question about lasers

[Traab]

Specifically the dangerous kind. Lets say you are firing a laser beam at a target some arbitrary distance away. Between the target and the beam there is an object that partially blocks the beam, lets say only a crescent moon shape of the beam continues on. Does that change anything about the laser and its effect when it reaches the target? Like, does the portion that reaches the target do less damage than it normally would to the part of the target hit? Does that effect how, not sure of the right term, coherent the beam remains? Like will there be some scattering from it no longer being a solid beam? Or does it have no effect and there is just a crescent moon shape burned into the target rather than a full circle?

[theNater]

There's a lot of factors at play here, which can cause a lot of different effects. First, while the blocking device probably won't affect the coherence of the laser, lasers aren't perfectly coherent to start with. So depending on the distance to the target, your crescent moon shape may be preserved, or get fuzzy, or (if far enough away) just become a circle again. You can get a sense of this with a flashlight, which is not very coherent at all, by shining it through a piece of cardboard with a shape cut out. Cardboard close to target, you keep the shape, but the further you go the less clear the shape is; lasers do the same thing over (much) longer distances.

As for damage, it depends on how your laser was damaging your target in the first place. Generally, lasers deposit energy that turns into heat. Less laser getting through means less heat, but if you're instantly burning holes in things then applying less heat to a smaller area will just burn through the smaller area. Slower burns, though, mean the heat has some time to dissipate through the rest of the object and into the atmosphere(if any). Depending on exactly how that works out, it may take longer to burn through or not burn through at all.

[McGarnagle]

I'd say that the damage would decrease in proportion to the fraction of the cross-sectional area of the beam that is blocked. The mask/obstacle/whatever is casting a shadow.

All finite sized laser beams diverge from being perfectly collimated as they travel through space, this is what sets the range of the weapon. Encountering an obstacle or mask can enhance this de-collimation effect due to diffraction. This would result in a reduction of the range of the laser.

[Traab]

I'd say that the damage would decrease in proportion to the fraction of the cross-sectional area of the beam that is blocked. The mask/obstacle/whatever is casting a shadow.

All finite sized laser beams diverge from being perfectly collimated as they travel through space, this is what sets the range of the weapon. Encountering an obstacle or mask can enhance this de-collimation effect due to diffraction. This would result in a reduction of the range of the laser.

Ah thank you, both of you really, that was the answer I was looking for. I wasnt sure if a partial obstruction would effect the de-collimation of the beam or not. Honestly, it started off as an odd thought that popped into my head. An article talking about how the starkiller base would have taken years to hit the worlds it targeted and I wondered if that would mean they could plant something in the path of the beam that would either break up the death beam, weaken it, or refract it in some safe way. I didnt want to bring that question up here I was just curious about lasers in general and what effect a partial block would have on them.

[veti]

An article talking about how the starkiller base would have taken years to hit the worlds it targeted and I wondered if that would mean they could plant something in the path of the beam that would either break up the death beam, weaken it, or refract it in some safe way.

Only if they can get news of the laser's firing at them before the laser light itself arrives, which is impossible per the physics I know but then what's that worth.

Otherwise they'd have to pre-emptively block it when the weapon started to point vaguely in their direction, before it was (as far as they knew) actually fired.

I have always thought this "it takes years" idea is really quite misleading. People say stuff like "we're seeing the stars as they were x years ago", but what does that really mean? As far as we're concerned, what we see is how they are now, because how else can you even define "now"?

[Traab]

Only if they can get news of the laser's firing at them before the laser light itself arrives, which is impossible per the physics I know but then what's that worth.

Otherwise they'd have to pre-emptively block it when the weapon started to point vaguely in their direction, before it was (as far as they knew) actually fired.

I have always thought this "it takes years" idea is really quite misleading. People say stuff like "we're seeing the stars as they were x years ago", but what does that really mean? As far as we're concerned, what we see is how they are now, because how else can you even define "now"?

I mean, they have faster than light travel ability, and its a giant laser, meaning it can only move at light speed till it hits its target. there is a reason distance in space is measured in light years. Inside the solar system it might work as a terror weapon, as the distances at light speed are far closer to reasonable (I think a laser from pluto would hit us in 2 days time? Im not sure if I messed up a zero somewhere) but between systems it takes light years to travel. And if you are in star wars you can travel that vast distance in hours at most, giving plenty of time for preparation to be taken. Even if steps to stop the incoming attack couldnt be taken, thats still literal years to evacuate the planet.

[Peelee]

I mean, they have faster than light travel ability, and its a giant laser, meaning it can only move at light speed till it hits its target

Canonically, it is not a laser and does not move at light speed.

[JNAProductions]

Canonically, it is not a laser and does not move at light speed.

What is it, then?

[Peelee]

What is it, then?

Watsonian answer: a beam that tunnels through hyperspace and can be seen on other planets due to rifts in hyperspace tunneling.

Doylist answer: stupid bull**** because Abrams is a hack.

[Brother Oni]

I didnt want to bring that question up here I was just curious about lasers in general and what effect a partial block would have on them.

Bear in mind that there's primarily two designs of weaponised lasers - pulsed and continuous.

Most sci-fi deal with continuous lasers as it's the simplest to understand thanks to years of pop-science and industrial laser cutters. In terms of energy delivery, they're comparatively slow, allowing the target material to heat up and vaporise allowing the beam burns through.

Pulsed lasers deliver energy much more quickly, so quick in fact that only the uppermost layers of the target are burnt off. However the vaporised material is ablated (burnt off) so quickly, it's converted to plasma, which then explodes, causing more damage than expected. On tested animals and humans, there's a secondary side effect that the EM radiation also triggers pain receptors, causing intense pain for comparatively little tissue damage: wikipedia link.

[Rydiro]

Doylist answer: stupid bull**** because Abrams is a hack.

While I agree, Star Wars was always full of space magic.
Honestly, the most unbelievable thing is that someone built that base in the first place. Intel of the former rebellion was really bad.
Like, Princess Leia, educated in governance, must have been a total incompetent hack for her entire existence to screw up after the victory over the empire that badly. No military, no planetary control, no intel, still the same tech since a generation. She is outcompeted by a rival underground organization after a victory and a headstart.
The Order has more unexplained resources than the Joker had his entire career.

[McGarnagle]

I have always thought this "it takes years" idea is really quite misleading. People say stuff like "we're seeing the stars as they were x years ago", but what does that really mean? As far as we're concerned, what we see is how they are now, because how else can you even define "now"?

It means the light that we observe today from some distant object was emitted X years ago, as reckoned by observers on Earth, and that is how long it took that light to reach us. The light we see from a celestial object today is a snapshot of when that object emitted the light, just as a photo of me from five years ago is still five years old even if you're seeing it for the first time today.

[Peelee]

While I agree, Star Wars was always full of space magic.
Honestly, the most unbelievable thing is that someone built that base in the first place. Intel of the former rebellion was really bad.
Like, Princess Leia, educated in governance, must have been a total incompetent hack for her entire existence to screw up after the victory over the empire that badly. No military, no planetary control, no intel, still the same tech since a generation. She is outcompeted by a rival underground organization after a victory and a headstart.
The Order has more unexplained resources than the Joker had his entire career.

The book Bloodlines goes into detail about that, actually! And, short version, you hit the nail on the head except it's Mon Mothma who drops all the balls rather than Leia. But yeah, the New Republic is effectively "we hate the Empire so much we're not going to do anything it did, including any of the basic governance that literally all space governments do.

Also former Imperials who actively want the Empire back make up like 15% of the Senate and we see no problem with that at all."

[Tvtyrant]

While I agree, Star Wars was always full of space magic.
Honestly, the most unbelievable thing is that someone built that base in the first place. Intel of the former rebellion was really bad.
Like, Princess Leia, educated in governance, must have been a total incompetent hack for her entire existence to screw up after the victory over the empire that badly. No military, no planetary control, no intel, still the same tech since a generation. She is outcompeted by a rival underground organization after a victory and a headstart.
The Order has more unexplained resources than the Joker had his entire career.

I mean, the movies explanation made sense to me. War fatigue is a real thing, they fought two very large civil wars in 20 years. That the planetary governments and populations would lose interest in politics and conflict after that is reasonable. Leia is screaming into the dark while worn down, impoverished people label her a hawk and stick their heads in the ground until it is too late.

[Peelee]

I mean, the movies explanation made sense to me. War fatigue is a real thing, they fought two very large civil wars in 20 years. That the planetary governments and populations would lose interest in politics and conflict after that is reasonable. Leia is screaming into the dark while worn down, impoverished people label her a hawk and stick their heads in the ground until it is too late.

Not how it happened.

[Tvtyrant]

Not how it happened.

In The Force Awakens the government forces are literally sitting aside waiting for proof that there is a terrorist organization, and then dies when it turns out there is? Isn't that the whole point of the Resistance, they are the people that care that nefarious things are going on while most of the government and population has basically tuned out.

[veti]

It means the light that we observe today from some distant object was emitted X years ago, as reckoned by observers on Earth, and that is how long it took that light to reach us. The light we see from a celestial object today is a snapshot of when that object emitted the light, just as a photo of me from five years ago is still five years old even if you're seeing it for the first time today.

Yes, so people have been telling me since I was about ten. But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means. The photo analogy is flawed because the photo isn't you, it's a photo of you, and you and the photo are independent objects each with your own lifespans.

When you say that "the light we see from a celestial object was emitted five years ago", you're presupposing the existence of some frame of reference that allows - something, clearly not us - to observe both the light leaving the distant star and the light arriving at our position, and measure the time between these events. (Of course there is a frame of reference that observes both these events, it's the reference of the light itself - but at that frame, I'm pretty sure all events are instantaneous anyway.)

[DavidSh]

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means.

You understand that many people who have thought about matter disagree about that?

Would you also say that when you listen to an object, you hear it, and the "it" that you hear is in the condition of "right now"? If you do, how you do account for not-terribly distant objects having a different "right now" for vision vs sound? If not, why do you prioritize vision?

[asda fasda]

Yes, so people have been telling me since I was about ten. But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means. The photo analogy is flawed because the photo isn't you, it's a photo of you, and you and the photo are independent objects each with your own lifespans.

When you say that "the light we see from a celestial object was emitted five years ago", you're presupposing the existence of some frame of reference that allows - something, clearly not us - to observe both the light leaving the distant star and the light arriving at our position, and measure the time between these events. (Of course there is a frame of reference that observes both these events, it's the reference of the light itself - but at that frame, I'm pretty sure all events are instantaneous anyway.)

On contrary the photo analogy is perfect, because in reality you never see a "thing", what you see is light that bounces out of that thing and has traveled to your eye, so in a way what you see is always a photo of the item, not the item itself. The longer the distance from the item then travel time makes this photo imbedded in light waves less accurate of the current state of the item. Pretty much in the same way as if I was to send you my picture over post what you would receive wouldn't be exact me at the moment when you have received the package.

[Peelee]

In The Force Awakens the government forces are literally sitting aside waiting for proof that there is a terrorist organization, and then dies when it turns out there is? Isn't that the whole point of the Resistance, they are the people that care that nefarious things are going on while most of the government and population has basically tuned out.

In The Force Awakens the government forces are literally on screen for all of twenty seconds, the entirety of which is their planets exploding. There are a few references, that's it.

Conversely, there is an entire book about specifically the government forces after the Rebellion and before the First Order, which I have already talked about in this very thread.

Yes, so people have been telling me since I was about ten. But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means. The photo analogy is flawed because the photo isn't you, it's a photo of you, and you and the photo are independent objects each with your own lifespans.

Blind Bob and I are both at a baseball game. I have seats right behind home base, Blind Bob is up in the nosebleed section by the outfield. Batterman swings and hits. I hear the crack of the bat immediately. Blind Bob does not hear the crack of the bat until a couple seconds have passed. When Blind Bob hears the crack, he knows that Batterman got a hit, but the balli is pretty honkin' far away from the at at that moment.

Is that better?

[Tvtyrant]

In The Force Awakens the government forces are literally on screen for all of twenty seconds, the entirety of which is their planets exploding. There are a few references, that's it.

Conversely, there is an entire book about specifically the government forces after the Rebellion and before the First Order, which I have already talked about in this very thread.

Yeah but
1. That has nothing to do with whether the movie explanation made sense, which is what I said.

and 2. I have absolutely no interest in the extended universe. I barely cared about the first two for that series.

[Peelee]

Yeah but
1. That has nothing to do with whether the movie explanation made sense, which is what I said.

and 2. I have absolutely no interest in the extended universe. I barely cared about the first two for that series.

1. There was no movie explanation. This is one of the many, many problems with TFA/the sequel trilogy as a whole.

2. It is irrelevant whether or not you have interest in the extended universe. For example, Palpatine returned in the extended universe, and yet that matters for the movies. Your lack of interest in Fortnite (and my own lack of interest in Fortnite, for that matter) doesn't change anything.

2a. I completely understand your lack of interest.

[Rydiro]

1. There was no movie explanation. This is one of the many, many problems with TFA/the sequel trilogy as a whole.

Agree.
ALL Governments having no intel / military is absurd.
I mean, come on, even the backwater Naboo had a functional military in the prequels. And they had active diplomatic relations with the rest of the republic.

[Tarmor]

Yes, so people have been telling me since I was about ten. But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means. The photo analogy is flawed because the photo isn't you, it's a photo of you, and you and the photo are independent objects each with your own lifespans.

If it truly makes no sense to you, then there's probably no point in any of us trying to explain it to you in another way, but...

Do you accept the idea of the speed of light, and the distance involved between stars?
If you don't like the photo analogy, how about a letter... (back when people wrote to each other). If I mail you a letter right now , then it's delivered to you... you will receive it "now", but you can see the date on my letter, and the post mark, and it's obvious that your "now" isn't the same as mine. It's taken time to get to you.

[AstralSeal]

Yes, so people have been telling me since I was about ten. But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

When you look at an object, you see it, and the "it" that you see is in a condition that we call "right now". That's what "now" means. The photo analogy is flawed because the photo isn't you, it's a photo of you, and you and the photo are independent objects each with your own lifespans.

When you say that "the light we see from a celestial object was emitted five years ago", you're presupposing the existence of some frame of reference that allows - something, clearly not us - to observe both the light leaving the distant star and the light arriving at our position, and measure the time between these events. (Of course there is a frame of reference that observes both these events, it's the reference of the light itself - but at that frame, I'm pretty sure all events are instantaneous anyway.)

It might help to look at an example with electromagnetic energy, but also one you can relate to. Since you have probably experienced a thunderstorm, lightning might help. Thunder is the sound of lightning. When the energy or whatever heats the air it causes a shock wave, which makes a noise. Depending on how far you are from the lightning, the thunder seems to occur longer after the flash. This is because both sound and light have to travel to you, but light is faster than sound. The thunder and lightning did happen at the same time (or very close to the same time), but the further away you are, the longer the gap between the thunder will seem, due to sound being much slower than light. If I am standing right next to something which got hit by the lightning and you are a kilometre away, I am going to say that it was pretty much the same time, while you will say that the thunder happened about 2.9 seconds after the lightning, because it would take sound about 2.9 seconds to travel that distance in air. We are both on the earth, and both standing still, which makes us in the same reference frame, but the light and the sound barely traveled to reach me, while they travel a distance to reach you, creating the perception that the slower one occurred after the faster one, when they actually occurred simultaneously.

[Lord Torath]

When you say that "the light we see from a celestial object was emitted five years ago", you're presupposing the existence of some frame of reference that allows - something, clearly not us - to observe both the light leaving the distant star and the light arriving at our position, and measure the time between these events. (Of course there is a frame of reference that observes both these events, it's the reference of the light itself - but at that frame, I'm pretty sure all events are instantaneous anyway.)

So let's posit a star goes nova 5 light-years away. (Let's also assume that's far enough away that we won't be harmed by the explosion - it's probably not, but we'll pretend it is). Very near the star going Nova is a space station with both excellent shielding and excellent telescopes. When the star goes nova, the light from it reaches the space station almost instantly. Five years later, it reaches us on earth, lighting us up, turning night into day, etc. Five years after that, the crew of the space station turns their excellent telescope toward Earth, and see it brighten as the light of the nova hits us. So they know that five years earlier, the light from the nova reached Earth.

We currently see this with Reflection Nebulae. We see a supernova, and then several years later, we see clouds of dust behind (but near) the supernova light up.

Does that work for your frame of reference?

[veti]

If it truly makes no sense to you, then there's probably no point in any of us trying to explain it to you in another way, but...

Do you accept the idea of the speed of light, and the distance involved between stars?

Oh absolutely, yes. I have no problem with the science, as far as that goes. It's the way people keep trying to explain it that I think is fraught with ambiguity, misunderstanding and downright error. In particular this "we are seeing the stars x years ago" meme, seems to me to rely on some very odd definitions of words like "seeing" and "ago", which are at variance with the way those words are normally used.

(See above example:

On contrary the photo analogy is perfect, because in reality you never see a "thing", what you see is light that bounces out of that thing and has traveled to your eye, so in a way what you see is always a photo of the item, not the item itself.

- that's simply not consistent with how anyone anywhere uses the word "see". You don't "see" light - well, maybe when you're trying to decide whether a building is occupied by looking at the windows, but that's not the same thing - you "see" the things that reflect/transmit/emit light.)

So let's posit a star goes nova 5 light-years away. (Let's also assume that's far enough away that we won't be harmed by the explosion - it's probably not, but we'll pretend it is). Very near the star going Nova is a space station with both excellent shielding and excellent telescopes. When the star goes nova, the light from it reaches the space station almost instantly. Five years later, it reaches us on earth, lighting us up, turning night into day, etc. Five years after that, the crew of the space station turns their excellent telescope toward Earth, and see it brighten as the light of the nova hits us. So they know that five years earlier, the light from the nova reached Earth.

We currently see this with Reflection Nebulae. We see a supernova, and then several years later, we see clouds of dust behind (but near) the supernova light up.

Does that work for your frame of reference?

It's the best explanation I've seen yet. For the most part I agree with the principle and I do understand the speed of light, honest. My issue is with the loose use of terms such as "ago" and "earlier".

Assuming the crew of the space station were interested in Earth, and assuming they speak English with usage and idiom that is something like what the people around me have always used, it seems to me that - ten years after the nova, when they're training the telescope in our direction, they'd probably be saying things to each other along the lines of "it'll hit them any minute now".

"Now" can be defined as "on the light cone of an event". It's a little unintuitive, but no more so than everything else we've been talking about, and it has the virtue of being consistent both with the normal everyday usage of the word and with relativity.

[Lord Torath]

I'm glad that example worked, because, my next one was going to be the appearance of the Eye of Terror in the 40k universe. It appeared 10,000 years before the 'present day', but since the galaxy is 50,000 light years across, there are many places that have not yet seen it. but since 40k has FTL travel, it's... inconsistent... with our universe.

[Rydiro]

For veti, "now" encompasses the past light cone, as far as I understand.
I honestly prefer it over most other relativistic definitions of now.

[Lord Torath]

Veti, here's another example: Gravitational Lensing. The light from a single event follows multiple paths, and thus the light from the same event arrives at the same place (our telescope) at different times.