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gooddragon1
2016-05-23, 02:56 AM
Preface: I have no problem with cat girls. Some of my favorite anime characters are cat girls.

So, how would a dragon's cold breath weapon work? I've heard some theories that it takes the heat out of the air. Would it be a substance? Could it be a type of energy that counters thermal energy? What sort of physics involved (or otherwise) answers can you think up?

Cat girls deleted my WoW character. Whether or not this was intentional makes no difference to me.

Lorsa
2016-05-23, 03:27 AM
Why are you interested in a Dragon's cold breath in particular? D&D holds so many fantastic and magical things it makes me curious about the motivation behind this.

The answer is: you have to ask whoever placed Dragons in the D&D Monsters Manual. Nobody knows the physics of D&D really.

veti
2016-05-23, 03:35 AM
I've always imagined it as, simply, a breath of extremely cold air. Like air conditioning turned up to 11. Cold enough to freeze small volumes of water. As to how, exactly, the dragon produces such a thing - well, in much the same way as other dragons produce fire, I guess.

Fri
2016-05-23, 03:37 AM
Evocation school of magic exist. you can evoke (or disappear) energy out of nowhere in DnD.

Elemental planes exist, one of them is elemental plane of ice. You can conjure things out of it, cold ice cubes included.

Karnack
2016-05-23, 03:42 AM
A dragon's lungs are charged with elemental energy from an organ in has next to the heart called the draconis fundamentum. It then projects said elemental energy from it's lungs when it uses the breath weapon. In the case of cold breath weapons you may notice this does not follow the laws of thermodynamics. This is because it's magic and it ain't got time for science, it's too busy being awesome.

ThinkMinty
2016-05-23, 04:01 AM
It's the magic equivalent of liquid nitrogen. So cold it HURTS.

Takewo
2016-05-23, 04:27 AM
They were fire dragons, but cat girls broke their hearts and now they are as cold as ice, their breath weapon is simply the defrosting feature.

Lorsa
2016-05-23, 04:30 AM
It's the magic equivalent of liquid nitrogen. So cold it HURTS.

This is bad. Now I wonder how much liquid nitrogen actually hurts. I mean, I haven't tried, have you?

To be honest, liquid H20 also hurts at some temperatures (well, it hurts at a lot of temperatures really, but in two different ways).

I don't think it has so much to do with actual temperature as its ability to effectively steal heat from your body.

Storm_Of_Snow
2016-05-23, 05:00 AM
It's the magic equivalent of liquid nitrogen. So cold it HURTS.
More like so cold it instantly freezes the water in the cells of your body, causing them to rupture, solidifies the blood in your veins, so that your circulation fails and your extremities suffer oxygen deprivation and die, and reduces the reaction rates of the enzymes in your body so that your metabolic processes shut down and your body is unable to sustain itself.

The pain comes later, and then only if your nerves defrost enough for you to be able to feel it.

Yuki Akuma
2016-05-23, 05:05 AM
In D&D, it works exactly the same way as Cold energy spells work, because it's also literally magic.

I can't think of any other settings with dragons that breathe cold. Elder Scrolls, I guess? In which case... same answer.


This is bad. Now I wonder how much liquid nitrogen actually hurts. I mean, I haven't tried, have you?

Incidental contact with liquid nitrogen doesn't hurt. It evaporates more or less instantly and is kept away from the surface of your skin in the same way water floats above the surface of a really hot pan. At worst you might get a little chilly if you spill some on your hand. Getting it soaked into your clothes is bad, because then it sticks around to keep leaching heat out of you, but you can get out of your clothes fast enough that it won't do anything major.

Full immersion in liquid nitrogen for more than a few seconds will freeze the water in your cells and cause severe frostbite, which hurts like hell, and can lead to major necrosis, so don't do that.

gooddragon1
2016-05-23, 05:23 AM
I liked the idea of air conditioning until I turned it up to an arbitrarily large number in my head. If you chill air to a low enough temperature it will become a liquid and then a solid.


Why are you interested in a Dragon's cold breath in particular? D&D holds so many fantastic and magical things it makes me curious about the motivation behind this.

I can almost guarantee that it has nothing to do with my profile pic whatsoever.

On second thought, I don't like how that sounds (it doesn't feel nice): it's because of my profile pic.

Lorsa
2016-05-23, 06:05 AM
I can almost guarantee that it has nothing to do with my profile pic whatsoever.

Ok. I'm glad we cleared that up.

(the above is also sarcasm btw)



Incidental contact with liquid nitrogen doesn't hurt. It evaporates more or less instantly and is kept away from the surface of your skin in the same way water floats above the surface of a really hot pan. At worst you might get a little chilly if you spill some on your hand. Getting it soaked into your clothes is bad, because then it sticks around to keep leaching heat out of you, but you can get out of your clothes fast enough that it won't do anything major.

Full immersion in liquid nitrogen for more than a few seconds will freeze the water in your cells and cause severe frostbite, which hurts like hell, and can lead to major necrosis, so don't do that.

I'm curious how I could accomplish full immersion in liquid nitrogen. I mean, if it was a bathtub full of it, chances are the room would contain almost exclusively N2 anyway, making it somewhat hard to breathe. If you spill out a high-pressure bottle of liquid N2 in an elevator, it's not the cold that will kill you, it's the resulting lack of oxygen.

Storm_Of_Snow
2016-05-23, 06:47 AM
Incidental contact with liquid nitrogen doesn't hurt. It evaporates more or less instantly and is kept away from the surface of your skin in the same way water floats above the surface of a really hot pan. At worst you might get a little chilly if you spill some on your hand. Getting it soaked into your clothes is bad, because then it sticks around to keep leaching heat out of you, but you can get out of your clothes fast enough that it won't do anything major.

Full immersion in liquid nitrogen for more than a few seconds will freeze the water in your cells and cause severe frostbite, which hurts like hell, and can lead to major necrosis, so don't do that.
There's the trick of dipping a flower into liquid Nitrogen for a few seconds, then pulling it out and smashing it with a hammer.

And there was also the craze a few years back for bars to use liquid Nitrogen in drinks to make them form a fog from it, and the woman who didn't wait for it to boil off (and she wasn't told, and the bar got heavily fined IIRC), and wound up having her stomach and lower oesophagus removed (although mostly because the gas expanded and split her stomach open rather than from the effect of the cold). :smalleek:

Brother Oni
2016-05-23, 06:47 AM
I'm curious how I could accomplish full immersion in liquid nitrogen. I mean, if it was a bathtub full of it, chances are the room would contain almost exclusively N2 anyway, making it somewhat hard to breathe. If you spill out a high-pressure bottle of liquid N2 in an elevator, it's not the cold that will kill you, it's the resulting lack of oxygen.

Large quantities of liquid nitrogen don't evaporate that quickly (it's complex (https://en.wikipedia.org/wiki/Accidental_release_source_terms#Evaporation_of_boi ling_cold_liquid_pool)) and you'd be more than able to jump into a bathtub of liquid nitrogen if freshly poured - by the time the room's O2 had been completely displaced, the bathtub would probably be empty.

That said, the O2 concentration being displaced to dangerous (sub 19%) would happen far more quickly but I don't have time at the moment to crunch the numbers.

Lorsa
2016-05-23, 07:08 AM
Large quantities of liquid nitrogen don't evaporate that quickly (it's complex (https://en.wikipedia.org/wiki/Accidental_release_source_terms#Evaporation_of_boi ling_cold_liquid_pool)) and you'd be more than able to jump into a bathtub of liquid nitrogen if freshly poured - by the time the room's O2 had been completely displaced, the bathtub would probably be empty.

That said, the O2 concentration being displaced to dangerous (sub 19%) would happen far more quickly but I don't have time at the moment to crunch the numbers.

Yeah, I do realize that "almost exclusively N2" was a bit of an overstatement. In any case, I mostly wanted to highlight that there are other considerations regarding liquid nitrogen than just the cold.

I don't think the numbercrunching is necessary unless any of us feel the need to take the nitrogen bath.

I guess liquid nitrogen is more "safe" in the oxygen displacement department than a regular high-pressure N2 gas tube. As you said, it takes a while for it to evaporate.

goto124
2016-05-23, 07:13 AM
How likely is a dragon able to toss out liquid nitrogen? Makes a bit more sense for a sci-fi dragon robot, maybe?

Storm_Of_Snow
2016-05-23, 07:34 AM
How likely is a dragon able to toss out liquid nitrogen? Makes a bit more sense for a sci-fi dragon robot, maybe?
If it can chill it down to about -200C, it's perfectly possible. Liquid CO2 at -57C's more likely though, with the rest of the visible effect being frozen water vapour. :smallamused:

Darth Tom
2016-05-23, 09:17 AM
I always kind of assumed Cold Dragons are cryogenic, and their breath weapon is a liquid helium quench. Those can certainly be nasty enough to freeze a bunch of people wearing nice conductive metal armour.

Why are they cryogenic? I don't know. Our body core temperature is different from that of, say, a fish, (I think), so surely in a high magic environment just going a little further is reasonable. OK it's on the order of ~300K, but nonetheless its the same principle right?

Brother Oni
2016-05-23, 09:21 AM
If it can chill it down to about -200C, it's perfectly possible. Liquid CO2 at -57C's more likely though, with the rest of the visible effect being frozen water vapour. :smallamused:

There's a video on Liveleak of a pool party where the organisers decided to go for an atmospheric fog effect by pouring large quantities of liquid nitrogen into the pool with people in it (they had quite a few 10L dewars). It didn't end well.

Presumably there's an optimal temperature range where the liquid is still cold enough to cause damage, but the temperature differential to the target isn't sufficient to cause the Leidenfrost effect (the aforementioned reason why incidental contact of liquid nitrogen isn't harmful) on incidental contact.

goto124
2016-05-23, 09:46 AM
And there was also the craze a few years back for bars to use liquid Nitrogen in drinks to make them form a fog from it, and the woman who didn't wait for it to boil off (and she wasn't told, and the bar got heavily fined IIRC), and wound up having her stomach and lower oesophagus removed (although mostly because the gas expanded and split her stomach open rather than from the effect of the cold). :smalleek:


There's a video on Liveleak of a pool party where the organisers decided to go for an atmospheric fog effect by pouring large quantities of liquid nitrogen into the pool with people in it (they had quite a few 10L dewars). It didn't end well.

Are frost dragons just human-created liquid nitrogen dragon robots that went out of control?

GNO GNOMES! Oh wait...

Joe the Rat
2016-05-23, 12:28 PM
Enh. It's just sound waves.

Cold-breath dragons produce a hypersonic roar in a focused cone that works at the vibrational frequencies of water molecules - specifically in counter vibration, so it reduces molecular motion, making the water vapor in the air, and by extension the air itself colder. Really powerful dragons can shift to N2, letting them freeze things in a desert.

gooddragon1
2016-05-23, 01:44 PM
Enh. It's just sound waves.

Cold-breath dragons produce a hypersonic roar in a focused cone that works at the vibrational frequencies of water molecules - specifically in counter vibration, so it reduces molecular motion, making the water vapor in the air, and by extension the air itself colder. Really powerful dragons can shift to N2, letting them freeze things in a desert.

How about a counter vibration that adjusts for all matter and perhaps even energy rather than just air molecules?

JAL_1138
2016-05-23, 02:49 PM
Dragons that breathe "acid" (used incorrectly as a catchall term for corrosive chemicals) use the same principle as bombardier beetles--they have two sacs, each with a different chemical (that's harmless to the dragon (and indeed on its own), that when combined produce an extremely caustic compound. Each chemical is aerosolized by both the force with which it is expelled through a small orifice within in the dragon's mouth, and the dragon's exhalation of air, and combine to form the caustic compound a short distance past the dragon's mouth.

Dragons that breathe fire have the same structures, but the chemicals in the sacs, when expelled, combine and produce a strongly-exothermic reaction instead of forming a corrosive compound.

Dragons that breathe cold, same thing except they have chemicals that produce a strongly endothermic reaction.

Dragons that breathe poison (actually venom, but D&D and colloquial English have always been bad at that distinction) simply have venom in the sacs, expelled and aerosolized by the same method as the others.

The real question is how in blazes the dragons that breathe lightning work. Presumably with an organ similar to that of an electric eel, although that doesn't explain how they're able to direct it in a line. Perhaps some sort of guided plasma-channel?

Psyren
2016-05-23, 03:06 PM
How about a counter vibration that adjusts for all matter and perhaps even energy rather than just air molecules?

All you need for that is to alter the pitch/resonance, so yeah.

But the real answer (which was already stated, and doesn't contradict the above either) is that yes, they are breathing out a substance - really, really cold air.

Kelb_Panthera
2016-05-23, 04:35 PM
If you're looking for an answer that fits with modern physics, don't. It's not there to find.

D&D physics works on a classical, four elements model of reality. Cold is not the absence of heat but a confluence of elemental air and water. That's what's coming out of the white or silver dragon's mouth just like the elemental fire of a red or gold dragon or the confluence of air and fire that is a blue dragon's lightning breath.

It's either accept the above, or learn about quantum physics and exotic particles and energies. Some kind of carrier particle that inhibits molecular interaction is pretty much the only thing in modern physics that could explain "cold energy" and then you have the fun of trying to figure out how an organic body could produce such an effect. Good luck.

TheThan
2016-05-23, 04:37 PM
I figured it's just a really, really big breath mint.

gooddragon1
2016-05-23, 04:40 PM
Dragons that breathe "acid" (used incorrectly as a catchall term for corrosive chemicals) use the same principle as bombardier beetles--they have two sacs, each with a different chemical (that's harmless to the dragon (and indeed on its own), that when combined produce an extremely caustic compound. Each chemical is aerosolized by both the force with which it is expelled through a small orifice within in the dragon's mouth, and the dragon's exhalation of air, and combine to form the caustic compound a short distance past the dragon's mouth.

Dragons that breathe fire have the same structures, but the chemicals in the sacs, when expelled, combine and produce a strongly-exothermic reaction instead of forming a corrosive compound.

Dragons that breathe cold, same thing except they have chemicals that produce a strongly endothermic reaction.

Dragons that breathe poison (actually venom, but D&D and colloquial English have always been bad at that distinction) simply have venom in the sacs, expelled and aerosolized by the same method as the others.

The real question is how in blazes the dragons that breathe lightning work. Presumably with an organ similar to that of an electric eel, although that doesn't explain how they're able to direct it in a line. Perhaps some sort of guided plasma-channel?

Toss up between this one and the counter vibration one. Slightly in favor of the counter vibrations because it doesn't involve stored matter.

Âmesang
2016-05-23, 05:12 PM
I just sort of pictured cold breath weapon attacks resembling a super-cooled snow blower attack, at least with regards to appearance. Perhaps resembling Sub-Zero's ice blasts from Mortal Kombat:
https://www.youtube.com/watch?v=fEYibbsDvAw


The real question is how in blazes the dragons that breathe lightning work.
POWER! UN-LIM-I-TED POWER!!

For the longest time I didn't know what he was saying until I turned on the subtitles.

Knaight
2016-05-23, 06:16 PM
An obvious possibility is just an extremely endothermic chemical reaction, where an aerosol spray is used to distribute the particles, which then stick to surfaces and react, leeching heat from the surfaces themselves. A similar possibility would be a gas expansion which cools the gas as it expands, which is a property of a number of real substances (https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect) within certain temperature/pressure ranges. For a lot of these it isn't too dramatic a change, but that isn't a big deal.

If you look at the nitrogen curve in particular, you'll see that there's a cooling process within any reasonable temperature, up to pretty ridiculous pressures. So, the dragon breathes in a huge quantity of air, pressurizing it with a biological pump as they do so. The dragon itself avoids heating up during the compression process by sweating, getting it a large amount of compressed gas. Then, it breaths out, throttling as it does so. That gets a cold gas, and it can also remain at pretty high pressures and thus high densities. High density gases traveling quickly have very high convection heat coefficients, so it will cool anything in the area extremely quickly. Thus, cold damage.

Cluedrew
2016-05-23, 06:34 PM
The real question is how in blazes the dragons that breathe lightning work. Presumably with an organ similar to that of an electric eel, although that doesn't explain how they're able to direct it in a line. Perhaps some sort of guided plasma-channel?If I remember BIO 347 correctly they have a pair of sacs like the others except the sacs have a highly uneven lining (like your intestine). As the liquid in the sacs sloshes around they become ionized, that is positively and negatively charged. Unlike the other dragons they expel liquids from these sacs one after the other. One (I believe the positively charged one) in a stream for a fraction of a second. Then they release the other in a burst, the positive negative charges cancel and a they do the moving voltage creates "lightning" along the initial stream. Also, I never actually took BIO 347 and if I did I don't think they would have talked about this.

Madbox
2016-05-25, 04:37 AM
1. Magic

2. Super lungs. Gasses heat up when compressed and cool when they expand. Ever use a can of compressed air on a humid day and see frost form? Like that. The dragon has lungs that work on some sort of pump-like mechanism rather than a diaphragm. This lets them use their lungs like an air compressor. Their blood carries away the heat from the compression (incidentally giving them heated blood that boosts cold resistance), and when they exhale, the expansion of air causes a drastic drop in temperature.

Frozen_Feet
2016-05-25, 06:20 AM
The dragon's metabolism creates two different chemicals stored in different pouches. When it "breathes" out, jets of these chemicals mix and cause an endothermic chemical reaction (https://www.youtube.com/watch?v=MyAzjSdc3Fc) which absorbs heat from the surroundings.

DigoDragon
2016-05-25, 07:08 AM
This is bad. Now I wonder how much liquid nitrogen actually hurts. I mean, I haven't tried, have you?

Personal experience: It feels like a very uncomfortable burning sensation on the skin, up until all the nerve endings there die. Useful way to get rid of warts and skin tags though, since the dead bits just fall off in a few days.

I like Frozen Feet's idea above. Makes some sense and the chemicals can be some fantasy stuff that works super efficiently.

Âmesang
2016-05-25, 08:10 AM
Come to think of it I believe that's how cone of cold is described as working, it drains the heat from the area; though I'm not sure if that means it's a blast of cold air/snow and ice or just a rapid decrease in temperature (I'd guess the former due to the similar Keraptis' flamecone).

Joe the Rat
2016-05-25, 09:09 AM
It's the same principle as Darkness - you are (somehow) subtracting energy, though in the parlance of D&D thaumophysics, you are blasting/releasing anti-heat / anti-light energy. Apparently D&D refrigerators work by reversing the polarity of the neutron flow.

Max_Killjoy
2016-05-25, 09:17 AM
After reading this thread, I'm reminded of why it's called "magic" and not "physics".

Jay R
2016-05-25, 10:36 AM
Describing magic in terms of real-world physics makes bad physics and bad magic.

Clearly, D&D magic doesn't follow the laws of thermodynamics. I assume that it's a blast of cold - the stuff from the Plane of Cold. In a D&D world, cold isn't merely the absence of heat, it is an opposed energy. It's not something that exists in the real world, just like the two opposing forces of Law and Chaos don't exist in our world.

JAL_1138
2016-05-25, 03:32 PM
If I remember BIO 347 correctly they have a pair of sacs like the others except the sacs have a highly uneven lining (like your intestine). As the liquid in the sacs sloshes around they become ionized, that is positively and negatively charged. Unlike the other dragons they expel liquids from these sacs one after the other. One (I believe the positively charged one) in a stream for a fraction of a second. Then they release the other in a burst, the positive negative charges cancel and a they do the moving voltage creates "lightning" along the initial stream. Also, I never actually took BIO 347 and if I did I don't think they would have talked about this.

That was it. Generally for current to travel through a liquid, it has to be an unbroken stream, but the dragon's not trying to transmit current through the liquid as a conductor. Instead it's creating two highly-charged streams, such that what appears to be a single "lightning bolt" is actually thousands of sparks of static discharge occurring between individual strongly-charged droplets. This is why the path of the breath weapon is fairly wide (5 feet, generally) also why there's no thunderclap associated with it (and why there's no Thunder damage from it in editions that separate the two damage types).

bulbaquil
2016-05-25, 05:13 PM
It manifests cold, which in this campaign setting is a quantifiable and measurable physical phenomenon and not merely "the absence of heat", from whichever organ produces the fire for a fire-dragon's flame breath weapon, by metabolizing the energy from the last batch of adventurers/explorers/innocent civilians it killed and either simultaneously or subsequently consumed. The laws of thermodynamics need not apply. Especially that one.

Cernor
2016-05-25, 05:36 PM
My preferred explanation is a bit pseudo-sciencey, but it works for me! First off, the three major assumptions I make...

1) Absolute zero is technically impossible to achieve, it is possible to functionally achieve absolute zero (for the precision relevant to a D&D campaign).
2) There exists an Elemental Plane of Cold, the depths of which are functionally absolute zero.
3) Some creatures, such as Phase Spiders or Ghosts, are innately capable of accessing multiple planes of existence.

Do you see where I'm going with this? Good!
If not... Inside the throat of each dragon is the Dragonheart, a roughly fist-sized organ which gives the dragon the ability to use its breath weapon. In the case of white dragons, this organ allows it to tap into the energy suffusing the Elemental Plane of Cold, exposing its current locale to what amounts to absolute zero. While the dragon itself is immune to the effect, being naturally imbued with the energy of the Plane, the air in its throat (and anything in a cone outside its mouth) has the heat sucked from it almost instantaneously, resulting in the phenomenon known as cold damage.

Cealocanth
2016-05-26, 12:13 AM
The endothermic reaction explanation mentioned above is what I generally use when asked. As far as the in-game explanation goes, dragons are elemental creatures just like salamanders and phoenixes and elementals. Their very existence is tied closely to the elemental plane which they are connected to, and the access to this magic grants them the ability to manipulate that particular magical energy. The breath weapon is one of the first elemental spells that dragons master because their brains and bodies are hardwired to be able to channel elemental magic in that particular fashion. This is why most dragons, when they get old enough, become wizards or something similar. Magic is intrinsic to their very being, and it is much easier for a dragon to become a mage than for a human to do the same.

In other words, magic is a form of energy that comes in different elemental flavors, and the manipulation of this energy causes the magical effects we are familiar with. This does raise the question that how, in the absence of divine intervention, would a human ever evolve as we currently know them in a world where animals can develop to gain access to such immense power?

Jay R
2016-05-26, 08:04 AM
This does raise the question that how, in the absence of divine intervention, would a human ever evolve as we currently know them in a world where animals can develop to gain access to such immense power?

Because they would need hands, eyes, and brains to survive in such a world. As the animals were evolving greater power, their prey would evolve as well. [They would also need to evolve into a magic-using race.]

Khedrac
2016-05-26, 08:15 AM
Interestingly we can cheat and do it another way.

Many physicists define temperature by the distribution curve of the vibrating molecules in a substance (after all we all know that heat is the molecular vibration).

Well this means they get to cheat and allows substances to have temperature measured in negative degrees Kelvin.
Yes, seriously negative Kelvin - i.e. below absolute zero.

This can be achieved by using laser set up so that they will only affect molecules travelling at certain speeds enabling them to rig the constituents of a substance (usually a gas) to have a reversed speed distribution - something that they have defined to be a "negative temperature".

(Personally I think that is a load of bunk and the gas is hot, especially as it will otherwise break the second law of thermodynamics by transferring excess energy to its surroundings.)

Anyway, a "cold breath" could be a cloud a "negative Kelvin gas" or a magic field that inverts energy distributions to give the affected material a negative temperature in K. Unfortunately this should still do heat damage not cold, but hey, magic.

DJ Yung Crunk
2016-05-26, 08:26 AM
http://www.mentosarabia.com/sites/default/files/mentos-pure-fresh-gum-freshmint-blt.png

Psyren
2016-05-26, 08:42 AM
This discussion reminds me of a FR novel where some militant elves doused an encroaching human squadron in Wintermint extract, causing a hungry white dragon to resolve the problem for them in short order.

Âmesang
2016-05-26, 09:37 AM
Crunk's post makes me want to ask the folks behind Dairy Queen's Blizzard; apparently they know all about SCIENCE!!

Knaight
2016-05-26, 04:13 PM
Well this means they get to cheat and allows substances to have temperature measured in negative degrees Kelvin.
Yes, seriously negative Kelvin - i.e. below absolute zero.

This can be achieved by using laser set up so that they will only affect molecules travelling at certain speeds enabling them to rig the constituents of a substance (usually a gas) to have a reversed speed distribution - something that they have defined to be a "negative temperature".

That's not how negative temperatures work (http://www.physicscentral.com/explore/action/negative-temperature.cfm) at all. It's a definitional quirk that relates to behavior in very particular high energy states, and is anything but cold. As for it breaking the second law of thermodynamics, it doesn't do that either.

Khedrac
2016-05-27, 01:54 AM
That's not how negative temperatures work (http://www.physicscentral.com/explore/action/negative-temperature.cfm) at all. It's a definitional quirk that relates to behavior in very particular high energy states, and is anything but cold. As for it breaking the second law of thermodynamics, it doesn't do that either.
Interesting - that is very different to the explanation I was (badly) remembering and makes a lot more sense.

Most interesting is:
It is important to note that the negative temperature region, with more of the atoms in the higher allowed energy state, is actually warmer than the positive temperature region. If this system were to be brought into contact with a system containing more atoms in a lower energy state (positive temperatures) heat would flow from the system with the negative temperatures to the system with the positive temperatures. So negative temperatures are warmer! And all this has to do with the how we define temperature.The statement that "the negative temperature region is warmer than the positive temperature region" is automatically nonsense as temperature is how warmth is measured. If a negative temperature is warmer then it's not negative (perhaps it is a complex temperature, e.g. 200iK?)
Saying a negative temperature can be "hotter" than a positive temperature means they broke the definition of "negative temperature", not that the temperature is actually negative from the definition of negative.

Knaight
2016-05-27, 03:23 AM
Saying a negative temperature can be "hotter" than a positive temperature means they broke the definition of "negative temperature", not that the temperature is actually negative from the definition of negative.

It doesn't break the rigorous statistical mechanics definition, just an imprecise lay definition - and those tend to have holes in them.

Khedrac
2016-05-27, 08:01 AM
It doesn't break the rigorous statistical mechanics definition, just an imprecise lay definition - and those tend to have holes in them.
On the contrary, what is "hotter" than something else is defined by temperature, therefore when the 'lower value' is 'hotter' that the 'higher value' someone has broken the definition. They are welcome to use a "rigorous statistical mechanics definition" of temperature, but if they do they cannot claim that the negative temperature is "hotter" - as that is defined by the temperatures so it is not hotter, it is colder.

If it isn't "colder" then the "negative temperature" state has been misapplied - which takes me back to my opinion that calling it a "negative temperature" is dangerously wrong.

Edit: checking back with the original quote replace "hotter" with "warmer" as that is the term the article used.
Edit 2: I also tried to say not that they had broken the definition of "temperature" but of "negative". Remember if they are following basic mathematics given equal masses with equal and opposite temperatures and perfect insulation from outside sources, over time the two bodies should average out at zero - something no-one I think is trying to say will happen here. They may have come up with one clever piece of logic which gets the tmeperatures to work as negative numebrs from that one perspective, but it is only from that perspective and that does not mean that the temperatures are acutally negative - there are too many counter examples from other perspectives which show this not to be an acceptable use of the word "negative".

Lord Torath
2016-05-27, 08:35 AM
On the contrary, what is "hotter" than something else is defined by temperature, therefore when the 'lower value' is 'hotter' that the 'higher value' someone has broken the definition. They are welcome to use a "rigorous statistical mechanics definition" of temperature, but if they do they cannot claim that the negative temperature is "hotter" - as that is defined by the temperatures so it is not hotter, it is colder.

If it isn't "colder" then the "negative temperature" state has been misapplied - which takes me back to my opinion that calling it a "negative temperature" is dangerously wrong.This is precisely the imprecise "lay" definition that Knaight was talking about. "Lay" in this case meaning "commonly understood". In general practice, higher temperature means hotter. But in the precise defined definition used by physicists, temperature is defined by vibration on atomic (or possibly molecular?) levels, and by that precise definition, negative temperatures are possible.

DigoDragon
2016-05-27, 09:56 AM
So if I understand what the article is telling me, Negative temperature on the Kelvin scale is a Quantum Mechanics thing, and makes sense if I define temperature as a relationship of energy and entropy, not as the simpler definition of the average kinetic energy of a system.

Knaight
2016-05-27, 03:51 PM
So if I understand what the article is telling me, Negative temperature on the Kelvin scale is a Quantum Mechanics thing, and makes sense if I define temperature as a relationship of energy and entropy, not as the simpler definition of the average kinetic energy of a system.

Oh it's absolutely a quantum mechanics thing, and it can only exist in the context of bound energy levels.

Khedrac
2016-05-28, 01:26 AM
Oh it's absolutely a quantum mechanics thing, and it can only exist in the context of bound energy levels.

First off, I want to apologise to Lord Torath as I sent him a pm saying I thought we should drop this side topiec as it is a thread derail - I had not noticed it was him not Knaight with whom I had been having the main conversation.

Secondly I want to say thank-you for this post, as I think it actually mainly answers my problem with "negative temperatures":
can only exist in the context of bound energy levelsI am bad at quantum mechanics, and I am not a physicist - I am a mathematician and I can see many problems with calling something a "negative temperature" when it does not behave like a conventional negative (let's start with its effect on specific heat capacity). Here your post admits that it does not work outside the very narrow defined boundaries which is basicaly the acknowledgement I thought it was lacking. I am going to assume (don't worry about correcting me if I am wrong) that it is being used as a way of describing these circumstances because it makes the maths for these specific circumstances work, rather than for any other reason.

I was going to try to move this conversation over to the Mad Science and Grumpy Technology forum as a new thread but I see no need now. If people do want further discussion then I suggest they start a new thread there...

Thank-you.

Regitnui
2016-05-28, 01:54 AM
We're not talking thermodynamics, but thaumodynamics. Of course, I'm just a bard, not a wizard.