time regarding the really early big bang is perfectly linear and normal, and not, you know, quantum physics and general relativity to the degree that we still don't fully understand it.

Talk about physics? Does my physics midterm tomorrow count? Weeee optics and inductance!

hiryuu

2012-02-28, 07:36 PM

Added bonus: sounds to me like he's assuming that all of time regarding the really early big bang is perfectly linear and normal, and not, you know, quantum physics and general relativity to the degree that we still don't fully understand it.

Yeah, I didn't even want to get into time dilation and the fact that, well, while he appears to be a math genius, the math stops working the same way when you get closer to the time period immediately following the big bang.

Talk about physics? Does my physics midterm tomorrow count? Weeee optics and inductance!

Sweet. One of my players works on laser systems and does engineering and whatnot. It's half the reason I need to have a better explanation than "it's magic, just let it fly" for any game I run, because my players all want to know how the magic works. The other half is that my other two players consist of a physicist and a biologist... they don't care if it's different physical model, again, they just want to know how the magic/different physical model works. It leads to a lot of campaigns that are basically about that setting's version of Galileo/Captain Cook/Darwin. Which I have no complaints about, but it requires lots of work.

Grinner

2012-02-28, 08:28 PM

From the layman's point of view, the Big Bang theory makes absolutely no sense anyway. The entirety of existence just erupted from a single point of energy?

IthilanorStPete

2012-02-28, 08:39 PM

From the layman's point of view, the Big Bang theory makes absolutely no sense anyway. The entirety of existence just erupted from a single point of energy?

The Big Bang theory doesn't actually address how things began; it doesn't go all the way back to t=0, but a miniscule amount of time after that. And it actually does make sense once you know the universe is expanding...just run things backward, and that's what you get.

JCarter426

2012-02-28, 08:49 PM

Well, it actually doesn't make sense if you know the universe is expanding, because it should have stopped expanding by now.

Drolyt

2012-02-28, 09:47 PM

From the layman's point of view, the Big Bang theory makes absolutely no sense anyway. The entirety of existence just erupted from a single point of energy?
Kind of. Thing is, we don't know what caused the big bang. What we know is that the universe is expanding. We also know that it has been expanding for quite some time. There is evidence of this; if you like I can try to explain some of it. Now, what do we expect to happen if we turn back time and keep going? Well, if the universe is expanding when time is moving forward then it should contract when it moves backwards, and eventually the entire universe would end up in a single extremely dense and energetic point. To test this theory we have looked for what we would expect if this is true and we have found some evidence for it. It is however by no means a universally accepted theory.

Well, it actually doesn't make sense if you know the universe is expanding, because it should have stopped expanding by now.
How do you figure?

JCarter426

2012-02-28, 10:05 PM

How do you figure?
Well, not me, personally, but... matter attracts matter - all the stuff in the universe is drawn to each other, and should slow the expansion. But that's not what's happening. In fact, it's speeding up. That goes against everything we know about explosions, and mechanics in general. There is some source of energy that's causing the expansion. We can't see it, and we don't know what it is, so we call it dark energy. Woooooooooooo!

Astrella

2012-02-28, 10:17 PM

Drolyt

2012-02-28, 10:23 PM

Well, not me, personally, but... matter attracts matter - all the stuff in the universe is drawn to each other, and should slow the expansion. But that's not what's happening. In fact, it's speeding up. That goes against everything we know about explosions, and mechanics in general. There is some source of energy that's causing the expansion. We can't see it, and we don't know what it is, so we call it dark energy. Woooooooooooo!
Note: What follows is my reasoning. You may already know this stuff, but bear with me. Newton's First Law states that "Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." So if all the objects in the universe start moving away from each other we would expect them to continue doing so unless a force acts upon them. Of course we know a force does act on them, gravity. The problem is that we would expect either a) the expansion of the universe will slow down and eventually reverse or b) eventually everything would get far enough apart to overcome gravity and thus keep expanding forever. Now, what you said originally was that the universe should have stopped expanding by now, and that isn't really clear from the data we have. The actual problem, as you have pointed out, is that the universe appears to be expanding faster, which is odd.

It's the structure of the universe that's expanding though. It's not really that galaxies are moving apart, just that the space between them expands.
Correct, but that is a subtle distinction that doesn't really affect the problem of "why is the expansion speeding up".

Grinner

2012-02-28, 10:37 PM

Kind of. Thing is, we don't know what caused the big bang. What we know is that the universe is expanding. We also know that it has been expanding for quite some time. There is evidence of this; if you like I can try to explain some of it. Now, what do we expect to happen if we turn back time and keep going? Well, if the universe is expanding when time is moving forward then it should contract when it moves backwards, and eventually the entire universe would end up in a single extremely dense and energetic point. To test this theory we have looked for what we would expect if this is true and we have found some evidence for it. It is however by no means a universally accepted theory.

No, I can't imagine that is. For example, while it explains how the universe may have formed, it fails to explain where this energy came from, how it formed, and how it could exist at all. But I haven't looked into the matter too closely. Sometimes, it seems like theoretical science is just a permutation of religion.

Now, can anyone explain the basics of aeronautics?

JCarter426

2012-02-28, 10:38 PM

Note: What follows is my reasoning. You may already know this stuff, but bear with me. Newton's First Law states that "Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." So if all the objects in the universe start moving away from each other we would expect them to continue doing so unless a force acts upon them. Of course we know a force does act on them, gravity. The problem is that we would expect either a) the expansion of the universe will slow down and eventually reverse or b) eventually everything would get far enough apart to overcome gravity and thus keep expanding forever. Now, what you said originally was that the universe should have stopped expanding by now, and that isn't really clear from the data we have.
Sorry, I did mean the slowed down/reversed scenario. Not just stop completely, yeah.

It's the structure of the universe that's expanding though. It's not really that galaxies are moving apart, just that the space between them expands.

Correct, but that is a subtle distinction that doesn't really affect the problem of "why is the expansion speeding up".
I'm also not sure if there is a proven fundamental distinction. We understand gravity as the curvature of the universe, after all. The space between stuff might as well just be the structure of the universe. If you kept going and going and going, would you hit a wall at some point?

No, I can't imagine that is. For example, while it explains how the universe may have formed, it fails to explain where this energy came from, how it formed, and how it could exist at all.
There's a whole 'nother branch of physics for that. Most theories are consistent with the Big Bang, some challenge it.

But I haven't looked into the matter too closely. Sometimes, it seems like theoretical science is just a permutation of religion.
I can understand why you feel that way; a lot of theoretical science seems to exist just to tell you what you can't do, rather than why things are. Much like religion. However, at least one of them admits it doesn't know everything and is subject to change in the future. But you have to understand that just over a hundred years ago, we thought we knew everything about the universe. And a hundred years before that, and a hundred years before that. If we didn't have theoretical physics to challenge the status quo, we wouldn't have computers, television, GPS, lasers, and a whole bunch of other cool stuff.

Gwyn chan 'r Gwyll

2012-02-28, 10:48 PM

My physics homework (http://www.oecd-nea.org/dbdata/data/mass-evals2003/mass.mas03)

Why.
WHYYYYYYYYY. :smallfurious:

IthilanorStPete

2012-02-28, 10:50 PM

My physics homework (http://www.oecd-nea.org/dbdata/data/mass-evals2003/mass.mas03)

Why.
WHYYYYYYYYY. :smallfurious:

*blinks* What.

PirateMonk

2012-02-28, 10:53 PM

Reh-he-heally? That sounds like it would show up in all the astronomy and physics blogs I follow. I wonder why it hasn't?

So. Some google searching turns up what this kid has to say about the big bang.

I have to stop here. WAIT, WHAT? This is WRONG. First, about 98.5% of Earth's crust isn't Carbon (those elements happen to be Oxygen, Silicon, aluminium, iron, calcium, sodium, potassium, and magnesium; for the record, the core's pretty much all nickel and iron, all molten and chewy). And second, yes, Carbon DOES come out of dying stars. In fact, the fusing of heavier elements in the core of dying stars (like iron and carbon!) is one of the symptoms of preparation for going nova.

But what's this about Carbon being created during the big bang? Sure, there's probably lots of high energy Carbon-13 going around being formed really rapidly, I mean, we see that in dying stars all the time, why wouldn't it be made during the big b-

Oh, I see. ...wat?

Based on what figures? He doesn't say, and I can't find anywhere that he does say. He just asserts this. That the carbon couldn't have formed in a high energy state. BUT IT DOES. ALL THE TIME. WE CALL IT THE HOYLE STATE. ARGH.

Kid basically assumes that every element as we see it comes from the big bang, and that the universe is essentially steady state, so therefore, big bang wrong. You heard it here first, folks! All our photos of deep space objects that require our knowledge of the big bang to be right to even calibrate for are basically just wrong. Uh, yeah, so, no.

Wow. From what I've read about him, it seems like he'd at least make less obvious mistakes. Overenthusiastic reporters? Kid trying to learn too fast? Something else?

Drolyt

2012-02-28, 10:54 PM

My physics homework (http://www.oecd-nea.org/dbdata/data/mass-evals2003/mass.mas03)

Why.
WHYYYYYYYYY. :smallfurious:
I'm confused. That just looks like a poorly formatted page listing some properties of atoms (or something, I don't really get it). What is your homework exactly?

Gwyn chan 'r Gwyll

2012-02-28, 10:56 PM

Drolyt

2012-02-28, 11:14 PM

I have to use that data, pick independent and dependent values, analyze, graph, etc. BUT THERE'S SO MUCH. It's just overwhelming. I think I'm gonna do N-Z as my independent, and Atomic Binding Energy per Nucleon as my dependent.

I still have no clue what the O column means though.
Beats me. I've only just decided what I think the table is. I think it is a list of properties of nuclides, is that right? Your assignment doesn't sound that bad, except for the part where you have to use that awful table.

Gwyn chan 'r Gwyll

2012-02-28, 11:15 PM

Well, and the fact that it's all due tomorrow, and it's a lot of work. And aye, it's properties of nuclides. I only just finished picking the values I'm using for my stuff :/

hiryuu

2012-02-28, 11:28 PM

Wow. From what I've read about him, it seems like he'd at least make less obvious mistakes. Overenthusiastic reporters? Kid trying to learn too fast? Something else?

I think the answer is a combination of factors, with reporters on one end and the fact that the kid might not have been exposed to the current research. It's very easy to be good at math, it's much harder to go over the vast volume of data already available; getting a doctorate isn't a matter of being smart, it's a matter of contacts and being certified to have already gone over the available material, something a 13 year old hasn't done yet. Give him a few years to be exposed to the actual information, and he'll probably make way fewer mistakes.

Grinner

2012-02-29, 12:05 AM

I can understand why you feel that way; a lot of theoretical science seems to exist just to tell you what you can't do, rather than why things are. Much like religion. However, at least one of them admits it doesn't know everything and is subject to change in the future. But you have to understand that just over a hundred years ago, we thought we knew everything about the universe. And a hundred years before that, and a hundred years before that. If we didn't have theoretical physics to challenge the status quo, we wouldn't have computers, television, GPS, lasers, and a whole bunch of other cool stuff.

When I imagine a scientist, or at least the mainstream sort, I often think of intellectual intolerance. When I imagine any sort of organized religion, I see a similar intolerance. Additionally, science and religion often try to fulfill the same role, claim the capability of omniscience, and are both very much subject to change. Frankly, I don't see the difference at all.

Nix Nihila

2012-02-29, 12:14 AM

hiryuu

2012-02-29, 12:15 AM

Gwyn chan 'r Gwyll

2012-02-29, 12:15 AM

I would disagree. Most all scientists I have met do NOT claim that science claims any sort of omniscience. Isn't that the whole point of science? That we DON'T know everything? That we might be proved wrong? I mean, your average scientific experiment is as follows

I wonder what happens if I do X to Y. It would make sense if it did Z, considering factors A, B and C happen.
*stuff*
Oh man, Z doesn't happen at all, N happens! What now!

And most scientists I know aren't intellectually intolerant. Well, unless by intellectually intolerant you mean "Have considered, weighed, and tested opposing hypotheses and found them lacking, and have adopted their own hypothesis which they find much more likely", as opposed to religions "Our hypothesis is right because it is right."

I've already shown how scientific views change. How does theology change though? The Bible is the Bible. It cannot change. If it is the Truth, than the truth cannot change. All that can change is INTERPRETATIONS of the truth, and even then...

Grinner

2012-02-29, 12:29 AM

Whoa, whoa, whoa. Now, I'm not a scientist, but unless I've got it all wrong, science does not claim omniscience. For goodness sake, just look at the uncertainty principle. All good science involves healthy doses of skepticism and acknowledgement that we don't know everything. I don't want to go into a discussion on religion, what with the board rules and all, but I don't see how you can make that comparison.

I said nothing about omniscience. I said the capability of omniscience. There's a fine difference between the two.

Anyway, when I wrote that, I had been thinking of a scientist who claimed to have built a device capable a generating propulsion solely through electricity. No fuels required. This sparked a backlash in the scientific communities in which he was involved. The only published refutation to his paper was another paper, which claimed it violated a scientific law. One of Newton's, I think. However, I heard no reports of anyone actually attempting to build one of these devices.

Basically, there's a point at which rationalism fails, but it seems like scientists are often unwilling to admit that.

Edit: Found the Wikipedia article (https://en.wikipedia.org/wiki/Emdrive).

I've already shown how scientific views change. How does theology change though? The Bible is the Bible. It cannot change. If it is the Truth, than the truth cannot change. All that can change is INTERPRETATIONS of the truth, and even then...

You got it.

Gwyn chan 'r Gwyll

2012-02-29, 12:39 AM

I said nothing about omniscience. I said the capability of omniscience. There's a fine difference between the two.

Anyway, when I wrote that, I had been thinking of a scientist who claimed to have built a device capable a generating propulsion solely through electricity. No fuels required. This sparked a backlash in the scientific communities in which he was involved. The only published refutation to his paper was another paper, which claimed it violated a scientific law. One of Newton's, I think. However, I heard no reports of anyone actually attempting to build one of these devices.

Basically, there's a point at which rationalism fails, but it seems like scientists are often unwilling to admit that.

On CAPABILITY of omniscience: that is the GOAL of science. Does any scientist seriously believe, in the modern age, that we will ever reach a point where everything that can be known will be? I don't think so.

As for that 'scientist', I've read the same thing. As far as I can remember, he never showed an actual scientist or published in a legitimate journal with an explanation of how it worked, or how to build a device, but I can't find anything on it...

And there is NOT a point where rationalism fails, I don't think. Your example above is not of rationalism failing, but, if everything is as you remember, of scientists failing to apply their rationalism evenly to everything. That is IRRATIONALISM failing there.

Edit: Ah, but the thing is, is that science admits the possibility that it's raw data may be wrong. Or that other sets of raw data may offer alternative, equally or more viable explanations. Religion does not. In religion, the raw data cannot be wrong, and other sets of raw data cannot offer alternative, equally, or more viable explanations, and must be out-right rejected.

And I was thinking of the Dean drive, not the EmDrive. Reading now. Will edit with more reply.

Drolyt

2012-02-29, 12:44 AM

I think the answer is a combination of factors, with reporters on one end and the fact that the kid might not have been exposed to the current research. It's very easy to be good at math, it's much harder to go over the vast volume of data already available; getting a doctorate isn't a matter of being smart, it's a matter of contacts and being certified to have already gone over the available material, something a 13 year old hasn't done yet. Give him a few years to be exposed to the actual information, and he'll probably make way fewer mistakes.
I'd mostly blame the reporters. It is not uncommon for people who are just getting into theoretical physics to start coming up with all their own ideas before eventually being corrected and having it explained to them that no, they aren't going to outsmart thousands of scientists who have been working on these problems for the better part of a century, at least not until they've got a few years under their belt and examined all the research that came before, probably not even then.

As for this Jacob Barnett, I type the kids name in and the first google result is "Autistic boy,12, with higher IQ than Einstein develops his own theory of relativity". The hell? I mean, sure the kid seems to be a prodigy, at the very least he knows more than I knew at his age, but that isn't the same as being a genius. Genius is the rare ability to see things in a new way. This kid, as far as I can tell, just thinks of normal things at a much younger age than normal. This is probably a result of his Asperger's, it sounds like he studies this stuff constantly instead of doing what normal kids his age do, it isn't that surprising that he managed to grok it eventually. I'm not saying I'm not impressed. I think in 10 or 20 years he might be a great physicist. I just don't get why people are acting like he's smarter than Einstein, or why anyone would take his theory seriously.

Nix Nihila

2012-02-29, 12:45 AM

I said nothing about omniscience. I said the capability of omniscience. There's a fine difference between the two.

My understanding is that the uncertainty principle states that we can't know everything, such as both the position and momentum of a particle (hence no capability of omniscience). But you should probably talk to someone who knows physics more thoroughly than I do. However, I don't see the harm in trying to reach a point of omniscience (and I don't think most physicists would claim that we can ever reach such a point). Even if we can't reach it, it will certainly improve our knowledge of the universe.

Grinner

2012-02-29, 12:46 AM

On CAPABILITY of omniscience: that is the GOAL of science. Does any scientist seriously believe, in the modern age, that we will ever reach a point where everything that can be known will be? I don't think so.

As for that 'scientist', I've read the same thing. As far as I can remember, he never showed an actual scientist or published in a legitimate journal with an explanation of how it worked, or how to build a device, but I can't find anything on it...

And there is NOT a point where rationalism fails, I don't think. Your example above is not of rationalism failing, but, if everything is as you remember, of scientists failing to apply their rationalism evenly to everything. That is IRRATIONALISM failing there.

No, what I meant was that the only refutation claimed it broke some hallowed law of science, but rationalism means little in comparison to empiricism, the foundation of science. Empiricism would have actually disproved it.

Anyway, I get the feeling you're getting a little worked up over this, so I'll drop the matter.

However, I don't see the harm in trying to reach a point of omniscience (and I don't think most physicists would claim that we can ever reach such a point). Even if we can't reach it, it will certainly improve our knowledge of the universe.

I agree.

Talakeal

2012-02-29, 12:51 AM

My knowledge may be out of date, but from the cosmology course I took in 2005 I thought that vacuum fluctuations* accounted for dark energy and could also have accounted for the conditions that triggered the big bang.

*IIRC the stream of random particles which are constantly appearing and disappearing in seemingly empty space.

grimbold

2012-02-29, 04:53 AM

so i'm starting the International Baccealaureate next year

what classes do i take to get into king's college cambridge or manchester?

factotum

2012-02-29, 07:42 AM

You're probably best off contacting UCAS in the UK to ask them--their website is here, but no idea if it covers what you need:

http://www.ucas.co.uk/

Gwyn chan 'r Gwyll

2012-02-29, 08:44 AM

so i'm starting the International Baccealaureate next year

what classes do i take to get into king's college cambridge or manchester?

don't do it.

I mean, ahem. Is King's College very specialized? I'm taking English A HL, History HL, Theatre HL, French B SL, Physics SL, Math SL, and I got into 2 of the 3 best universities in the entire country. The third one I just didn't apply for.

Gravitron5000

2012-02-29, 09:38 AM

Anyway, when I wrote that, I had been thinking of a scientist who claimed to have built a device capable a generating propulsion solely through electricity. No fuels required. This sparked a backlash in the scientific communities in which he was involved. The only published refutation to his paper was another paper, which claimed it violated a scientific law. One of Newton's, I think. However, I heard no reports of anyone actually attempting to build one of these devices.

Your premise overlooks the concept of cost.

How much would it cost to build this machine? Does the technology even exist to build it? If not, can we infer what pieces of tech are missing from the complete assembly, and what would it take to get there. How much would it cost to develop these missing pieces? And if the pieces are missing pieces ... well, technology progresses at its own pace.

There is also the cost of time. If a cursory look at the proposed drive seems to contradict a principle or theory with a lot of evidence backing it, how likely is it to really work? Looking at it from a scientists point of view, even if I was working in a related field, would I drop everything to work on this or would I keep working on whatever I am currently working on? There are only so many hours in the day so I would need to use some factor as a filter to weed out ideas that may not pan out. A large body of evidence against it working seems like a good criterion.

grimbold

2012-02-29, 10:14 AM

don't do it.

I mean, ahem. Is King's College very specialized? I'm taking English A HL, History HL, Theatre HL, French B SL, Physics SL, Math SL, and I got into 2 of the 3 best universities in the entire country. The third one I just didn't apply for.

King's College is Newton's college...
it's hard to get in...

JCarter426

2012-02-29, 11:46 AM

When I imagine a scientist, or at least the mainstream sort, I often think of intellectual intolerance. When I imagine any sort of organized religion, I see a similar intolerance. Additionally, science and religion often try to fulfill the same role, claim the capability of omniscience, and are both very much subject to change. Frankly, I don't see the difference at all.
It was certainly like that one time, indeed many times, but I don't see it today at all. The Asimov Memorial Debates are a good example of this - a bunch of scientists in a room, who don't agree with each other on nearly anything, but are still friendly and still open to new ideas.

My knowledge may be out of date, but from the cosmology course I took in 2005 I thought that vacuum fluctuations* accounted for dark energy and could also have accounted for the conditions that triggered the big bang.

*IIRC the stream of random particles which are constantly appearing and disappearing in seemingly empty space.
That's zero point energy (AKA vacuum energy or negative energy). Totally different, as far as I know. I'm sure there are theories that have one account for the other, but there's nothing universally accepted yet.

shawnhcorey

2012-02-29, 01:34 PM

for the record, the core's pretty much all nickel and iron, all molten and chewy

Actually because it's under tremendous pressure, it's very runny. If it had any place to go, it could travel at several thousand miles per hour.

I would disagree. Most all scientists I have met do NOT claim that science claims any sort of omniscience. Isn't that the whole point of science? That we DON'T know everything? That we might be proved wrong? I mean, your average scientific experiment is as follows

I though they were more like this (https://xkcd.com/242/).

Talakeal

2012-02-29, 01:46 PM

That's zero point energy (AKA vacuum energy or negative energy). Totally different, as far as I know. I'm sure there are theories that have one account for the other, but there's nothing universally accepted yet.

As I said, my knowledge is only from undergraduate level college courses six years ago, and I could well be wrong, but I was taught that vacuum energy was believed to be the cause of universal expansion, and a quick wikipedia search reveals that this is still one of the leading theories.

JCarter426

2012-02-29, 02:06 PM

The thing is vacuum energy is also unaccounted for. And I don't think it would account for dark energy entirely - it would just explain one part of the phenomenon (the speeding up part). Basically, it sets a precedence for energy to exist in a vacuum, but it's still within the realm of "well, something is doing it", which is why it's just one of the leading theories.

Grinner

2012-02-29, 07:51 PM

How much would it cost to build this machine? Does the technology even exist to build it? If not, can we infer what pieces of tech are missing from the complete assembly, and what would it take to get there. How much would it cost to develop these missing pieces? And if the pieces are missing pieces ... well, technology progresses at its own pace.

The guy supposedly built a working prototype. Even posted a video of it on YouTube. It's hardly anything conclusive though.

According to Wikipedia, the device is comprised of a magnetron and a near-conical cavity resonator. Pretty much a microwave oven and a specially-shaped metal box.

There is also the cost of time. If a cursory look at the proposed drive seems to contradict a principle or theory with a lot of evidence backing it, how likely is it to really work? Looking at it from a scientists point of view, even if I was working in a related field, would I drop everything to work on this or would I keep working on whatever I am currently working on? There are only so many hours in the day so I would need to use some factor as a filter to weed out ideas that may not pan out. A large body of evidence against it working seems like a good criterion.

That's a fair point. It's just that he probably lost a ton of credibility over it, despite the design seeming pretty simple.

PirateMonk

2012-02-29, 08:43 PM

I think the answer is a combination of factors, with reporters on one end and the fact that the kid might not have been exposed to the current research. It's very easy to be good at math, it's much harder to go over the vast volume of data already available; getting a doctorate isn't a matter of being smart, it's a matter of contacts and being certified to have already gone over the available material, something a 13 year old hasn't done yet. Give him a few years to be exposed to the actual information, and he'll probably make way fewer mistakes.

Certainly, but press accounts claim that he's been into astrophysics since he was three. That's more than enough time to learn the basics, even given that he was busy with math, elementary school, and having a childhood. On the other hand, a Real Physicist from the Institute for Advanced Study says about Barnett's GR work:

"I'm impressed by his interest in physics and the amount that he has learned so far," Tremaine wrote in an email, provided by the family. "The theory that he's working on involves several of the toughest problems in astrophysics and theoretical physics.

"Anyone who solves these will be in line for a Nobel Prize."

He then encouraged Jake to spend as much time as possible to learn more and to further develop his theory.

This notably doesn't include anything about the theory being at all correct as currently formulated.

It's just that he probably lost a ton of credibility over it, despite the design seeming pretty simple.

Anyone who comes up with a design that obviously violates conservation of momentum, which appears to be closely tied to the constancy of the laws of physics over translations in space (http://en.wikipedia.org/wiki/Noether%27s_theorem), without either explaining how it doesn't actually violate CoM or articulating a coherent physical theory without CoM, deserves to be ridiculed.

Grinner

2012-02-29, 09:52 PM

Anyone who comes up with a design that obviously violates conservation of momentum, which appears to be closely tied to the constancy of the laws of physics over translations in space (http://en.wikipedia.org/wiki/Noether%27s_theorem), without either explaining how it doesn't actually violate CoM or articulating a coherent physical theory without CoM, deserves to be ridiculed.

And if it works regardless?

Which is more important? The cause, or the effect?

JCarter426

2012-02-29, 10:03 PM

Grinner

2012-02-29, 10:19 PM

PirateMonk

2012-02-29, 10:56 PM

And if it works regardless?

Which is more important? The cause, or the effect?

As noted previously, exhaustively exploring every possible violation of well-established physical laws is horrifically wasteful. If he builds the prototype anyway, and it seems to work, the first thing to do is thoroughly check if something not prohibited by the basic laws of physics as we understand them is going on, and then we can start talking about completely reworking all of physics. There is so much evidence behind CoM that, even if his model appears to work, it is far more likely that he is misinterpreting the results than that CoM is wrong.

Yora

2012-03-01, 06:03 AM

shawnhcorey

2012-03-01, 07:53 AM

According to Wikipedia, the device is comprised of a magnetron and a near-conical cavity resonator. Pretty much a microwave oven and a specially-shaped metal box.

That would make it a maser, a laser that uses microwaves. Laser propulsion has been proposed before but since they have such a low impulse, they would only be good for long-term (as in centuries), unmanned probes.

Another proposal would be using the reverse Casimir effect (http://scienceblog.com/13866/scientists-reverse-casimir-effect/) to create a reactionless[1] drive. (And don't say it violates Newton's laws; relativity violates Newton's laws so Newton does not have the final say.)

[1] Sorry, I used the wrong word; initially I called it an inertia-less drive. I apologize for any confusion.

factotum

2012-03-01, 07:54 AM

Actually because it's under tremendous pressure, it's very runny.

Depends which part you're talking about--the outer core is liquid, and it's movements within that molten material that generate the Earth's magnetic field. The inner core is solid, despite the much greater temperature, due to the higher pressure.

Gwyn chan 'r Gwyll

2012-03-01, 08:49 AM

Here a much simpler question since it's not about sub-atomic physics (I believe).

How does vibration in a single piece of material, lets say metal, actually work? Take a pipe, bang it against something hard, and it will vibrate and produce a humming noise. What actually happens to the object?

Well, you're applying a force to the object. The force moves the object slightly, but the elasticity of the object moves the object back to where it was, and then over-shoots it in the opposite direction. This is what the actual vibration is. The humming noise is because the object is displacing air in a frequency detectable by human ears. Imagine it like a wave, only instead of looking at it sideways, it's a wave coming straight at you, caused by successive peaks (when the metal pipe vibrates TOWARDS you) and troughs (when the metal pipe vibrates AWAY from you).

Gravitron5000

2012-03-01, 09:35 AM

The cause. The effect only gives short term benefits. Understanding the cause leads to potential long term benefits. To recycle my Tyson example, the the concept of magnetic resonance was developed by an astrophysicist studying nuclei in space. If he only cared about the effect, we wouldn't have magnetic resonance imagers today.

Or on a more practical scale, if his invention works, but no one knows why it works, it only has one possible application. It cannot be improved upon, nor can its design philosophy be applied to anything else.

If you have an effect, but don't understand the cause, then you have found something interesting to study. Both cause and effect are linked. Without cause there would be no effect. Without effect the concept of cause is nonsensical.

That said, I would have to agree with you. As far as usefulness and understanding goes, the why is more important than the what.

Castaras

2012-03-01, 12:52 PM

My physics homework (http://www.oecd-nea.org/dbdata/data/mass-evals2003/mass.mas03)

Why.
WHYYYYYYYYY. :smallfurious:

Google chrome tried to get me to translate that from Romanian into English. :smalltongue:

PirateMonk

2012-03-01, 02:54 PM

That would make it a maser, a laser that uses microwaves. Laser propulsion has been proposed before but since they have such a low impulse, they would only be good for long-term (as in centuries), unmanned probes.

Another proposal would be using the reverse Casimir effect (http://scienceblog.com/13866/scientists-reverse-casimir-effect/) to create a reactionless[1] drive. (And don't say it violates Newton's laws; relativity violates Newton's laws so Newton does not have the final say.)

[1] Sorry, I used the wrong word; initially I called it an inertia-less drive. I apologize for any confusion.

First, relativity still conserves (relativistic) momentum. Second, so does this:

Professor Ulf Leonhardt and Dr Thomas Philbin of the University’s School of Physics & Astronomy believe that they can engineer the Casimir force of quantum physics to cause an object to repel rather than attract another in a vacuum.

My bolding.

One possibility for an almost-reactionless drive might be to deposit the counterbalancing momentum in virtual particles. Does anyone know if this would actually work? Apparently I need to read better.

shawnhcorey

2012-03-01, 03:06 PM

One possibility for an almost-reactionless drive might be to deposit the counterbalancing momentum in virtual particles. Does anyone know if this would actually work?

Yes, it's called the reverse Casimir effect. The Casimir effect reduces the energy of the vacuum, the amount of virtual quanta, in a volume of space. The difference in energy pushes any body (or light) from the greater energy volume to the lesser. This is reactionless movement: a body moves but no other body moves as a result of this. There is an action but no reaction. The simplest way to look at a reactionless drive is that the drive dumps a whole bunch of energy into space just behind the vehicle and as it disperses, it drives the ship forward.

grimbold

2012-03-01, 03:22 PM

Yes, it's called the reverse Casimir effect. The Casimir effect reduces the energy of the vacuum, the amount of virtual quanta, in a volume of space. The difference in energy pushes any body (or light) from the greater energy volume to the lesser. This is reactionless movement: a body moves but no other body moves as a result of this. There is an action but no reaction. The simplest way to look at a reactionless drive is that the drive dumps a whole bunch of energy into space just behind the vehicle and as it disperses, it drives the ship forward.

wasn't their a sci am article on this a few years ago?

technically you don't even need to push anything off the body, if it moves in the right way it can autopropel

JCarter426

2012-03-01, 04:02 PM

The Casimir effect is another name for zero point/vacuum/negative energy, incidentally.

shawnhcorey

2012-03-01, 04:16 PM

The Casimir effect is another name for zero point/vacuum/negative energy, incidentally.

Sorry, no. Space has positive energy. The Casimir effect reduces the energy but it always remains positive.

JCarter426

2012-03-01, 04:41 PM

Er... that's what negative energy is. :smallconfused: The Casimir effect is a case of local energy density being in the negative values. There's nothing there, and yet there's negative pressure.

shawnhcorey

2012-03-01, 04:57 PM

Asta Kask

2012-03-01, 05:01 PM

You can have negative absolute temperature in highly artificial systems, but the energy is still higher than at absolute zero. This has to do with the definition of temperature.

JCarter426

2012-03-01, 05:28 PM

Sorry, no. It's like saying the freezing point of water is zero and anything colder has negative temperature. In reality, temperature has an absolute zero it can't go below (or even get as cold as). Everything else is positive temperature. Same with the energy of space; there always some there. You can reduce it but you can't make it negative. You can't even make it zero.
I assure you that you can have negative temperatures, and also that the concept of temperature is not what you are describing; the concept you are describing is no different to the concept of zero point energy - the lowest state of energy you can possibly reach - also known as vacuum energy or the Casimir effect. All of these are considered to be negative energy - which is predicted in relativity - due to the localized energy density, which is negative. Some visual examples here (http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=18823.0;attach=173 418;image) and here (http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=18823.0;attach=173 421;image).

(If you want to get technical, vacuum energy is the zero point energy of a vacuum, and the Casimir effect is the process of this energy decreasing, making it, paradoxially, negative energy).

teratorn

2012-03-01, 06:12 PM

No, what I meant was that the only refutation claimed it broke some hallowed law of science, but rationalism means little in comparison to empiricism, the foundation of science. Empiricism would have actually disproved it.

I don't get your reasoning. The guy isn't going from an experiment towards
a theory, he starts from the current theory and accepts it. The author states that in framework of existing relativistic theory one can generate trust in a closed cavity. He doesn't advance a new theory, he claims thrust is a consequence of existing theory. In this regard breaking a law of science derived in the framework of the theory he uses is enough to show his derivations are flawed. Why would one need empiricism to «disprove» bad math?

shawnhcorey

2012-03-01, 06:53 PM

I assure you that you can have negative temperatures, and also that the concept of temperature is not what you are describing; the concept you are describing is no different to the concept of zero point energy - the lowest state of energy you can possibly reach - also known as vacuum energy or the Casimir effect. All of these are considered to be negative energy - which is predicted in relativity - due to the localized energy density, which is negative. Some visual examples here (http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=18823.0;attach=173 418;image) and here (http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=18823.0;attach=173 421;image).

(If you want to get technical, vacuum energy is the zero point energy of a vacuum, and the Casimir effect is the process of this energy decreasing, making it, paradoxially, negative energy).

Yes, you can have temperature lower than 0C. You can't have them lower than absolute zero. You can have negative energy with respect to some reference but there is only so much energy in a volume of space and you can't take out more than what's there. You can't even get all of it out; some is always left behind. Yes, there is reduced energy but there is no such thing as negative energy. It doesn't exist in our universe.

JCarter426

2012-03-01, 07:10 PM

You can't even get all of it out; some is always left behind. Yes, there is reduced energy but there is no such thing as negative energy. It doesn't exist in our universe.
This is precisely what the Casimir effect is thought to do. So don't argue with me, argue with Casimir.

Grinner

2012-03-01, 07:33 PM

I don't get your reasoning. The guy isn't going from an experiment towards
a theory, he starts from the current theory and accepts it. The author states that in framework of existing relativistic theory one can generate trust in a closed cavity. He doesn't advance a new theory, he claims thrust is a consequence of existing theory. In this regard breaking a law of science derived in the framework of the theory he uses is enough to show his derivations are flawed. Why would one need empiricism to «disprove» bad math?

We're talking about two different people now. You're referring to Roger Shawyer, the inventor in question. That quote refers to one John Costella, who wrote a paper arguing against Shawyer's reasoning.

Regarding rationalism versus empiricism, I had meant that Costella had been arguing against Shawyer via rationalism. However, rationalism is, in the end, educated speculation.

What irks me about the affair is that no one attempted to recreate the device. If it works, then who cares? It seems like his peers cared more about their precious, admittedly well-founded preconceptions than the results.

But! We've already been through this....Check page 2 of the thread.

shawnhcorey

2012-03-01, 08:35 PM

This is precisely what the Casimir effect is thought to do. So don't argue with me, argue with Casimir.

You keep insisting that the mathematics are the physics. They're not. With the mathematics, it doesn't matter if you remove positive energy or add negative energy; the result is the same. But the Casimir effect only removes positive energy. And there is a limit of how much energy it can remove. There's a point that the Casimir effect can't go below; something it could do if negative energy existed.

JCarter426

2012-03-01, 08:54 PM

I don't think you're getting exactly what it does. Zero point energy is the point where you can't get any lower. That seems to be the natural energy level of the fabric of the universe. The Casimir effect is a process by which that is still lowered, somehow. Whatever energy causes this is, therefore, negative. The energy density is negative - it is less than the natural level.

This is not the same thing as the temperature dropping below zero. It's not just mathematical semantics.

No, it's not a case of some sci-fi anti-particles, or whatever. It's caused by physical stuff. But it's still a case of an energy that is negative in value - something that is predicted by relativity, and is required for certain things like wormholes to be possible in reality rather than just on paper.

shawnhcorey

2012-03-01, 09:18 PM

I don't think you're getting exactly what it does. Zero point energy is the point where you can't get any lower.

No, that is not the zero point. Quantum mechanics says vacuum always has positive energy. It can never be zero.

JCarter426

2012-03-01, 09:22 PM

grimbold

2012-03-02, 06:03 AM

That's what zero point energy is. :smallconfused: It's like absolute zero. It's not a literal value of zero; it's the closest you can get to zero. You really could spend your time more wisely by reading on these subjects than arguing with me over the semantics.

i beg to differ (sort of

theoretically absolute zero and zero point energy exist as values of 0

they're just unobtainable

slight semantic difference,, sorry..

Asta Kask

2012-03-02, 06:13 AM

Yes, you can have temperature lower than 0C. You can't have them lower than absolute zero.

Yes, we can! (http://www.newscientist.com/article/mg20827893.500-how-to-create-temperatures-below-absolute-zero.html)

Yora

2012-03-02, 08:29 AM

Which is true for the concept of temperature that physicists use, but not for the concept that's used in every day appliances. But under -200° C, every day applications no longer exist anyway. :smallbiggrin:

teratorn

2012-03-02, 08:54 AM

In accepted general relativity the equations work equally well if one includes negative mass, and that would imply negative energy. It was never observed, anti-matter seems to have positive mass. I'm not aware of any evidence for negative mass or energy. The closest one gets is indeed with the Casimir effect.

Zero point energy discussion requires people to be familiar with quantum field theory. The formalism is quite hard to grasp because it involves infinite quantities (like the zero point energy) that are dealt with a thing called field renormalization. Basically one subtracts infinite quantities from each other to obtain finites. It's sort of hocus-pocus, and always made me feel uncomfortable with quantum electrodynamics. Technically, zero point energy is the minimum energy the system can achieve, but the energy density is also technically infinite.

Just to finish my exchange with Scotchland. I'm not sure you are aware of the large number of people having proposed things to harness zero point energy in what would effectively be a perpetual motion device, violating the laws of thermodynamics. They were all shown to be crackpots, some of them quite sophisticated. There are a few hints identifying most of them. Not acknowledging obvious errors in their math is the most important, and in itself an indication one should not waste time with them.

What irks me about the affair is that no one attempted to recreate the device. If it works, then who cares? It seems like his peers cared more about their precious, admittedly well-founded preconceptions than the results.
But why should one waste time with an experiment that is based on a guy not knowing how to solve equations? If one of my students fails to solve a simple set of equations I won't give him money to run an experiment, I'll flunk him.

It would have been quite different if he arrived first with a conclusive experiment and said, «I got this funny result and we need to change our theories.» Getting conservation of energy but non-conservation of momentum would mean one needed to adjust current knowledge. The problem is that the guy doesn't get it, he keeps saying he isn't violating anything, that he can derive everything from accepted theories. This talks of ignorance or, worst, fraud. The guy can't be trusted.

He said «as a result of well known physics if I put microwaves inside this apparatus it'll start moving, now give me funding.» He was lying, theory predicts no such thing. He got his funding, made an inconclusive experiment (as expected) and said «wow, give me more funding to put this in lower temperatures, then it will work.» There are many examples of «establishment» not accepting revolutionary results, but this is something different.

JCarter426

2012-03-02, 12:29 PM

i beg to differ (sort of

theoretically absolute zero and zero point energy exist as values of 0

they're just unobtainable

slight semantic difference,, sorry..
As soon as I finished saying that, I knew someone would point that out. :smalltongue:

It's sort of hocus-pocus, and always made me feel uncomfortable with quantum electrodynamics. Technically, zero point energy is the minimum energy the system can achieve, but the energy density is also technically infinite.

Just to finish my exchange with Scotchland. I'm not sure you are aware of the large number of people having proposed things to harness zero point energy in what would effectively be a perpetual motion device, violating the laws of thermodynamics. They were all shown to be crackpots, some of them quite sophisticated. There are a few hints identifying most of them. Not acknowledging obvious errors in their math is the most important, and in itself an indication one should not waste time with them.
Agreed, but this doesn't change the validity of the phenomenon; there are devices that utilize the Casimir effect, and couldn't possibly work if it were hocus-pocus. It requires more study, certainly. There are still people who feel this way about relativity.

Rockphed

2012-03-02, 01:32 PM

Yora

2012-03-02, 01:54 PM

If negative temperatures do exist, how can you not go past zero?

Grinner

2012-03-02, 02:20 PM

If negative temperatures do exist, how can you not go past zero?

I get the feeling this discussion has become muddled with different systems of measurement.

Edit: A distant memory and some brief research have revealed that zero Kelvins is absolute zero, as opposed to zero degrees Celsius, which is the freezing point of water.

Gwyn chan 'r Gwyll

2012-03-02, 03:50 PM

PirateMonk

2012-03-02, 03:58 PM

Actually, the Planck temperature is the truly scientific measure of temperature*. It's utterly impractical, of course.

I get the feeling this discussion has become muddled with different systems of measurement.

Edit: A distant memory and some brief research have revealed that zero Kelvins is absolute zero, as opposed to zero degrees Celsius, which is the freezing point of water.

No one is talking about Celsius. Read Asta Kask's link. Calling that phenomenon negative absolute temperature seems like something of an abuse of the term, but we should probably just take the scientists' word for it.

*Edit: This sentence is not intended fully seriously.

JCarter426

2012-03-02, 04:09 PM

Kelvin is the truly scientific measurement of temperature. Celsius is the same, the only difference being Kelvin is measured from Absolute 0, while Celsius is measured from the freezing point of water.

Fahrenheit is totally different and confusing and not worth learning unless you're in America.
I'd say they're both pretty worthless for different reasons. Fahrenheit is also based on the freezing point of water, incidentally - salt water, though. The freezing point of water isn't all that important in most aspects of science, rendering both Fahrenheit and Celsius problematic. Kelvin suffers from its relationship with Celsius, as well - the system of degrees they use is just not very practical in most applications, because it creates arbitrarily low numbers and requires decimals in many situations. And don't even get me started on the metric system.

Gwyn chan 'r Gwyll

2012-03-02, 04:22 PM

My point about Fahrenheit and Celsius isn't that they're both based on water, that's all basically arbitrary anyways. The fact remains, however, that
a) Kelvin is the universally accepted scientific measure of temperature
b) Celsius shares a scale with Kelvin
c) The only places in the world that uses Fahrenheit is the USA, Belize, and the Cayman Isles.

All numbers on any scale is arbitrary. To me, Fahrenheit has arbitrarily high numbers in many situations. I have never seen a Celsius/Kelvin number with a decimal outside of incredibly specific science.
There are no inherent advantages or disadvantages to either scale, other than my 3 points above.

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.
Again, however, in the end, everything there is completely arbitrary. You just use whatever measuring system you grew up with. Hell, in Canada we unofficially measure PEOPLE only in feet and pounds. Why? I have no clue.

The third point for Fahrenheit is just as applicable for non-Metric systems either. Metric is the universally accepted scientific measurement system, and is used in all countries across the world, officially, except the USA, Liberia, and Myanmar. The only reason these countries haven't changed over to Metric is because, for Liberia, they're too poor for it to have a difference, Myanmar, they're too isolationist for it to have a difference, and for the USA, they're so economically huge that all the economic problems that arise from having a complete different system of measuring EVERYTHING can be successfully absorbed.

JCarter426

2012-03-02, 04:40 PM

All numbers on any scale is arbitrary. To me, Fahrenheit has arbitrarily high numbers in many situations. I have never seen a Celsius/Kelvin number with a decimal outside of incredibly specific science.
I'm talking more day to day uses, such as weather. Fahrenheit is more precise. Its precision might not make a lick of sense, but it is more precise.

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.
I said don't get me started on the metric system. :smalltongue:

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.
The only thing I hate more than the metric system is this. Well, and the number seven.

But my problem with the metric system isn't the logic - it's a fair attempt at a logical system. My problem is 10 is just a horrible, horrible, horrible number to work with. Yeah, it's "easier to remember", but it only has four divisors. It's practically a prime number. You can't divide it into thirds or even quarters without going into fractions or decimals, or rounding. And that makes it annoying to the point of useless in many applications - publishing, carpentry, cooking, just to name a few. It might work in strictly scientific settings, but it's impossible to use (for me at least) day to day.

Anyway, I agree, it's all arbitrary. But I still think both Fahrenheit and Celisus are inherently flawed. They were designed for specific things, not as universal systems of measurement. Kelvin has problems, as I said, due to its connections to Celsius; the degree system could be easily changed to make absolute zero and the freezing point of water (or some Earth temperature) align better. As it is now, Kelvin is still designed to have exactly 100 degrees between the freezing point of water and the boiling point of water, which really doesn't make any sense when both those temperatures aren't even integers on this scale.

PirateMonk

2012-03-02, 06:06 PM

All numbers on any scale is arbitrary.

Right, but √(ħc5/(Gk2))=1 is less arbitrary than the alternatives.

Drolyt

2012-03-02, 06:08 PM

Yora

2012-03-03, 06:28 AM

And the metric system is simply easier to remember. That's what it comes down to. It's internally consistent, and easy to remember because everything's a multiple of 10. 10 millimeters in a centimeter, 10 centimeters in a decimeter (though nobody really uses those), 10 decimeters or 100 centimeters in a meter, etc.

As opposed to 12 inches in a foot, 4 feet in a yard, 145 yards in a mile... I actually don't even know anything other than how many inches are in a foot, to be honest.
Again, however, in the end, everything there is completely arbitrary. You just use whatever measuring system you grew up with. Hell, in Canada we unofficially measure PEOPLE only in feet and pounds. Why? I have no clue.
With non-metric measurements, you can often still see it in the name, that they were not origiginally meant to be accurate measurements on a specific scale, but estimations for people who had no measuring devices. Obviously the foot, but also the mile comes from lating "thousand paces".
In a world were accurate measurement devices are everywhere, those scales are redundant.

Form

2012-03-03, 07:38 AM

I've been staying out of the temperature discussion because I'm not that knowledgable about it but... I'll go ahead and explain how I think it works.
On the Kelvin scale 0 is absolute zero, no temperature at all. In classical physics this was a real but unreachable value, while in modern, quantum physics it is believed to simply be non-physical since 0 temperature would imply 0 energy which is impossible in quantum mechanics. That said, negative temperature is possible. It has to do with what temperature is, which is kind of complicated. I won't go into that since I haven't actually figured it all out, but realize that negative temperature isn't less than 0 regular temperature, it defines an unusual variant of regular temperature. This can be observed in the fact that negative temperature doesn't necessarily mean cold. It is just like negative charge, it doesn't mean less than 0 charge, it means a different kind of charge.

Statistical mechanics and thermodynamics course powers ho! The temperature of a system is defined in terms of how the energy depends on the entropy of the system. It is defined as the derivative of the energy w.r.t. the entropy. The larger the energy increase due to an increase in entropy, the higher the temperature.

When talking about negative temperature a better parameter to use is coldness or the inverse temperature. The coldness of a system is of course, by definition, related to how the entropy depends on the energy. Systems with negative coldness exist and are hotter (lower value of coldness == less cold) than systems with positive coldness. One can easily convert coldness to temperature and vice versa since they're each other's inverse, but the coldness parameter is usually more useful because the temperature can get a bit funky about 0. Anyway, systems with negative temperature are hotter than systems with positive temperature.

I'd also like to stress that 0 temperature does not mean 0 energy. The energy of the ground state of a system can be non-zero. The energy of the ground state of a harmonic oscillator, for instance, is non-zero. A system consisting of particles behaving like harmonic oscillators will have a non-zero energy at T=0. The particles will occupy the ground state, but the ground state energy is non-zero and so the energy of the system will be non-zero.

(Purely for completeness: For a system of bosons all particles will inhabit the ground state at T=0. For a sytem of fermions the situation is different.)

The above should be correct, but if someone more knowledgeable detects an error or feels the need to elaborate on any of my points they should of course feel free to do so.

Yora

2012-03-04, 06:11 PM

Grinner

2012-03-04, 06:30 PM

Why is purple such a rare color in nature? There's lots of reds, browns, yellows, and greens, and even some stuff that is blue. But pink and light purple colors seem to appear only in a couple of flowers. Is there a reason there are so few substances that reflect purple light?

While a can't give a scientific perspective on it, I can tell you that violet paints are exceedingly difficult to mix by hand. So, the particular pigments required for violet must be in precise ratios.

But that's just my guess.

Yora

2012-03-04, 08:30 PM

Grinner

2012-03-04, 08:40 PM

How is that? When working with pigments, it is more difficult than making green or orange? Or is it, that our minds classify shades as either blue or red, with only a small segment that we would call "purple"?

Probably more the former.

I once made a painting entirely in violet (some might call it "purple"), and I would often compare mixes I thought were true violet to a color wheel. Most of the time, they were either too blue or too red.

It just seems like green and orange have larger margins of error.

Yora

2012-03-04, 08:43 PM

So you think it's not that certain wavelengths are rarely reflected in nature, but that we just don't think of them as purple?

Grinner

2012-03-04, 08:48 PM

*I* don't have a fully-formed opinion on the matter. I'm just stating what I've experienced.

PirateMonk

2012-03-04, 09:47 PM

So you think it's not that certain wavelengths are rarely reflected in nature, but that we just don't think of them as purple?

That wouldn't quite work, since there's fairly little that's blue, either.

JCarter426

2012-03-04, 10:48 PM

Different things are colored in different ways, so I don't think there's a unilateral answer. But plants are simple enough. Plants absorb light, so the color we see is the spectrum of light that they're not absorbing. Now, what colors they choose to absorb depends on what are reaching the plants, which depends on the type of star - what colors it produces in the first place - and the type of planet - what colors are filtered by the atmosphere, as well as how valuable the light is to plants in the first place.

On Earth, most of the green part of the spectrum is filtered out; most that reaches us is red, so most plants absorb red light. A lot of them also absorb blue light, since it's energetic and thus valuable - although not all of them do, because high energy light can also be damaging.

Now, I'm not botanist, but I think flowers have different colors because they're designed to catch extra light when it's needed - when the flowers are in bloom, the plant's reproducing so it needs extra energy for all the hot plant sex. There's a lot more variety of flower color than there is of stem and leave color, so obviously it's a more complex phenomenon.

Leaves change color with the seasons because photosynthesis becomes harder in the winter, both due to the colder temperatures and the lower light levels. The plants essentially go to sleep and don't need as much energy, so the color can change drastically. I think it depends more on other factors, because of how little photosynthesis is occurring.

So in short, you don't see a lot of purple plants here on Earth because most plants are absorbing red light, because it's the most prominent. And since blue is high energy, a lot of them are absorbing blue light too; and to see purple, they'd have to be absorbing everything except blue and red. Now, it's different in pigments that aren't involved in photosynthesis, but I'm sure there's still some similarity. Purple is just a tricky color to make because it means you have a lot of high energy light and a lot of low energy light, but not much in between.

Oh, and here's a pretty cool article on alien plants (http://www.solstation.com/life/a-plants.htm).

Grinner

2012-03-04, 10:53 PM

Oh, and here's a pretty cool article on alien plants (http://www.solstation.com/life/a-plants.htm).

I'm thinking it's high time somebody started an "Exobiology in the Playground" thread. :smalltongue:

Yora

2012-03-05, 06:43 AM

So in short, you don't see a lot of purple plants here on Earth because most plants are absorbing red light, because it's the most prominent. And since blue is high energy, a lot of them are absorbing blue light too; and to see purple, they'd have to be absorbing everything except blue and red. Now, it's different in pigments that aren't involved in photosynthesis, but I'm sure there's still some similarity. Purple is just a tricky color to make because it means you have a lot of high energy light and a lot of low energy light, but not much in between.
Is high energy and low energy the same thing as wavelength? And is purple color a mix of light with very different wavelength?

factotum

2012-03-05, 07:44 AM

Is high energy and low energy the same thing as wavelength?

In the case of electromagnetic energy, yes--all electromagnetic waves travel at the same speed (lightspeed in a vacuum), so what differentiates a low-energy wave from a high-energy one is its frequency and thus wavelength.

As for what colours are available, as I understand it, the human eye sees colours via receptors that are "tuned" to particular frequencies of electromagnetic radiation, so it's largely the relative magnitudes of those frequencies that determine which colours we see.

shawnhcorey

2012-03-05, 08:34 AM

Is high energy and low energy the same thing as wavelength? And is purple color a mix of light with very different wavelength?

No. Purple is a mixture of two wavelengths: red & blue. It is the only colour that is.

Yora

2012-03-05, 08:43 AM

Now that's interesting and the kind of thing I was thinking about. Because I can't think of anything that isn't a flower that would have such a color by nature. Except those snails they collected in massive numbers to make tiny amounts of purple dye, which is another indicator that it is extremely rare in nature if people thought it's worth to go through all the trouble.

teratorn

2012-03-05, 08:48 AM

On Earth, most of the green part of the spectrum is filtered out; most that reaches us is red, so most plants absorb red light. A lot of them also absorb blue light, since it's energetic and thus valuable - although not all of them do, because high energy light can also be damaging.

Not exactly. The solar spectrum on the ground, after all the absorption from the atmosphere, peaks around the green-yellow, and is relativelly flat between blue and red, that is between 400 to 700 nanometers. Chlorophyll a, the major pigment used by green plants has peaks in absorption at ~420 nm and 660 nm, and shows a huge gap in between. An absorption spectrum of chlorophyll a can be found here (http://omlc.ogi.edu/spectra/PhotochemCAD/html/122.html). Chlorophyll wastes the whole middle part of the spectrum (more than 50% of photons are not used).

There are other pigments (bacteriorhodopsin) used by bacteria that have the peaks in absorption exactly in between the two chlorophyll peaks, and one of the explanations proposed is that originally chlorophyll based bacteria were restricted to higher depths and had to use what light those other bacteria didn't absorb.

EDIT: oh, and bacteriorhodopsin is purple!

grimbold

2012-03-05, 11:35 AM

so what would the most efficient color of plant be?

i think i once heard red... but i'm not so sure...

Terumitsu

2012-03-05, 12:08 PM

Going strictly by energy output for different wavelengths, the most 'efficient' color would be so far into the infra-red spectrum so as to appear black to us. Like, you could measure the color in the same area of the spectrum as you would measure radio waves. And I'm not even sure that such a thing would even have the potential to happen in nature. The radio wave thing, I mean...

Tangent:
Although, that sounds like a really nifty idea for a Sci-Fi plant. That is, it grows in urban areas where high levels of radio wave traffic take place. Oh! And it could be a huge problem so one would have to send cleaners out every so often... Though for a plant to survive off that energy alone, there would need to be a lot of it... And too much of that radiation is dangerous to humans...

Right, I claim this idea for a short story now. Dibs.

However, to account for our sun's output wavelengths, then red would suffice quite well.

Yes, we can! (http://www.newscientist.com/article/mg20827893.500-how-to-create-temperatures-below-absolute-zero.html)

And this was just bothering me but reading the article, it simply puts forth a hypothesis on how to achieve such a thing. Experiments have not yet been performed. Also, stopping an atom from vibrating, as has been stated before, simply results in 0 energy. How one gets negative energy from there is hard for me to understand as any motion is positive energy. In short. (http://www.fbclick.com/wp-content/plugins/wp-o-matic/cache/012c3_funny-captions-how-the-hell-do-you-manage-this.jpg)

teratorn

2012-03-05, 12:10 PM

so what would the most efficient color of plant be?

i think i once heard red... but i'm not so sure...

That's easy, plants should be black, that is, they should absorb essentially the whole visible spectrum. Some red algae are in fact nearly black.

JCarter426

2012-03-05, 03:59 PM

Not exactly. The solar spectrum on the ground, after all the absorption from the atmosphere, peaks around the green-yellow, and is relativelly flat between blue and red, that is between 400 to 700 nanometers. Chlorophyll a, the major pigment used by green plants has peaks in absorption at ~420 nm and 660 nm, and shows a huge gap in between. An absorption spectrum of chlorophyll a can be found here (http://omlc.ogi.edu/spectra/PhotochemCAD/html/122.html). Chlorophyll wastes the whole middle part of the spectrum (more than 50% of photons are not used).
Huh, that's not what I've read. So plants are just wasteful?

There are other pigments (bacteriorhodopsin) used by bacteria that have the peaks in absorption exactly in between the two chlorophyll peaks, and one of the explanations proposed is that originally chlorophyll based bacteria were restricted to higher depths and had to use what light those other bacteria didn't absorb.
Ah, that makes sense. So yeah, as I said, there are other factors at play; there is no simple, universal answer.

so what would the most efficient color of plant be?

Going strictly by energy output for different wavelengths, the most 'efficient' color would be so far into the infra-red spectrum so as to appear black to us.

That's easy, plants should be black, that is, they should absorb essentially the whole visible spectrum. Some red algae are in fact nearly black.
Indeed, a black body is a theoretical object that absorbs all radiation; it gets its name because it would appear black to us. Likewise, black holes. It's the same principle.

teratorn

2012-03-05, 07:42 PM

Huh, that's not what I've read. So plants are just wasteful?

In a way sort of the opposite. The rhodopsin bacteria that survive today can't use co2 to build their carbon skeletons, but only dissolved organic carbon sources like acetate. Those would be plentiful in a world with no free molecular oxygen. The green bacteria had to survive on scraps. They used the light the others didn't use and some of them were adapted to use co2 to build their carbon skeletons.

A residue from the activity of green bacteria was molecular oxygen and when that accumulated in the atmosphere aerobic heterotrophs were able to «burn» the dissolved organic carbon for energy, so rhodopsin bacteria had limited raw materials to use and are now restricted to a few habitats. All of this is over-simplistic speculation but it suggests we're really lucky, it's not clear something like this will happen in other worlds, they may remain stuck with anaerobic environments.

The energetic efficiency of plants is not that impressive, typically 1.5 to 3% of the radiant energy is converted into biomass. Particular efficient plants, like sugarcane can reach up to 7-8%. Plants adapted to use other pigments, like carotenoids, to «fill» the chlorophyll gap but their role remains minor in terrestrial plants. Apparently the limiting factor for plant growth is not light, so they didn't go all the way to have black leaves, and remain mostly green. Oh, and by the way, flower color is not related to photosynthesis but to insect vision.

For those interested in solar energy harvesting outside of plants and bacteria, there is a group of wasps, the oriental hornet Vespa orientalis, that seems to do it. There's published research from 2010.

Yora

2012-03-06, 10:45 AM

Why does water interfere with ordinary fire?

Fire needs fuel, heat, and oxygen. I don't think water would be able to remove the fuel, so it's either heat or oxygen.

shawnhcorey

2012-03-06, 10:47 AM

Why does water interfere with ordinary fire?

Fire needs fuel, heat, and oxygen. I don't think water would be able to remove the fuel, so it's either heat or oxygen.

It's both. Water can suffocate small fires but firefighters use water to remove heat from big fires. When the water turns to steam, it absorbs a lot of energy.

Douglas

2012-03-06, 01:48 PM

Why does water interfere with ordinary fire?

Fire needs fuel, heat, and oxygen. I don't think water would be able to remove the fuel, so it's either heat or oxygen.
If I remember my high school chemistry correctly, water has a very high heat capacity compared to most substances - it takes a lot more energy to heat up water by 1 degree than it does to heat up the same amount of most other materials by 1 degree. Thus, if you dump cold water on something hot (or burning), the hot thing will have to cool down a lot before the water heats up enough to match its temperature, and the end result will be a lot colder than the rough midpoint temperature you might otherwise expect. The conversion to steam also takes a lot of heat, as shawnhcorey noted, but taking the liquid from cold to boiling may actually be the bigger factor.

Short version, it's mainly the heat.

Yora

2012-03-06, 04:58 PM

I assume very hot water works just as well. But compared to fire, even 100 degrees is still rather cold.

Grinner

2012-03-06, 05:29 PM

I don't think water simply deprives fire of heat. Rather, it eliminates the oxygen supply and then siphons off heat as the fire tries to spread back.

Take this video (http://www.youtube.com/watch?v=Fj5ex0cDTUs) for instance. Skip to 2:00.

The oil is a liquid of different density than water, and so it only mixes with the water. As it has fuel, residual fire, and oxygen, it then reignites as it separates, and the water has no effect on it.

factotum

2012-03-06, 05:36 PM

The oil is a liquid of different density than water, and so it only mixes with the water. As it has fuel, residual fire, and oxygen, it then reignites as it separates, and the water has no effect on it.

"Mixes" is the wrong term there. Oil and water are immiscible, so they don't mix, and since oil is lighter than water the burning oil will just float on top of the water--this is one of the reasons why water is not very useful for putting out an oil fire!

Grinner

2012-03-06, 05:38 PM

"Mixes" is the wrong term there. Oil and water are immiscible, so they don't mix, and since oil is lighter than water the burning oil will just float on top of the water--this is one of the reasons why water is not very useful for putting out an oil fire!

Mixes? As in "to form a mixture", as opposed to a solution? C'mon, I'm not that bad at this.

Edit: I might stand corrected.

World English Dictionary
mixture (ˈmɪkstʃə)

— n
1. the act of mixing or state of being mixed
2. something mixed; a result of mixing
3. chem a substance consisting of two or more substances mixed together without any chemical bonding between them
4. pharmacol a liquid medicine in which an insoluble compound is suspended in the liquid
5. music an organ stop that controls several ranks of pipes sounding the upper notes in a harmonic series
6. the mixture of petrol vapour and air in an internal-combustion engine

JCarter426

2012-03-06, 06:47 PM

I assume very hot water works just as well. But compared to fire, even 100 degrees is still rather cold.
It's not just about raising the temperature of the water; once water reaches 100 degrees Celsius, it doesn't just magically turn into steam - you need to apply energy to transform matter from one state to another, before it continues to rise in temperature. It's not just about the boiling point of the water - mater isn't all the same, different materials have different conductive properties. Water isn't a good conductor, it's very bad at this sort of thing compared to other materials.

Here's a real world example for you practical types: Water-based central heating works on this principle. You have hot water inside metal pipes. Water takes a while to cool down, ensuring that the heat will flow throughout the whole building, and the metal casing allows the heat to escape from the water easily so every room gets a bit of the heat.

Yora

2012-03-07, 05:50 AM

"Mixes" is the wrong term there. Oil and water are immiscible, so they don't mix, and since oil is lighter than water the burning oil will just float on top of the water--this is one of the reasons why water is not very useful for putting out an oil fire!

Also the water will evaporate and expand rapidly, spilling the burning liquid everywhere, making the fire much worse.

factotum

2012-03-07, 07:15 AM

Also the water will evaporate and expand rapidly, spilling the burning liquid everywhere, making the fire much worse.

Yeah, that's why I said "one of the reasons"--the Mythbusters episode where they experimented with dumping water into a burning pan of oil was quite scary!

Drolyt

2012-03-07, 07:34 AM

Yeah, that's why I said "one of the reasons"--the Mythbusters episode where they experimented with dumping water into a burning pan of oil was quite scary!
Never saw that episode. I do however remember the time a pan full of oil set on fire right in my kitchen. Well, it only had a little bit of oil and I still don't understand how it caught fire, but I managed to put it out by smothering it with towels. I guess that was depriving it of oxygen. Ruined my towels.

On a completely unrelated note, I'd like to thank everyone for all this interesting info. I haven't had anything worth posting but I love reading this thread.

Yora

2012-03-07, 07:38 AM

They tried to make the largest possible pillar of flame (http://www.youtube.com/watch?v=JmjSUlKoMXE) from an oil fire in a frying pan. They got it really quite big. :smallbiggrin:

MeanMrsMustard

2012-03-07, 07:23 PM

Yay for physics! I am proud to say that I finally get this joke (http://www.giantitp.com/comics/oots0838.html). (First panel, V's line)
(Yes, we talked about magnetism in class. Why do you ask?)

Knaight

2012-03-08, 04:21 AM

It's not just about raising the temperature of the water; once water reaches 100 degrees Celsius, it doesn't just magically turn into steam - you need to apply energy to transform matter from one state to another, before it continues to rise in temperature. It's not just about the boiling point of the water - mater isn't all the same, different materials have different conductive properties. Water isn't a good conductor, it's very bad at this sort of thing compared to other materials.

Heat of vaporization is the key point, yes. However, very broadly speaking, materials with higher specific heat have higher heats of vaporization*, and that makes it a useful indicator. The reduction in available oxygen is, as stated prior, the other main factor.

*This is somewhat simplified terminology.

Caesar

2012-03-08, 10:29 AM

Statistical mechanics and thermodynamics course powers ho! The temperature of a system is defined in terms of how the energy depends on the entropy of the system. It is defined as the derivative of the energy w.r.t. the entropy. The larger the energy increase due to an increase in entropy, the higher the temperature.

When talking about negative temperature a better parameter to use is coldness or the inverse temperature. The coldness of a system is of course, by definition, related to how the entropy depends on the energy. Systems with negative coldness exist and are hotter (lower value of coldness == less cold) than systems with positive coldness. One can easily convert coldness to temperature and vice versa since they're each other's inverse, but the coldness parameter is usually more useful because the temperature can get a bit funky about 0. Anyway, systems with negative temperature are hotter than systems with positive temperature.

I'd also like to stress that 0 temperature does not mean 0 energy. The energy of the ground state of a system can be non-zero. The energy of the ground state of a harmonic oscillator, for instance, is non-zero. A system consisting of particles behaving like harmonic oscillators will have a non-zero energy at T=0. The particles will occupy the ground state, but the ground state energy is non-zero and so the energy of the system will be non-zero.

(Purely for completeness: For a system of bosons all particles will inhabit the ground state at T=0. For a sytem of fermions the situation is different.)

The above should be correct, but if someone more knowledgeable detects an error or feels the need to elaborate on any of my points they should of course feel free to do so.

Correct, but confusing to the layman I would venture. Might want ammend it, but no matter what, it ends up sounding confusing due to the statistical definitions of temperature, heat, and cold. I'll give it a shot, regardless:

No system can be colder than absolute zero; this is not what negative temperature means. Absolute zero is absolute. Nothing goes below it.A system that has a negative temperature is actually hotter than a system with any positive temperature. This is not the same as infinite temperature (another impossiblity, but due to constraints of special relativity), but the difference is academic. Heat will always flow from any negative energy system into any positive energy system.

Grinner

2012-03-08, 05:15 PM

How does irradiation work? How do the effects of nuclear contamination linger so long?

Jimorian

2012-03-08, 05:39 PM

How does irradiation work? How do the effects of nuclear contamination linger so long?

There are 2 ways that "stuff" can become a radiation hazard. The first, and most common way is for already radioactive elements to get spread around and infiltrate other materials. "Dirty" radioactive weapons work this way, by spreading enough nuclear material around in dust form that it gets in *everything* So even though the overall average radioactive level is small, there's no way to avoid being exposed no matter how well you try to clean everything, though surface cleaning (and making sure THAT material is properly disposed of), will mitigate the majority of the problem. This is the main problem facing areas after a nuclear reactor explodes (from steam pressure and/or hydrogen gas released from the coolant water, NOT from the kind of critical mass fission blast from a nuclear weapon).

The other way is for the alpha, beta, and gamma particles from radioactive decay to strike other atoms, thus converting them from stable isotopes to unstable, radioactive ones. This is typically only a problem after LONG periods of exposure, so the most common materials here are containers, pipes, and other construction materials near nuclear reactors, plus I believe some medical equipment that uses radiation for treatment.

The length of time something remains dangerous depends on what elements are involved, and what the half-life of their decay is. The most radioactive materials typically have short half lives, which means they "burn" faster spitting out more particles per unit weight, but for a shorter time than long half life materials. Uranium is slow burn, so it usually takes a lot to produce enough direct radiation to be a danger. What usually makes a contaminated site bad is that you have a mix of fast, medium and slow, and some of the fast may decay into OTHER unstable isotopes with various rates, so it takes a long time for the whole combination to finally settle to a safe level. This is also why plans for radioactive storage sites think in the long term.

One proposed way of handling that dilemma is to break down radioactive waste to its elemental components, then chemically separate them all so that you can isolate fast isotopes in one pile, slow in another, and then the vast, vast majority of safe isotopes into yet another, thereby decreasing the volume of waste needed to be stored to a tiny fraction of what we haul away now.

JCarter426

2012-03-08, 05:53 PM

Heat of vaporization is the key point, yes. However, very broadly speaking, materials with higher specific heat have higher heats of vaporization*

*This is somewhat simplified terminology.
Right, and this increases water's effectiveness exponentially.

How does irradiation work?
Well, that's a complicated question. But basically radiation is tiny particles flying incredibly fast. The harmful kind is generally ionizing radiation; this means that when it hits matter, it can force an electron out of its orbit. This can be rather damaging. But this is an oversimplification. Photons (i.e. the stuff light is made of) are ionizing, but it's only harmful at certain frequencies. There are a lot of variables, and a lot of them seem counter intuitive.

How do the effects of nuclear contamination linger so long?
This is easier to answer. Radioactive substances are basically matter that's heavy and unstable, so it breaks down over time. Radioactive decay is exponential; after a given amount of time, half of the substance will decay. After the same amount of time, half of what's left will decay, and again, and again. As the substance gets smaller and smaller, it still takes just as long for half of it to decay. The half-life - the time it takes for half of it to decay - varies depending on the substance; for some it's decades, for others it's fractions of a second. But generally, the exponential nature is a major contributing factor to the length.

Form

2012-03-08, 06:00 PM

How does irradiation work? How do the effects of nuclear contamination linger so long?

There are a bunch of atomic nuclei which are simply unstable. To obtain a more stable state they can decay through several different processes in which they emit a particle. The nucleus is changed as a result and radioactive decay will continue until it finally reaches a stable configuration. The particles emitted during this decay process are what constitutes radiation.

Different decay processes yield different types of radiation. Alpha decay results in the emission of helium nuclei, beta(-) decay in electrons and anti-neutrinos, beta(+) decay in positrons and neutrinos and gamma decay in the emission of a photon.

The problem with radioactivity is not just the radiation itself, but also the atoms that produce it. Radioactive nuclei will continue to produce radiation until they all reach a stable state. The half-life associated with radioactive substances is how long it takes for 50% of the nuclei to decay. A longer half-life means a group of radioactive nuclei will produce radiation for a longer time. Half-life times can vary from less than microseconds to much more than billions of years.

An area contaminated by a radioactive substance will remain so until that substance has been removed or until enough of the substance has decayed to a stable state so that the remainder doesn't produce enough radiation to be dangerous.

shawnhcorey

2012-03-08, 08:02 PM

Photons (i.e. the stuff light is made of) are ionizing, but it's only harmful at certain frequencies. There are a lot of variables, and a lot of them seem counter intuitive.

And just to make things more confusing, they got intense light to act like its a photon (http://arstechnica.com/science/news/2012/03/thousands-of-photons-carry-the-photoelectric-effect-into-the-classical-regime.ars).

Knaight

2012-03-09, 01:53 AM

The half-life - the time it takes for half of it to decay - varies depending on the substance; for some it's decades, for others it's fractions of a second. But generally, the exponential nature is a major contributing factor to the length.

Decades? Decades is barely anything, for some it is hundreds of thousands of years.

factotum

2012-03-09, 02:28 AM

Decades? Decades is barely anything, for some it is hundreds of thousands of years.

Maybe, but stuff with a half-life that long probably isn't very radioactive to start with!

JCarter426

2012-03-09, 03:09 AM

I was specifically thinking of fission products associated radiation poisoning (i.e. the stuff that will actually kill you in a realistic scenario). Most of them decay in days, but two common ones - strontium and caesium - have half-lives of about thirty years. That means the fallout from Chernobyl hasn't even reached its first half-life yet.

But yeah, the heavy stuff used in nukes - certain isotopes of uranium and plutonium - have half-lives of several millennia.

Wait, what do you mean "millennia" isn't a word? Curse you, Mozilla!

Er, sorry about that. Anyway...

On the other end we have the unun-whatsits, synthetic elements too trendy to have names, with half-lives measured in milliseconds.

Grinner

2012-03-09, 03:34 AM

I was specifically thinking of fission products associated radiation poisoning (i.e. the stuff that will actually kill you in a realistic scenario). Most of them decay in days, but two common ones - strontium and caesium - have half-lives of about thirty years. That means the fallout from Chernobyl hasn't even reached its first half-life yet.

But yeah, the heavy stuff used in nukes - certain isotopes of uranium and plutonium - have half-lives of several millennia.

Actually, I had been particularly interested in radiation poisoning when I asked the question. I remember having read a book where people affected by a nuclear blast began spontaneously bleeding weeks after the event.

I assume this is the result of being perforated by loose atomic particles.

However, I also remember reading that when travelling in radioactive areas, people will take potassium iodide tablets. The potassium iodide is supposed to back up in the thyroid gland and prevent the absorption of radiation into the body.

Yora

2012-03-09, 05:30 AM

It does not protect against radiation, but prevents certain radioactive particles to be absorbed by the body and instead just passing through. With nuclear accidents, you have radioactive dust everywhere that gets into your nose and mouth. Because of the chemical structure, they bond very well with certain parts of the body. If you take the tablets, you make sure the parts where the radioactive particles can bond with body cells are already occupied.
It doesn't help against radiation from the environment, but severely reduces ongoing radiation from inside your own body after you left the irradiated area.

Alchohol apparently makes a decent substitute, as it will make the radioactive particles bond with the alcohol-molecules and also prevents them from being absorbed by the gland.

factotum

2012-03-09, 07:17 AM

Actually, I had been particularly interested in radiation poisoning when I asked the question. I remember having read a book where people affected by a nuclear blast began spontaneously bleeding weeks after the event.

That's more because the damage is caused fairly quickly, but it takes time for the physiological consequences to manifest themselves. You could receive a fatal radiation dose that stops immediately (e.g. no on-going radiation) and yet take days or even weeks to die.

(Think of paracetamol poisoning as something similar-ish--if you take an overdose of that you'll have suffered fatal liver damage within 6-12 hours, but you might not even notice any symptoms until after the point of no return).

shawnhcorey

2012-03-09, 08:02 AM

Maybe, but stuff with a half-life that long probably isn't very radioactive to start with!

It's very radioactive. What it isn't is concentrated. Long-life radioactive material like radium and uranium naturally appear is very small concentrations. When it's concentrated, it becomes dangerous. Marie Curie died from this.

Yora

2012-03-09, 08:10 AM

I've read that in Japan, the worst contamination was prevented because they were able to maintain containment long enough for the most dangerous substances to mostly decay, as they are only created inside reactors and have a very short half-life.

That's more because the damage is caused fairly quickly, but it takes time for the physiological consequences to manifest themselves. You could receive a fatal radiation dose that stops immediately (e.g. no on-going radiation) and yet take days or even weeks to die.
If the radiation affects the DNA of cells, the effect would only become obvious when the old working cells are replaced by new faulty ones. Which can take some time.

Eirala

2012-03-09, 08:52 AM

And just to make things more confusing, they got intense light to act like its a photon (http://arstechnica.com/science/news/2012/03/thousands-of-photons-carry-the-photoelectric-effect-into-the-classical-regime.ars).

That seems to me like old information. I'm studying nano science and technology and they taught us that like over a year ago, and there are already microscopes using this technology. If the intensity is sufficiently high you can still liberate electrons, even if the energy of a single photon doesn't suffice.

But it's not that surprising. The only problem with liberating electrons with more than one photon (multiple photon excitation) is, that if you excite an electron but don't liberate it, it will go back into the ground state after a very short time. Only with a very intense photon source, the chance that a second photon hits the electron during the small time frame in which the electron is excited is high enough that you can observe it.

I've read that in Japan, the worst contamination was prevented because they were able to maintain containment long enough for the most dangerous substances to mostly decay, as they are only created inside reactors and have a very short half-life.

Yes, it could have been much worse. But in Chernobyl they also managed to prevent the by far worst outcame. There are still plenty of after effects, but it could have been MUCH worse - threatening all of life on earth.
Sadly, saying "It could have been worse" doesn't mean anything if it's about nuclear reactors :/

factotum

2012-03-09, 11:19 AM

There are still plenty of after effects, but it could have been MUCH worse - threatening all of life on earth.

Got some proof of that? Chernobyl was fairly nasty, yes, but not even the most pessimistic projections I've seen suggests it could have threatened the entire planet. As for what actually *did* happen at Chernobyl, I think the total number of confirmed deaths from that accident is into the low hundreds, even now nearly 30 years after it happened; how many people have been killed from the pollution put into the atmosphere from coal-fired power plants over that time, I wonder?

JCarter426

2012-03-10, 02:36 AM

Actually, I had been particularly interested in radiation poisoning when I asked the question. I remember having read a book where people affected by a nuclear blast began spontaneously bleeding weeks after the event.

I assume this is the result of being perforated by loose atomic particles.

That's more because the damage is caused fairly quickly, but it takes time for the physiological consequences to manifest themselves. You could receive a fatal radiation dose that stops immediately (e.g. no on-going radiation) and yet take days or even weeks to die.
Additionally, you can be exposed via food contamination weeks later and miles away. Spinach and mushrooms have been found to absorb caesium, for example. There were cases of milk contaminated by iodine in Japan last year. The grass absorbs the iodine, the cows eat the grass, and it ends up in the milk. Discarded milk in Japan caused iodine levels in rainwater here in Massachusetts to rise.

However, I also remember reading that when travelling in radioactive areas, people will take potassium iodide tablets. The potassium iodide is supposed to back up in the thyroid gland and prevent the absorption of radiation into the body.

It does not protect against radiation, but prevents certain radioactive particles to be absorbed by the body and instead just passing through. With nuclear accidents, you have radioactive dust everywhere that gets into your nose and mouth. Because of the chemical structure, they bond very well with certain parts of the body. If you take the tablets, you make sure the parts where the radioactive particles can bond with body cells are already occupied.
It doesn't help against radiation from the environment, but severely reduces ongoing radiation from inside your own body after you left the irradiated area.

Alchohol apparently makes a decent substitute, as it will make the radioactive particles bond with the alcohol-molecules and also prevents them from being absorbed by the gland.
Yeah, iodine tablets only protect you from iodine - they fill up your thyroid with safe, non-radioactive iodine so there's no room for it to absorb the radioactive kind. Radioactive iodine is one of the most common threats, but it's still just one of many. Radioiodine is pretty nasty, though; its short half-life means if you ingest it, you'll be exposed to a lot of radiation before it's out of your system. And the thyroid readily accepts it because it needs iodine to function and can't tell the difference between isotopes.

Oh, and while we're on the subject of Chernobyl and mushrooms, there are mushrooms that feed off the radiation like plants do on light (http://www.scienceagogo.com/news/20070422222547data_trunc_sys.shtml).

Grinner

2012-03-10, 02:42 AM

Oh, and while we're on the subject of Chernobyl and mushrooms, there are mushrooms that feed off the radiation like plants do on light (http://www.scienceagogo.com/news/20070422222547data_trunc_sys.shtml[/url).

That's pretty cool. Also, the link's broken.

JCarter426

2012-03-10, 02:46 AM

I didn't break it, you obstinate piece of electrons!

I mean, uh, it's fixed. :smallwink:

Eirala

2012-03-10, 07:58 PM

Got some proof of that?

Sorry, I don't remember where I read that. It was quite some time ago. I'll post it when I find it.

As for what actually *did* happen at Chernobyl, I think the total number of confirmed deaths from that accident is into the low hundreds, even now nearly 30 years after it happened

Yeah, the actual amount of deaths is way higher. Of course it's hard to tell who died from the radiation and not from cancer with another cause. But in the years after Chernobyle, already a few thousand of the Liquidators died. Estimates for total deaths range from a few 10,000 up to 200,000.

It's amazing how the Russian government tried and still tries to cover the whole story up. Not until days later did they publicly announce that there was an accident, and they still tried to sell it as not that bad.

how many people have been killed from the pollution put into the atmosphere from coal-fired power plants over that time, I wonder?

Not that's a question i'd also like to hear an answer to.

Drolyt

2012-03-11, 04:05 AM

Yeah, the actual amount of deaths is way higher. Of course it's hard to tell who died from the radiation and not from cancer with another cause. But in the years after Chernobyle, already a few thousand of the Liquidators died. Estimates for total deaths range from a few 10,000 up to 200,000.
Actually this is a very controversial subject. The 200,000 number comes from Greenpeace and isn't terribly reputable among actual physicists. The Chernobyl Forum Report reported a much lower 9,000, although they have been accused of trying to minimize the disaster and, I believe, only counted deaths within the Soviet Union. UNSCEAR (the United Nations Scientific Committee on the Effects of Atomic Radiation) initially reported 27,000 worldwide based on the Linear No Threshold Model. Since then the Linear No Threshold Model has come under attack and some scientists believe that the actual death toll might be as low as 62; this is the number UNSCEAR now stands by. This lower number is based on a model where levels of radiation below a threshold are not harmful.

Eirala

2012-03-12, 09:01 AM

The 200,000 number comes from Greenpeace and isn't terribly reputable among actual physicists.
The Chernobyl Forum Report reported a much lower 9,000, although they have been accused of trying to minimize the disaster and, I believe, only counted deaths within the Soviet Union.
I wouldn't trust Greenpeace with their estimate either. 200,000 may be right for the total number of affected people, but not for the number of people who died, in my opinion.
The point is, it's impossible to determine the number of casualties. 200,000 seems overboard, but you can't rule it out completely either because of the long term effects of radiation and also the large area radioactive material was spread.
9,000 on the other hand seems a bit low. I think it's significantly more.
As always, the truth probably lies somewhere in between.

That's one possibility, it could refer to the casualties in the Soviet Union. Or the number only refers to people who very likely died from radiation or only in the next few years and ignored the uncertain deaths. Or it's just not accurate ^^

some scientists believe that the actual death toll might be as low as 62; this is the number UNSCEAR now stands by. This lower number is based on a model where levels of radiation below a threshold are not harmful.
62? That's already about the number of liquidators who died directly after the incident because of the high radiation dose. Thats just unrealistic.
And while there is a level of radiation that does not affect us, they probably set the bar way too high. The levels of radioactive radiation were tens of million times higher than normally, up to a billion times around the reactor core. A few minutes exposed to such intense radiation and you can be pretty sure that you'll get cancer, and most of the people just ignored the limited time they were allowed to work.

I think the number is around 30,000-50,000, but that's just my guess.

Drolyt

2012-03-12, 05:59 PM

62? That's already about the number of liquidators who died directly after the incident because of the high radiation dose.
Precisely. Some Physicists do not agree with the Linear No Threshold Model and instead believe that under a certain threshold radiation does not cause cancer. Realize that while huge amounts of radiation were released, beyond the immediate proximity the concentrations were in fact very low. It is possible that no one who was not actually on the site received any negative effects.

The problem with estimating the actual effects of radiation is that the deaths it causes will be hidden among the much larger number of cancer victims who would have had cancer anyways. So at best we can make educated guesses. Still, my understanding is that Physicists consider the Linear No Threshold Model to be the worst case scenario, and based on my understanding of the model that makes sense to me, so I tend to discount the larger estimates.

Yora

2012-03-16, 02:06 PM

I just saw another image of the hexagonal cloud on Saturn.

It looks weird, but isn't it just a sine-wave twisted into a circle?

Drolyt

2012-03-16, 04:25 PM

I just saw another image of the hexagonal cloud on Saturn.

It looks weird, but isn't it just a sine-wave twisted into a circle?
Did you read the same Cracked article?

grimbold

2012-03-16, 05:09 PM

came back from a lecture on the histroy of our understanding of the universe today...
very cool

Yora

2012-03-16, 05:11 PM

Did you read the same Cracked article?

I did, but it only reminded my that I had thought about it before and I could ask this thread for confirmation of that idea.

valsedecoconut

2012-03-16, 05:12 PM

I just posted something similar in the math thread, but here's the physics version.

http://physics.stackexchange.com/ is an amazing place for introductory college to post graduate level physics.

Do you want to teach yourself theoretical physics (and get a math minor while going about it)? http://www.staff.science.uu.nl/~hooft101/theorist.html Nobel prize winner t'Hooft has you covered.

Cheers!

irenicObserver

2012-03-22, 11:47 AM

I attended the Kyoto Laureate Symposium at the UCSD Price Center Ballroom yesterday. There was a lecture on the creation of the universe which was very heavy. Unfortunately I did not get enough sleep the night prior and kept dozing off, in the first couple rows! I hope the speaker didn't see me.

noparlpf

2012-03-26, 02:32 PM

I just found this old Physics lab. And I had a good laugh at my answer.
"Part 5: Polarisation. Using two polariser sheets, observe what happens when you rotate one sheet relative to the other. Now using four sheets, see if you can figure out a way to allow light to pass through where two sheets would not have allowed light through."
The right answer:
http://i41.tinypic.com/lccvt.jpg
My answer:
Well, when you hold two perpendicular to each other, they turn dark. But because of the plastic material, they're kind of reflective. So instead of trying to get light to go through the two sheets using the other two, I'm just going to make it go around the two sheets using the other two as a mirror.
http://i42.tinypic.com/2r5gv2p.jpg
The prof. thought it was cute, and it was the best answer anybody in the lab came up with; he had to show us the right answer.

IthilanorStPete

2012-03-26, 02:33 PM

noparlpf

2012-03-26, 02:38 PM

Why exactly does this work as a solution? Also, I really like your approach. :smallbiggrin:

Well, when two are perpendicular, they filter out all the light. But when you put two in the middle, twisted a bit, they reorient some of the light the first one polarises, so not all of that is blocked by the one that's perpendicular to the first. It's still not like looking through a clear sheet, but it's better than looking through two perpendicular polarising sheets.
Something like that.

IthilanorStPete

2012-03-26, 04:44 PM

Well, when two are perpendicular, they filter out all the light. But when you put two in the middle, twisted a bit, they reorient some of the light the first one polarises, so not all of that is blocked by the one that's perpendicular to the first. It's still not like looking through a clear sheet, but it's better than looking through two perpendicular polarising sheets.
Something like that.

Gotcha, thanks.

Yora

2012-03-29, 11:35 AM

Often when you look at large rivers, the surface seems to have to different types of "textures". Some parts have few waves and reflect the surroundings, while others are more frizzled and don't produce discernable mirror images.

Why is that? What's happening with the water?

Grinner

2012-03-29, 12:17 PM

Disruption of flow from underwater rocks and flora?

factotum

2012-03-29, 03:34 PM

Could have something to do with superposition--this basically says that when you have two waves affecting the same point, the result is what you get when you add the waves together. So, if one wave is at negative amplitude and the other is positive, they'll cancel each other out. Since you'll have a lot of waves crossing and re-crossing the water, the interaction of them all is going to produce some complex effects.

Grinner

2012-03-29, 10:28 PM

What is a photon, and how is related to the electromagnetic spectrum?

I understand that it's supposed to be the substance of light, but that's only a vague description.

Drolyt

2012-03-30, 12:19 AM

What is a photon, and how is related to the electromagnetic spectrum?

I understand that it's supposed to be the substance of light, but that's only a vague description.
How is it vague? If I understand properly (and there's a fair chance I might not) light is fundamental. It isn't made up of anything, the same way electrons aren't made up of anything. A photon is a unit of light. You can't have less light than one photon and any quantity of light must be a whole number of photons. Also, things get wacky with quantum mechanics, I don't really understand it well enough to explain, but light can be described as a wave despite being made up of photons. In fact a single photon is somehow a wave. Kind of.

As for the electromagnetic spectrum, that is more complicated. Light is electromagnetic radiation. Which is the same as saying that photons are electromagnetic radiation. The electromagnetic spectrum is simply the range of frequencies electromagnetic radiation can take, and we perceive different frequencies as being different colors, although we can only see a limited range.

Yora

2012-03-30, 05:21 AM

factotum

2012-03-30, 06:27 AM

I think in current physics, electrons are considered to be waves as well and not particles. The particle model works well enough for chemistry, but apparently has not much to do with what electrons really are.

I'm pretty sure the situation is that electrons can act as both particles and waves depending on the circumstances. For example, they will diffract when fired through a diffraction grating, but they can also collide with atomic particles and be affected by magnetic fields, neither of which a pure wave would do.

The Extinguisher

2012-03-30, 10:17 AM

I think in current physics, electrons are considered to be waves as well and not particles. The particle model works well enough for chemistry, but apparently has not much to do with what electrons really are.

They're considered both waves and particles. Everything is a wave and a particle. A "waveicle" as we call them. (We don't call them that)

Drolyt

2012-03-30, 01:22 PM

I'm pretty sure the situation is that electrons can act as both particles and waves depending on the circumstances. For example, they will diffract when fired through a diffraction grating, but they can also collide with atomic particles and be affected by magnetic fields, neither of which a pure wave would do.

They're considered both waves and particles. Everything is a wave and a particle. A "waveicle" as we call them. (We don't call them that)
My research leaves me to believe that this is a matter not entirely decided as of yet. The most certain thing is that they are not waves or particles in the standard sense. The most common interpretation, if I understand it correctly, is that electrons (and photons) are neither particles nor waves but can act as either depending on the circumstances. Another view is that they never act as either particles or waves, but as something inbetween that in the past we have merely succeeded as modelling as a particle or wave in different circumstances. A minority opinion I've heard, and I can't say whether it should be taken seriously or not, is that electrons and photons are in fact both just waves, but weird quantized waves that can be mistaken for particles.

noparlpf

2012-03-30, 01:24 PM

My understanding of the subject is that photons and electrons are confusing things of indeterminate nature that act similarly to both waves and particles but can probably do other things too, we just haven't discovered those other things yet.

RebelRogue

2012-03-30, 01:33 PM

My research leaves me to believe that this is a matter not entirely decided as of yet. The most certain thing is that they are not waves or particles in the standard sense. The most common interpretation, if I understand it correctly, is that electrons (and photons) are neither particles nor waves but can act as either depending on the circumstances. Another view is that they never act as either particles or waves, but as something inbetween that in the past we have merely succeeded as modelling as a particle or wave in different circumstances. A minority opinion I've heard, and I can't say whether it should be taken seriously or not, is that electrons and photons are in fact both just waves, but weird quantized waves that can be mistaken for particles.
It's very much decided. They're quantum objects (as is everything), which means that they're neither, but in different contexts they may act similar to particles or waves.

@noparlpf: Make no mistake. Just because we can't liken a photon or electron's behaviour to the way everyday, macroscopic stuff behaves, that doesn't mean we understand how they act to a freightening degree of accuracy. As for 'what they really are' it seems that such a question literally makes no sense. It's much like asking "how tall is the color green?"

teratorn

2012-03-30, 02:05 PM

What is a photon, and how is related to the electromagnetic spectrum?

I understand that it's supposed to be the substance of light, but that's only a vague description.

A photon is a boson (more exactly a gauge boson, but that is probably too much for you), a packet (quanta) of energy and momentum used to mediate electromagnetic interactions, characterized by a spin number of one (important for its statistically properties) and no mass. Basically what this means is that in electromagnetic interactions energy is transferred into small packets of energy, that is, light is made of a large number of discrete units. That's what «particle» means in this sense.

Strangely enough, despite being particles, their description requires the use of wave properties, with frequencies and wavelengths. The energy (E) of the photon defines a quantity called frequency (v) , by the simple relation E = hv, where h is the so called Planck constant. This frequency, or the corresponding wavelength (l = c/v, c being the speed of light) is what we associate with color. The electromagnetic spectrum is nothing more than the range of wavelengths of the photons.

noparlpf

2012-03-30, 02:09 PM

It's very much decided. They're quantum objects (as is everything), which means that they're neither, but in different contexts they may act similar to particles or waves.

@noparlpf: Make no mistake. Just because we can't liken a photon or electron's behaviour to the way everyday, macroscopic stuff behaves, that doesn't mean we understand how they act to a freightening degree of accuracy. As for 'what they really are' it seems that such a question literally makes no sense. It's much like asking "how tall is the color green?"

Well I've been wondering how tall the color green is for ages, but half my friends can't even see it. One guy sees it as blue instead, and another sees it as yellow or grey.

Yora

2012-03-30, 02:14 PM

The most common interpretation, if I understand it correctly, is that electrons (and photons) are neither particles nor waves but can act as either depending on the circumstances.
That's what I always assumed to be the most plausible solution.

When it has a bill like a duck and a body like a beaver, then it is probably neither. Most certainly not both!
So the "both a particle and a wave" explaination never seemed logical to me.

Drolyt

2012-03-30, 03:04 PM

It's very much decided. They're quantum objects (as is everything), which means that they're neither, but in different contexts they may act similar to particles or waves.
I'm not a physicist, so I'm just reporting what I've read, but what I've read is written by physicists so you'll forgive me if I'm not yet convinced of what you say. To be clear, I don't think anyone except the one wave guy strictly disagrees with you. But there are at least two ways of interpreting what you said. In what I understand to be the standard model wave-particles act exactly like waves in some circumstances and exactly like particles in others, but never like both at the same time. The other interpretation has them always acting like some sort of wave-particle which we only approximate as waves and particles in different circumstances. I don't think the math changes either way, this is just interpretation.

The Extinguisher

2012-03-30, 07:53 PM

A photon is a boson (more exactly a gauge boson, but that is probably too much for you), a packet (quanta) of energy and momentum used to mediate electromagnetic interactions, characterized by a spin number of one (important for its statistically properties) and no mass. Basically what this means is that in electromagnetic interactions energy is transferred into small packets of energy, that is, light is made of a large number of discrete units. That's what «particle» means in this sense.

Strangely enough, despite being particles, their description requires the use of wave properties, with frequencies and wavelengths. The energy (E) of the photon defines a quantity called frequency (v) , by the simple relation E = hv, where h is the so called Planck constant. This frequency, or the corresponding wavelength (l = c/v, c being the speed of light) is what we associate with color. The electromagnetic spectrum is nothing more than the range of wavelengths of the photons.

I'm fairly certain that you don't need to use wave properties to describe a photon, as the momentum of the photon will uniquely describe it as well.

But it's just easier to use frequency and wavelength, because light behaves more like a wave at macroscopic levels than it does a particle.

JCarter426

2012-03-30, 07:57 PM

Interesting point, but I thought it was the other way around - it's only when you break it down to individual particles that the wave-particle duality becomes obvious and thus necessary. The double-slit experiment, for example; it can be explained away as particles taking different routes and interfering with each other - in other words, a large group of particles can act like a wave, just like a large group of water molecules can - but when you only let a single particle through, you get the same interference pattern, which can only be explained by the wave-like nature of the particle itself (or possibly alternate universes or whatever, but never mind that).

Also, on an unrelated note, the forums just bugged out and replaced the little green arrow with an emoticon of the ancient black dragon. But when I refreshed, it went back to normal. Did my attempt to observe it change the outcome? :smallamused:

The Extinguisher

2012-03-30, 09:57 PM

Well yeah, that's what I meant. At macroscopic levels you're dealing with something that by all intents and purposes is a wave. Okay, it's a lot of particles that look like a wave, but it's easier to talk about it's frequency and wavelength in that context.

Yora

2012-03-31, 11:51 AM

The Extinguisher

2012-03-31, 12:57 PM

It's kinda simple once you understand it. The key to it all is that a moving electric field creates a magnetic field, and a moving magnetic field creates an electric field. So you start with an oscillating electric field. The creates a magnetic field oscillating perpendicularly, and you can see where that leads. What you get is mutually perpendicular oscillating fields propagating each other in the third perpendicular direction.

A.K.A, Electromagnetic Radiation.

irenicObserver

2012-03-31, 01:03 PM

Is Earth just spiralling the drain? By which I mean we are already losing .0...1 of a second (I'm not sure how many zeros) each day. One day, after all of Eath's systems die down, when all currents, spheres and rotations stop (which it will) will Earth just plummet into the sun?

Yora

2012-03-31, 01:06 PM

Well, the sun also loses mass while Earth loses velocity, this may results in both canceling each other out, or even cause the earth to move in increasingly larger circles. The moon is making increasingly larger circles around the earth.

However, I've read in several places that the earth will be crushed by gravity as the Sun grows larger (and at the same time less denser), so the effect of gravity on earth appears to be increasing. Which would mean yes, eventually it would fall in.

It's kinda simple once you understand it. The key to it all is that a moving electric field creates a magnetic field, and a moving magnetic field creates an electric field. So you start with an oscillating electric field. The creates a magnetic field oscillating perpendicularly, and you can see where that leads. What you get is mutually perpendicular oscillating fields propagating each other in the third perpendicular direction.

A.K.A, Electromagnetic Radiation.
You mean it's a series of alternating layers of magnetic and electronic fields?

factotum

2012-03-31, 04:07 PM

However, I've read in several places that the earth will be crushed by gravity as the Sun grows larger (and at the same time less denser), so the effect of gravity on earth appears to be increasing.

Er, what? The Sun may very well get larger as it gets older, but it won't get any more massive--where would the extra mass come from? Thus the gravitational field at Earth's distance would be exactly the same. What may well happen once the Sun becomes a red giant is that the star will be so large that the Earth will be orbiting through its outer layers, and even though those layers will be so tenuous they'll be a pretty good approximation of a vacuum, there'll still be enough drag to slow the Earth in its orbit and cause it to fall deeper into the Sun, eventually to be consumed.

As for the Earth's orbit slowing down, yes, it is, but this is because the Earth is actually moving further from the Sun. The rate at which this is happening is absolutely tiny, though, no need to worry about it for a good few thousand million years!

teratorn

2012-03-31, 06:35 PM

It's much like asking "how tall is the color green?"

It's about 550 nano-meters.

The Extinguisher

2012-03-31, 06:40 PM

It's about 550 nano-meters.

No, that's how wide the colour green is.

teratorn

2012-03-31, 07:03 PM

No, that's how wide the colour green is.

It depends if it's standing up or on the side.

noparlpf

2012-03-31, 07:04 PM

Just because green light has a wavelength of 550 nm doesn't mean the color green itself is that tall.

Rawhide

2012-03-31, 07:08 PM

Physics in the Playground! (http://www.youtube.com/watch?v=muVfidujxRg)

noparlpf

2012-03-31, 07:09 PM

Physics in the Playground! (http://www.youtube.com/watch?v=muVfidujxRg)

My favorite Physics presentation EVER.

RebelRogue

2012-03-31, 09:10 PM

It's about 550 nano-meters.
I half expected a response like this, but I ran with it anyway :smallbiggrin:

grimbold

2012-04-01, 06:40 AM

Physics in the Playground! (http://www.youtube.com/watch?v=muVfidujxRg)

made of win!

JCarter426

2012-04-01, 05:03 PM

I have to say that just the concept of a wave surpasses my comprehension. I understand a wave of liquid or a wave of pressure, or even a sine wave as a mathmatical function.

But a wave of electromagnetism? The wind tastes awfully blue today.
Then prepare to have your mind blown...

GRAVITATIONAL WAVES. (http://www.black-holes.org/gwa1.html)

Pie Guy

2012-04-01, 05:21 PM

Then prepare to have your mind blown...

GRAVITATIONAL WAVES. (http://www.black-holes.org/gwa1.html)

There are no words. (http://troll.me/images/i-dont-want-to-live-on-this-planet-anymore/i-dont-want-to-live-in-this-universe-anymore.jpg)

Battleship789

2012-04-03, 05:54 PM

I have to say that just the concept of a wave surpasses my comprehension. I understand a wave of liquid or a wave of pressure, or even a sine wave as a mathmatical function.

But a wave of electromagnetism? The wind tastes awfully blue today.

As others have mentioned, wave-particle duality (and waviness in general, especially with regards to forces [EM, gravity, etc.]) is a fundamental part of modern science. Everything has WPD, even macroscopic objects, however, the wavelengths of macroscopic objects are much to small in comparison to their physical size for this to be observed/meaningful.

Example: The wavelength of an object that masses 100 kg (say, an adult human) and is moving at 5 m/s (a meh running speed), has a wavelength of 1.32*10^-36 m, or ~10^36 times smaller than said human. Contrast this to an electron moving at 5 m/s (not going to happen, but it is useful for comparison), the wavelength is 1.45*10^-4 m, which is ~10^11 times larger than the classical radius of an electron. So we can observe wave properties for an electron, but not a human.

More fun with the dual slit experiment: if you dim the light down a lot, so that you know that only one photon is going through the experiment apparatus over some time, you still get a wave pattern if one collects data for a long period of time. However, each individual photon hits a single spot on the collection screen as if it were a particle (like a BB hitting a wall) but the locations where it can hit are determined by the probabilities due to the wave (due to the photon interfering with itself)! So a location where wave interferrence would cancel out (leaving a dark spot in a bright light experiment) will never have a "hit," and it is more likely for the hit to occur in a area where the wave interference adds up. You can never be sure where the photon will hit, only where it won't hit.

And the dual slit experiment has been done with stuff much larger than electrons: 60 Carbon buckyballs have been used in a dual slit experiment, and they did indeed show wave properties, despite their size.

You mean it's a series of alternating layers of magnetic and electronic fields?

Not really. To reiterate: A moving magnetic field induces (creates) an electric field that is perpendicular in direction to both the magnetic field and the direction in which the magnetic field is moving. (This is also true for the reverse, a moving electric field induces a magnetic field...) So the magnetic field and the electric field oscillate around a line (that points in the direction of movement) and are perpendicular to each other.

Here is a .gif that shows what is happening: gif (http://en.wikipedia.org/wiki/File:Electromagneticwave3D.gif).

Drolyt

2012-04-03, 08:33 PM

Okay, so I have a sort of odd question. In some works of fiction magic interferes with "technology". I put technology in quotes because it seems a bit random what counts as "technology". Anyways, a thought occurred to me, but I don't know enough physics to know if it would work. A magnetic field interferes with electronics, correct? So could we say that that magic produces an electromagnetic field, or even works on electromagnetic principles? If so, what effects would a field capable of interfering with electronics have in general? For example, would it have other effects besides interfering with electronics?

JCarter426

2012-04-03, 09:36 PM

I dare say it should interfere with compasses. But if you're looking for a scientific explanation for it, I'd say magnetism isn't the best one. Magnetic fields don't mess with technology in the way that magic does in certain fictional works - or at least, the strength of a magnetic field required for one to do such would be so enormous that a wizard would have other problems beside messing up someone's computer. Static electricity is less problematic; for one thing, it's directly harmful to a variety of technical components (magnets aren't, it's just that hard drives use magnets to store data, so other magnets would interfere, of course), but more importantly, you don't need a bolt of lightning to do it.

I prefer any explanation that involves quantum mechanics though, though. In any universe with science as we know it, magic must involve laws physics that are fundamentally different from that we understand. Assuming we haven't gotten everything in physics wrong, there must be entirely separate forces that we haven't discovered, and these forces must involve likewise undiscovered subatomic particles. And how they would react with any other system that involves the delicate manipulation of subatomic particles - e.g. all electronics - I can't say, since we haven't discovered these particles. But it's probably not something good.

Grinner

2012-04-03, 11:49 PM

On the other hand, if you're looking for a semi-scientific to pseudo-scientific explanation, much fringe science associates forms of magnetism with the supernatural. Take for instance E.V.P., certain interpretations of ghosts, a couple of takes on psychics, religious experiences, and life itself.

Knaight

2012-04-04, 01:10 AM

Okay, so I have a sort of odd question. In some works of fiction magic interferes with "technology". I put technology in quotes because it seems a bit random what counts as "technology". Anyways, a thought occurred to me, but I don't know enough physics to know if it would work. A magnetic field interferes with electronics, correct? So could we say that that magic produces an electromagnetic field, or even works on electromagnetic principles?

Given that "technology" seems to almost always include guns, and the magnet needed to screw up gunpowder ignition is going to cause much bigger problems than messing with "technology" (we're talking about something that is pressing the electrons of every atom towards the nucleus, or stripping them away) it doesn't seem to work.

factotum

2012-04-04, 01:57 AM

It would probably be easier to assume that an intense magical field warps the values of some physical constants in such a way that things stop working...of course, you then have to figure out how a living being remains that way inside such a field! In short, let's just say a wizard did it. :smallsmile:

Drolyt

2012-04-04, 08:13 AM

Given that "technology" seems to almost always include guns, and the magnet needed to screw up gunpowder ignition is going to cause much bigger problems than messing with "technology" (we're talking about something that is pressing the electrons of every atom towards the nucleus, or stripping them away) it doesn't seem to work.
Well, I was thinking it wouldn't include guns. At least, I can't imagine how it could. Guns, or at least some guns, aren't terribly complicated devices (I mean to say that the principle behind them is fairly basic, not that they are easy to engineer). I can't imagine how you would interfere with guns without interfering with almost everything.

In short, let's just say a wizard did it. :smallsmile:
That would be easier, yes, but it would also defeat the purpose of me asking. :smallamused:

noparlpf

2012-04-04, 09:31 AM

Well, I was thinking it wouldn't include guns. At least, I can't imagine how it could. Guns, or at least some guns, aren't terribly complicated devices (I mean to say that the principle behind them is fairly basic, not that they are easy to engineer). I can't imagine how you would interfere with guns without interfering with almost everything.

I don't know. A basic gun isn't hard to make. Remember that episode of Star Trek TOS? I think it was "The Arena", but I might be mixing up names.

Yora

2012-04-04, 12:29 PM

Mythbusters tried to build that thing Kirk had made, and theirs didn't work at all.
Not to say it can't be done, but it's certainly not as easy as Kirk makes it look like.

shawnhcorey

2012-04-04, 12:44 PM

Of course not. Kirk missed a step. It involves wine and urine (don't ask).

Grinner

2012-04-04, 01:03 PM

Of course not. Kirk missed a step. It involves wine and urine (don't ask).

I'm now asking. :smalltongue:

noparlpf

2012-04-04, 01:05 PM

Mythbusters tried to build that thing Kirk had made, and theirs didn't work at all.
Not to say it can't be done, but it's certainly not as easy as Kirk makes it look like.

I was joking by making the Star Trek reference. It's not quite that simple. It is still pretty simple to make a basic gunpowder projectile launcher.

Grinner

2012-04-04, 01:22 PM

I was joking by making the Star Trek reference. It's not quite that simple. It is still pretty simple to make a basic gunpowder projectile launcher.

The trick is not blowing your hands off while doing it. :smallwink:

Drolyt

2012-04-04, 01:40 PM

I'm now asking. :smalltongue:
I'm not exactly sure what he is referring to, but I have heard you can make gunpowder out of urine. I'm not clear on the details.

Grinner

2012-04-04, 01:51 PM

I'm not exactly sure what he is referring to, but I have heard you can make gunpowder out of urine. I'm not clear on the details.

I've heard something like that in the past.

The idea was that urine supposedly contains a high concentration of oxygen. If you mixed some blackpowder, urinated on it, and formed it into cakes, then the increased oxygen content would cause it to burn faster, thus more intensely.

I've never tried to verify that though. :smalleek:

shawnhcorey

2012-04-04, 02:02 PM

I've heard something like that in the past.

The idea was that urine supposedly contains a high concentration of oxygen. If you mixed some blackpowder, urinated on it, and formed it into cakes, then the increased oxygen content would cause it to burn faster, thus more intensely.

I've never tried to verify that though. :smalleek:

Actually, the urine was to make the cakes. Look up gunpowder in Wikipedia and search for "wine-drinking bishops".

noparlpf

2012-04-04, 03:32 PM

The trick is not blowing your hands off while doing it. :smallwink:

I've been warned of that before. Still have all ten fingers, and I'm nearly eighteen! This is a good record!

As for the urine--distilled spirits works better. It's the same principle of wet-mixing they use in pharmaceuticals these days.

Yora

2012-04-07, 04:07 PM

Looking on information how to make fascinating and realistic fantasy castles, I found this series on engineering on youtube (http://www.youtube.com/watch?v=va0NCwQ_XHE&feature=BFa&list=PL812DC4917391DC06&lf=BFp).
It's on a very basic level and rather brief on most topics, but if you don't have any previous knowledge except for what you figured out by playing with wood blocks, sticks, and mud, I find it a very easy introduction. Didn't think it's possible to understand the basic principle of buckling without an advanced degree in physics. :smallbiggrin:
But all those questions like "How do towers topple without being pushed over", "How tall can you stack bricks until the lowest brick at the bottom gets crushed by the weight", or "could you make multi-story buildings with floors made from stone beams without them snapping" (to which the answer is "not in a practical way") are all really not that difficult when someone explains to you the very basics of structural engineering.

Grinner

2012-04-07, 05:26 PM

Drolyt

2012-04-07, 08:10 PM

Looking on information how to make fascinating and realistic fantasy castles, I found this series on engineering on youtube (http://www.youtube.com/watch?v=va0NCwQ_XHE&feature=BFa&list=PL812DC4917391DC06&lf=BFp).
It's on a very basic level and rather brief on most topics, but if you don't have any previous knowledge except for what you figured out by playing with wood blocks, sticks, and mud, I find it a very easy introduction. Didn't think it's possible to understand the basic principle of buckling without an advanced degree in physics. :smallbiggrin:
But all those questions like "How do towers topple without being pushed over", "How tall can you stack bricks until the lowest brick at the bottom gets crushed by the weight", or "could you make multi-story buildings with floors made from stone beams without them snapping" (to which the answer is "not in a practical way") are all really not that difficult when someone explains to you the very basics of structural engineering.
Cool, thanks.

Rockphed

2012-04-07, 10:45 PM

It's very much decided. They're quantum objects (as is everything), which means that they're neither, but in different contexts they may act similar to particles or waves.

Technically, quantum means it exists only at integer multiples of Planck's constant.

Jimorian

2012-04-07, 11:35 PM

I'm not exactly sure what he is referring to, but I have heard you can make gunpowder out of urine. I'm not clear on the details.

It's the nitrates in urine, although human urine is far from the best source. Pig urine is much better, and guano is even better, which is the reason why the bird guano cliffs in Chile were considered one of the most important MILITARY resources in the world during WWI. Being cut off from this source was the single reason Germany developed artificial means of making nitrates in bulk.

shawnhcorey

2012-04-08, 07:04 AM

Technically, quantum means it exists only at integer multiples of Planck's constant.

Sorry, no. Quantum means they exists at discrete energy levels, as opposed to continuous ones. They are not integer multiples.

Asta Kask

2012-04-08, 07:18 AM

I've discovered that all animals with sensitive electrolocation live in water? Is this because water conducts electricity more efficiently or is there something else going on? Or is it just chance playing me for a fool?

factotum

2012-04-08, 11:23 AM

Air doesn't really conduct electricity *at all*. The only way to get it to do so is to use stupidly high voltages, and even then the charge will probably only travel a short distance in the form of a spark.

This is why lightning exists--if air conducted electricity then the built-up charge in a cloud could harmlessly conduct to earth without having to create an ionisation path and temperatures similar to the surface of the Sun!

Douglas

2012-04-08, 05:57 PM

Pure distilled water is also a very poor conductor, but most water on Earth is far from pure. Salt water is an excellent conductor, and that's what fills the oceans.

Fresh water is somewhere in the middle. Outside of a chemistry lab, almost all water has something dissolved in it in at least trace amounts, and it doesn't take all that much to make water conductive. More dissolved ions makes it more conductive, of course, and fresh water is much less conductive than salt water, but "more conductive than air" is a point that is passed at quite low concentrations.

irenicObserver

2012-04-11, 11:02 PM

On gravity and tidal forces, is there time dilation between the center and the event horizon?

The Extinguisher

2012-04-11, 11:58 PM

Long story short, we have no idea. No information can leave the event horizon (Hawking Radiation doesn't count), so we don't know what's going on there.

Grinner

2012-04-12, 08:50 PM

What do you all think time is?

The Extinguisher

2012-04-12, 09:04 PM

What do you all think time is?

Time is a dimension of spacetime, much like length and width.
That's like asking us what we think the x-coridinate is.

Now, our perception of time being different than our perception of space is the interesting question, but that probably has everything to do with universal entropy. It's probably easier to perceive things as going in the same direction as entropy than not. But physics is still the same when time is reversed.

Grinner

2012-04-12, 09:19 PM

Time is a dimension of spacetime, much like length and width.
That's like asking us what we think the x-coridinate is.

That's what time describes.

Gourtox

2012-04-12, 09:19 PM

My physics homework (http://www.oecd-nea.org/dbdata/data/mass-evals2003/mass.mas03)

Why.
WHYYYYYYYYY. :smallfurious:

So I'm reading through the thread and I come across this little link. I got to it and am thoroughly amused when my Google translate app for chrome tells me this page is in Romanian and asks if I want it translated. Funniest thing I've seen all day.

The Extinguisher

2012-04-12, 09:57 PM

That's what time describes.

Again, you're asking what width is. It's a dimension of spacetime. That's all it is.

Time, just like every other dimension of spacetime is distorted by motion and gravity. Time, just like every other dimension of spacetime, between two points is irrelevant when they aren't in each others light cones. It's not anything different.

The only reason we think of it as such is because we're accustomed to looking at time as only increasing, because of entropy, so it's easier for us. But time works the same way no matter what, just like length works the same no matter if it's increasing or decreasing.

Grinner

2012-04-12, 10:06 PM

Again, you're asking what width is. It's a dimension of spacetime. That's all it is.

Time, just like every other dimension of spacetime is distorted by motion and gravity. Time, just like every other dimension of spacetime, between two points is irrelevant when they aren't in each others light cones. It's not anything different.

The only reason we think of it as such is because we're accustomed to looking at time as only increasing, because of entropy, so it's easier for us. But time works the same way no matter what, just like length works the same no matter if it's increasing or decreasing.

I think we have a misunderstanding. I had been asking whether you all personally thought that time has an actual substance or is just a supposition of the human mind.

I get the impression that you are firmly in the latter camp.

The Extinguisher

2012-04-12, 10:14 PM

I think we have a misunderstanding. I had been asking whether you all personally thought that time has an actual substance or is just a supposition of the human mind.

I get the impression that you are firmly in the latter camp.

It does have an actual substance. It's part of spacetime, no different than length or width. Physics still works when time is reversed, so the fact that we only perceive time going in one direction is one thing, but it's the same effect as the direction up. We tend to use up to describe the opposite direction of a gravitational field, just like we tend to describe time going forward in relation to entropy.

Grinner

2012-04-12, 10:22 PM

The Extinguisher

2012-04-12, 10:59 PM

Space elves? Because that's the way the universe is made? The same reason physics still works when you flip the x-axis. It's just the way spacetime is. Sure, you can ask why the speed of a photon in a vacuum is constant across all reference frames, but it's hard to ask why things have a speed at all. It's going to get you nowhere.

Grinner

2012-04-12, 11:17 PM

Drolyt

2012-04-12, 11:39 PM

What makes you think time has a physical substance? What makes you think it can be manipulated? If you can't defend your answer or even just cite a text that will, then you might as well call your line of reasoning a religion. :smallsigh:
I'm too tired to enter this discussion this late, but if I remember correctly there was a good explanation of the theories of special and general relativity in the introductory chapters of Brian Greene's The Elegant Universe, with intuition and a discussion of the experiments that confirmed it. The rest of the book deals with string theory, but that intro is the best general introduction to relativity I know of, and you need to understand relativity to understand current theories of time. Also, if I recall correctly, discussion of the nature of spacetime and why we perceive time as flowing in one direction made up a large part of Stephan Hawking's A Brief History of Time, or perhaps it was the updated and condensed version A Briefer History of Time. These aren't exactly what you are asking for, but they should help.

Grinner

2012-04-12, 11:45 PM

I'm too tired to enter this discussion this late, but if I remember correctly there was a good explanation of the theories of special and general relativity in the introductory chapters of Brian Greene's The Elegant Universe, with intuition and a discussion of the experiments that confirmed it. The rest of the book deals with string theory, but that intro is the best general introduction to relativity I know of, and you need to understand relativity to understand current theories of time. Also, if I recall correctly, discussion of the nature of spacetime and why we perceive time as flowing in one direction made up a large part of Stephan Hawking's A Brief History of Time, or perhaps it was the updated and condensed version A Briefer History of Time. These aren't exactly what you are asking for, but they should help.

Cool. I'll look for them in the library.

shawnhcorey

2012-04-13, 07:51 AM

What do you all think time is?

Time is a mathematical illusion. It does not really exist. Time gets it dimensionality from space, by mapping the cyclic movement of a body to time. (We call these bodies clocks.)

JCarter426

2012-04-13, 06:29 PM

Looking on information how to make fascinating and realistic fantasy castles, I found this series on engineering on youtube (http://www.youtube.com/watch?v=va0NCwQ_XHE&feature=BFa&list=PL812DC4917391DC06&lf=BFp).
I finally finished watching. This is the most fascinating thing I've seen in a while.

As for the issue of time, I'm a believer in time but I don't think you can explain it away as simply being a dimension. My first reaction is "time exists so the universe doesn't happen all at once, because that would be silly". But then I consider that space exists as well, and in more than one dimension. So to say it's just a dimension a) does not explain the why the concept exists to begin with, since there's another kind of dimension that also exists, and b) does not explain why there is only one dimension of time and multiple dimensions of space (there are some models with multiple time dimensions, but most theoretical physicists believe in one temporal dimension, with the addition of the 3 spatial ones we can see and up to 7 we can't as of yet).

So the best I can think of, the only justification for time's existence, is entropy. Never mind the laws of thermodynamics though; entropy simply represents change. The fact that the universe can and does change means that time must exist. Moments exist. The universe as it is right now is different from how it was when you were reading the last sentence, and how it was when you started this sentence is different from how it is now that you're reading the end of it. And how it is right now is very different from how it was at the beginning of the Big Bang. That's what time is to me. I don't know why it is that way, but it is.

the_druid_droid

2012-04-13, 06:59 PM

It does have an actual substance. It's part of spacetime, no different than length or width. Physics still works when time is reversed, so the fact that we only perceive time going in one direction is one thing, but it's the same effect as the direction up. We tend to use up to describe the opposite direction of a gravitational field, just like we tend to describe time going forward in relation to entropy.

Actually, although special relativity unifies the treatment of space and time, it doesn't quite say that they're interchangeable. In particular, the time coordinate has a different sign from the spatial coordinates in the metric, which means it isn't quite on the same footing. In fact, if it wasn't for time being "the same, but different", special relativity wouldn't be that interesting, since the Lorentz transformation would have the same (hyper)spherical geometry as normal Euclidean rotations, instead of its actual hyperbolic geometry within SR.

One other point is that although there hasn't been experimental detection of time-reversal asymmetry, a lot of physicists think there have to be some processes which don't run the same backward as forward. Their reasoning is based on something called the TCP theorem, which says that for pretty much any allowable quantum field theory, the combined symmetry of the product of time-reversal, charge and parity symmetries has to be exact (i.e. non-violated). Experiments with neutrinos have already shown that CP symmetry is violated separately in at least some decay processes, so for TCP to remain a perfect symmetry, time-reversal invariance would also need to be broken (the reasoning for this is loosely because -1 * -1 = +1, so two violations together make for an unviolated product).

I hope that didn't come off like I'm trying to call you out on time stuff; I just thought I'd clarify a few points and throw out some new topics for discussion.

shawnhcorey

2012-04-13, 07:10 PM

the_druid_droid

2012-04-13, 07:38 PM

It's the square of the time coordinate that's subtracted from the sum of squares of the space coordinates. So it uses imaginary numbers. :smalltongue:

It's true that some books use this convention, and then the metric is just the unit matrix, however the point remains that the time coordinate is distinguished from the others; in this case by the presence of a factor of i. Also, most modern books avoid introducing the factor of i, because it isn't really necessary and more advanced work depends on the language of vectors, one-forms and metric tensors anyway. Using these tools, there is no need for the imaginary factor, since the negative sign is transferred to the metric and no square roots appear.

All the equations of quantum-particle physics are reversible. That means you can reverse them locally creating a small pocket where time appears to run backward. All the equations of quantum-wave physics are not reversible. A wave is an expanding sphere so creating a shrinking sphere that perfectly converges at a point is impossible.

This distinction is not actually a thing.

shawnhcorey

2012-04-13, 09:11 PM

All the equations of quantum-particle physics are reversible. That means you can reverse them locally creating a small pocket where time appears to run backward. All the equations of quantum-wave physics are not reversible. A wave is an expanding sphere so creating a shrinking sphere that perfectly converges at a point is impossible.

This distinction is not actually a thing.

I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider this diagram (http://www.zeitnews.org/chemistry-physics-and-material-sciences-research/quantum-physics-first-researchers-observe-single-photons-in-two-slit-interferometer-experiment.html) of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.

the_druid_droid

2012-04-13, 09:56 PM

I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider this diagram (http://www.zeitnews.org/chemistry-physics-and-material-sciences-research/quantum-physics-first-researchers-observe-single-photons-in-two-slit-interferometer-experiment.html) of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.

My point is that quantum mechanics applies equally to all physical phenomena, and it is the equations of the theory that must be invariant under TCP symmetry. The distinction between particle-like and wave-like behavior isn't an issue for the TCP theorem, because it deals with the structure of the theory itself, rather than a particular manifestation.

The Extinguisher

2012-04-14, 12:01 AM

Actually, although special relativity unifies the treatment of space and time, it doesn't quite say that they're interchangeable. In particular, the time coordinate has a different sign from the spatial coordinates in the metric, which means it isn't quite on the same footing. In fact, if it wasn't for time being "the same, but different", special relativity wouldn't be that interesting, since the Lorentz transformation would have the same (hyper)spherical geometry as normal Euclidean rotations, instead of its actual hyperbolic geometry within SR.

One other point is that although there hasn't been experimental detection of time-reversal asymmetry, a lot of physicists think there have to be some processes which don't run the same backward as forward. Their reasoning is based on something called the TCP theorem, which says that for pretty much any allowable quantum field theory, the combined symmetry of the product of time-reversal, charge and parity symmetries has to be exact (i.e. non-violated). Experiments with neutrinos have already shown that CP symmetry is violated separately in at least some decay processes, so for TCP to remain a perfect symmetry, time-reversal invariance would also need to be broken (the reasoning for this is loosely because -1 * -1 = +1, so two violations together make for an unviolated product).

I hope that didn't come off like I'm trying to call you out on time stuff; I just thought I'd clarify a few points and throw out some new topics for discussion.

That's interesting. I'll admit I've never gone too in depth with relativity (I decided to focus more on math than physics), but that's cool to know. Although, I wonder if it's possible to set special relativity up in such as way that, say, the x-dimension dilated, while the others contracted in motion. Is our choice of time as the separate dimension mostly arbitrary (I think it would be)

As for TCP symmetry, I didn't know CP symmetry had been violated. That's really cool.

I'm not sure what you mean by "a thing" but the distinction is real. Waves do not behave like particles. Consider this diagram (http://www.zeitnews.org/chemistry-physics-and-material-sciences-research/quantum-physics-first-researchers-observe-single-photons-in-two-slit-interferometer-experiment.html) of the double-slit experiment. Notice that the path of the particle bends when it passes thru one of the slits. But there is absolutely no reason for it to do so. A particle in motion remains in motion unless acted on by an outside force. There is no outside force here. The particle should continue in a straight path. Waves behave differently than particle and you cannot explain why the double-slit experiment behaves the way it does without them. You must make the distinction.

Yes, but in quantum physics, nothing is a wave or a particle, but a combination of both. The particle is a wave as well. That's why it works.

the_druid_droid

2012-04-14, 12:49 AM

That's interesting. I'll admit I've never gone too in depth with relativity (I decided to focus more on math than physics), but that's cool to know. Although, I wonder if it's possible to set special relativity up in such as way that, say, the x-dimension dilated, while the others contracted in motion. Is our choice of time as the separate dimension mostly arbitrary (I think it would be)

It is possible to set things up so that the space-like coordinates have negative signs in the interval expression (and the metric) and the time coordinate has a positive sign. However, the time and space coordinates are grouped separately, from what I understand, because the speed of light (being an invariant velocity) relates space and time in a specific way. However, it doesn't link two different spatial coordinates in the same way, so you couldn't have arbitrary components dilate and contract independently; they'd have to be linked by the underlying structure based on the invariance of light speed.

As for TCP symmetry, I didn't know CP symmetry had been violated. That's really cool.

Re-reading my sources suggests that I misspoke about neutrinos. The neutrino experiment showed that P symmetry alone was violated. It was actually experiments with kaon decays that showed CP was violated (B mesons have a similar effect). The point remains, but I thought I should mention it.

Interestingly, one of the proposed experiments for detecting T violation is to actually try and run certain decay processes 'backward', by firing the products together to re-form the starting particles. The problem is that only neutrino reactions have low enough background noise to be viable, but neutrinos are tricky to work with since they interact so weakly. There are a few indirect methods available, but so far nothing experimentally exists to prove that T is in fact a broken symmetry.

shawnhcorey

2012-04-14, 06:30 AM

Yes, but in quantum physics, nothing is a wave or a particle, but a combination of both. The particle is a wave as well. That's why it works.

But the whole point of my argument is that because it is a wave, time is irreversible. If you replaced the detector with a source and the source with a detector and send a quantum backward thru the experiment, the chances of it hitting the detector are slight. The wave properties of quanta make time irreversible.

Yora

2012-04-14, 07:39 AM

Rawhide

2012-04-14, 07:49 AM

Does anyone of you know what causes a column to buckle under compression? I used my google-fu to the point of exhaustion, but all I can find is demonstrations of how buckling is affected by different cross sections, the length of the column, and how the ends of the column is mounted, or people explaining the equation to calculate at what force buckling does occur.

But what causes it? If I have a perfectly shaped column and there is the same compression force at any point of a given cross section, I've seen the claim buckling would still occur. If there's a slight bend in the column, buckling would occur at a lower compression force, but it would still occur if it's perfectly straight and of homogenous composition.
Then what causes the mass to move sideways when the only forces acting on it are up and down?

None of the videos you posted earlier (see below) helped?

Looking on information how to make fascinating and realistic fantasy castles, I found this series on engineering on youtube (http://www.youtube.com/watch?v=va0NCwQ_XHE&feature=BFa&list=PL812DC4917391DC06&lf=BFp).
It's on a very basic level and rather brief on most topics, but if you don't have any previous knowledge except for what you figured out by playing with wood blocks, sticks, and mud, I find it a very easy introduction. Didn't think it's possible to understand the basic principle of buckling without an advanced degree in physics. :smallbiggrin:
But all those questions like "How do towers topple without being pushed over", "How tall can you stack bricks until the lowest brick at the bottom gets crushed by the weight", or "could you make multi-story buildings with floors made from stone beams without them snapping" (to which the answer is "not in a practical way") are all really not that difficult when someone explains to you the very basics of structural engineering.

Yora

2012-04-14, 07:53 AM

No, it just explains the math at what point buckling occurs and what to do to make it less likely to happen. No explaination what causes it.

shawnhcorey

2012-04-14, 07:56 AM

... but it would still occur if it's perfectly straight and of homogenous composition.
Then what causes the mass to move sideways when the only forces acting on it are up and down?

But in real life, nothing is perfect. There are always tiny flaws in the structure where failure can start.

The mass moves sideways because the flaws introduce shear forces at an angle. The shear forces can be composed into two components, one vertical, one horizontal. It's the horizontal one that causes sideways movement.

the_druid_droid

2012-04-14, 02:55 PM

But the whole point of my argument is that because it is a wave, time is irreversible. If you replaced the detector with a source and the source with a detector and send a quantum backward thru the experiment, the chances of it hitting the detector are slight. The wave properties of quanta make time irreversible.

Actually, there’s nothing about waves that makes them intrinsically time irreversible. For a completely classical example, Maxwell’s equations describe wave motion, but they behave just as well when you run time backward. I should also point out that I’m not talking about a specific solution to Maxwell’s equations (which for various reasons may be chosen to be time-asymmetric), but the equations themselves. This is the same point in quantum mechanics: we are talking about the underlying structure of the theory, not necessarily a specific solution to the set of equations.

The reason physicists think time-reversal symmetry is violated has nothing to do with the properties of waves vs. particles, and everything to do with the generality of the TCP theorem and the observed violation of CP symmetry. Even then, in interactions where CP isn’t violated (the strong and electromagnetic domain) time-reversal should continue to be an exact symmetry, again because of TCP.

shawnhcorey

2012-04-15, 09:50 AM

The reason physicists think time-reversal symmetry is violated has nothing to do with the properties of waves vs. particles, and everything to do with the generality of the TCP theorem and the observed violation of CP symmetry. Even then, in interactions where CP isn’t violated (the strong and electromagnetic domain) time-reversal should continue to be an exact symmetry, again because of TCP.

And what does TCP theorem have to do with the double-slit experiment? This is what I'm talking about. Physicists are trained to disrespect any and every mention of wave physics.

Thou shall not do wave physics.

If you shoot a photon backward thru the double-slit experiment, it doesn't return to its source like a particle should. Instead, you get another interference pattern. Waves are not reversible.

Douglas

2012-04-15, 11:59 AM

If you shoot a photon backward thru the double-slit experiment, it doesn't return to its source like a particle should. Instead, you get another interference pattern. Waves are not reversible.
I imagine this is because the photon "shot backward" would not, in fact, correspond to the original interference pattern. Of course shooting a photon with a known location back through the slits doesn't return to the source - that isn't reversing the experiment, it's doing a new and separate double-slit experiment.

In order for the experiment to be a valid test of time symmetry, you would have to construct a photon that matches the full interference pattern of the original experiment's output.

To illustrate with a more easily understood form of wave, consider dropping a pebble into water and the circular wave that expands outward from it as a ripple on the water surface. To study this wave in reverse, you would need to do something like drop a horizontal circular hoop into the water. I suspect that human scientists still have a long way to go before being able to produce the photonic equivalent of this hoop, especially with as complex a form as the double-slit interference pattern, but if it were done I would indeed expect a photon produced by it to return to the source.

the_druid_droid

2012-04-15, 07:32 PM

And what does TCP theorem have to do with the double-slit experiment? This is what I'm talking about. Physicists are trained to disrespect any and every mention of wave physics.

Thou shall not do wave physics.

If you shoot a photon backward thru the double-slit experiment, it doesn't return to its source like a particle should. Instead, you get another interference pattern. Waves are not reversible.

Couple of points here:

1.) The idea that physicists are trained to 'disrespect' waves is just patently false. The culmination of every book on electrodynamics is a detailed discussion of the electromagnetic wave equation, its solution for all sorts of boundary conditions, its consequences for radiation, etc. In fact, in many graduate books, wave material takes up nearly half the textbook volume. In quantum mechanics, traveling wave solutions are used extensively as well, particularly as a part of scattering theory, and owing to the prevalence of collider experiments in high-energy physics, scattering processes (and hence wave-like solutions) constitute the bulk of the subject! Even in classical mechanics, the theory of elastic solids makes extensive use of wave solutions and terminology. Physicists are most definitely not biased against waves, and are in fact dependent on wave formalism to describe a huge portion of natural phenomena.

2.) Waves are in fact time-reversible. For a trivial example, a right-moving wave in free space is just as physical as a left-moving one. If you were to watch a movie of both types, you wouldn't be able to tell whether the left-moving movie was just the right-moving one run backward, or an entirely separate wave. This is exactly what is meant by time reversibility.

3.) To address your specific example, I'll elaborate a bit on douglas' point, since he has the right of it:

I imagine this is because the photon "shot backward" would not, in fact, correspond to the original interference pattern. Of course shooting a photon with a known location back through the slits doesn't return to the source - that isn't reversing the experiment, it's doing a new and separate double-slit experiment.

In order for the experiment to be a valid test of time symmetry, you would have to construct a photon that matches the full interference pattern of the original experiment's output.

The problem for the two-slit experiment is that the final probability distribution is difficult to reconstruct experimentally as an initial condition. Additionally, there are interactions with the material of the screen during the photon's transit which are even more difficult to time-reverse. Strictly speaking, if you don't time-reverse the behavior of the entire system (screen included), you won't get back to the initial result, because you aren't actually time-reversing your experiment, only a part of it.

A better example that illustrates the time-reversibility of waves is a scattering experiment. Say I send a photon toward some scattering center, like an atom. I then measure where the photon comes out, with what momentum, etc. (Strictly speaking these would all be probability distributions, rather than single values in quantum mechanics, but I'm assuming we can navigate around this slight experimental complexity for this thought experiment) If I then send a photon back toward my scatterer with the measured final position, momentum, etc, I will recover the initial state of my photon at the other end of my apparatus. This is time-reversibility, plain and simple.

Yora

2012-04-18, 12:10 PM

Why does a pressurized container with a hole produce thrust?

It probably has something to do with "every action produces an equal and opposite reaction". In air, I would assume that the escaping gas is pushing agains the surrounding air, but this can't be it, as thrusters also work in space, as I understand.

But assuming a spherical container, the gas pressure on the container would be equal at each point and when a hole is opened, the pressure should decrease equally on the whole inside of the container. Why does the container start to move in the opposite direction of the hole? There is less pressure on the container in the area where the hole is, but that can't really be the source of the thrust, can't it?

Douglas

2012-04-18, 12:30 PM

There is less pressure on the container in the area where the hole is, but that can't really be the source of the thrust, can't it?
Actually, yes it is. If you have a large sphere filled with highly compressed air, the air is pushing evenly in all directions on all parts of the sphere's interior surface. If you then cut a 1 foot wide hole in the sphere, suddenly there is a 1 foot wide area on the opposite side of the sphere where the pressurized air is pushing on it with no corresponding push to cancel it out. Multiply the air pressure by the area of the hole, and you get the net force pushing the sphere away from the hole.

That last part might make you think that a bigger hole should make for a better rocket, but there are other factors that make it rather more complicated. A bigger hole would indeed make for a higher initial thrust, but it would also allow a lot more air to escape much faster, which would decrease the pressure of the remaining air and consequently decrease ongoing thrust later. Make the hole too big, and the thrust decreases too quickly for the initial surge to compensate. An ideal rocket would have extremely high pressure and a carefully calibrated not-too-big not-too-small hole. The shape of the hole also matters, but that's approaching details where you'd need to ask an actual rocket scientist.