4 ms·

Temperature is not really the average kinetic energy any more. The temperature of a system says something about how much the entropy increases as you add energy

by jules 14y ago

Temperature is not really the average kinetic energy any more. The temperature of a system says something about how much the entropy increases as you add energy. Negative temperature simply means that the entropy decreases as you add energy. This is possible if the number of high energy states is small, because then adding energy will result in the system being in one of a smaller number of high energy states, thus having lower entropy (the entropy of a system is basically the logarithm of the number of states we think the system could be in, so if the number of states is lower, then the entropy is lower). Another way of saying this is that is as follows. For positive temperatures our knowledge of the state of the system would decrease if we added energy (think about a box full of neatly arranged balls and giving it a kick). For negative temperatures our knowledge of the system would increase if we added energy. In the extreme case if the number of possible high energy states is 1, then adding energy can force the system into exactly that state, thus we would know all you can know about the system. In particular in the experiment described in the article, if you removed energy from the system the particles would not be in the lattice arrangement and instead would go around randomly. Thus the entropy would increase if you removed energy.

fusiongyro 14y ago
Your first three sentences are worth more than that entire article. Thanks!
balsam 14y ago
Are any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? As a physics student, let me check: 0. Probably not. 1. Not directly applicable 2. Nope. 3. Nope. [1] http://en.wikipedia.org/wiki/Laws_of_thermodynamics http://en.wikipedia.org/wiki/Laws_of_thermodynamics BTW, I now check HN for physics news too, as a BTW.
- bostonpete 14y ago
  > Are any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? That's a pretty tough criteria for judging an experiment to be interesting. Seems that must pretty much rule out all experiments in fact...
zeidrich 14y ago
So I understand it conceptually: A thought experiment where a cup of some substance manages to be observed at a stable negative temperature. This substance is a liquid at its current temperature. But it is colder than the ambient temperature of the room. As the liquid warms up from the ambient room temperature, it freezes. But taken outside on a cold day, it will melt and then evaporate? I know you're not going to get a cup of this stuff in reality, but am I at least understanding the idea? The "temperature" of the liquid would remain negative, it's not like it could ever "warm up" to the point it was positive? It could reach an equilibrium state with its environment, but it would still be negative?
- pdonis 14y ago
  An object with negative temperature is hotter than an object with positive temperature. So it's not possible for the cup of negative temperature liquid to be "cooler" than the ambient temperature of the room. The liquid's temperature will get more and more negative until it flips from negative infinity to positive infinity, and then cool down from there until it equilibrates with the ambient environment.
- jules 14y ago
  Yes, that's the right idea (assuming it has lower entropy when it's frozen than when it's liquid). In the context of the liquid you describe, and assuming it has otherwise normal properties then yes you can't warm it up (= add energy) and make it positive again. And if you took it outside, the temperature would first become more and more negative and then flip to positive. However, as long as we are talking about exotic stuff it is conceivable that as you add energy, the entropy first decreases (i.e. negative temperature) but if you add more and more energy, the entropy starts to go up again. For example lets say that if you add a lot of energy to the frozen stuff, it becomes a gas. I'm not sure if a system like that where you go from positive to negative to positive is physically possible, but I don't currently see any reason why it wouldn't be.

by jules 14y ago

fusiongyro 14y ago

Your first three sentences are worth more than that entire article. Thanks!

balsam 14y ago

Are any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? As a physics student, let me check: 0. Probably not. 1. Not directly applicable 2. Nope. 3. Nope. [1] http://en.wikipedia.org/wiki/Laws_of_thermodynamics http://en.wikipedia.org/wiki/Laws_of_thermodynamics BTW, I now check HN for physics news too, as a BTW.

bostonpete 14y ago
> Are any laws of thermodynamics violated here[1]? If not, why is this experiment interesting? That's a pretty tough criteria for judging an experiment to be interesting. Seems that must pretty much rule out all experiments in fact...

zeidrich 14y ago

So I understand it conceptually: A thought experiment where a cup of some substance manages to be observed at a stable negative temperature. This substance is a liquid at its current temperature. But it is colder than the ambient temperature of the room. As the liquid warms up from the ambient room temperature, it freezes. But taken outside on a cold day, it will melt and then evaporate? I know you're not going to get a cup of this stuff in reality, but am I at least understanding the idea? The "temperature" of the liquid would remain negative, it's not like it could ever "warm up" to the point it was positive? It could reach an equilibrium state with its environment, but it would still be negative?

pdonis 14y ago
An object with negative temperature is hotter than an object with positive temperature. So it's not possible for the cup of negative temperature liquid to be "cooler" than the ambient temperature of the room. The liquid's temperature will get more and more negative until it flips from negative infinity to positive infinity, and then cool down from there until it equilibrates with the ambient environment.
jules 14y ago
Yes, that's the right idea (assuming it has lower entropy when it's frozen than when it's liquid). In the context of the liquid you describe, and assuming it has otherwise normal properties then yes you can't warm it up (= add energy) and make it positive again. And if you took it outside, the temperature would first become more and more negative and then flip to positive. However, as long as we are talking about exotic stuff it is conceivable that as you add energy, the entropy first decreases (i.e. negative temperature) but if you add more and more energy, the entropy starts to go up again. For example lets say that if you add a lot of energy to the frozen stuff, it becomes a gas. I'm not sure if a system like that where you go from positive to negative to positive is physically possible, but I don't currently see any reason why it wouldn't be.