Can Temperature Ever Reach Absolute Zero?

[delplace]

Is it possible to obtain a temperature lower than 0 K

 

[Pengwuino]

Quick answer, no. Long answer, no. :)

 

[delplace]

why ?

 

[Pengwuino]

The temperature is related to the energy of a particle. In a simplistic explanation, a particle can't have a negative energy, thus it can't have a negative temperature. It's more complicated than that though, so hopefully someone else can bring in more detail.

 

[delplace]

thank you, :) but you know it depends on the reference you take. For ice, you can obtain solid water with a temperature lower than 0°C; for example I have ice at -18 °C in my fridge. At O K, all particles are freezed and I read that entropy = 0 (desorder is at its minimum). But I can not understand why we are not able to obtain -300 °C

 

[Pengwuino]

Remember, Kelvin is an absolute scale, it begins at 0. Celsius and Fahrenheit are relative scales where their 0's are at convenient temperatures and not actual absolute 0's.

 

[delplace]

Thank you Pengwuino :-) have a nice day

 

[elixirnova]

I would imagine this topic is getting into theoretical physics than what is known and used everyday. I sure do not know anything about it, but it is interesting to ponder.

Unfortunately without anyone being able to do it there is nothing to be known about it hah. All theory and from what other posters have said I would assume theory says it isn't possible which is what my thermodynamics class assumes.

 

[delplace]

Well, I agree with you, but I consider that limits can hide something unknown. Science history is full of that...
0 Kelvin is the limit where all subatomic particles stop moving. Why do they stop ? To stop something you need to brake. What is the force braking e- p+... Do photons continue moving at O K. I find this subject complex.

 

[Vanadium 50]

0 Kelvin is the limit where all subatomic particles stop moving.

No, it's the limit where the particles have the minimum energy. You can't have energy less than the minimum. (That's what minimum means)

 

[diazona]

It's a little different for photons because they're massless particles - they never stop moving, but I suppose their energy would become zero at temperature 0K. For example, if you plug T=0 into the Planck radiation law, it shows you that the intensity of radiation becomes zero for all nonzero photon frequencies.

 

[delplace]

Sorry but I thought that energy means movement in that case; kinetic energy giving rise to pressure. Is it true ?

 

[Pengwuino]

Edited for nonsense.

 

[delplace]

Sorry but if photons move, it means for me they have kinetic energy ! is it wrong ?

 

[Vanadium 50]

Photons don't have kinetic energy

Yes they do. A photon's kinetic energy is given by E = pc.

 

[delplace]

ouf !

 

[iamthegelo]

Here's another take...I learned this from Feynman (The Lectures).

0 K is not possible because it violates the uncertainty principle. At 0 K it is assumed that there is no motion, therefore we know its definite position and velocity (= 0) hence, it violates the uncertainty principle.

 

[delplace]

true ! but what does it mean ? it is not possible to reach 0 K ? is it a limit to the uncertainty principle ?
Definitely I find this question interesting !

 

[fatra2]

Hi there,

One of the best approach to understand the absolute temperature is to consider a "perfect gas". A gas where the particles are considered point-particle, and where they do not interact with the others in the gas.

If you retract energy from these particles, the gas will contract. If you keep on taking energy the gas will contract some more. What happens if you take all the energy from the particle, the volume of the gas will become a point in space. At this time, the temperature can be easily calculated to be 0K.

Cheers

 

[delplace]

ok, but by lowering the temperature of your gas, you will reach a change of state : gas --> liquide and finally solid !

 

[fatra2]

Not if you take the perfect gas theory in consideration. A gas only gets liquid because of the cohesion interaction between the different particles forming it.

If they don't act on each other, your gas will never turn into a liquid.

Remember: the perfect gas is a theory that helps understand gas, normally at higher temperature.

Cheers

 

[Borek]

https://www.physicsforums.com/showthread.php?t=347281

https://www.physicsforums.com/showthread.php?t=259970

https://www.physicsforums.com/showthread.php?t=296150

 

[delplace]

Thank you Borek; you are true, it is clearly a complicated problem

Best regards

 

[Vanadium 50]

At 0 K it is assumed that there is no motion

That's twice this has been said. It's not correct. At 0K there is the minimum motion.

 

[mazenmerhi]

what about negative potentiel energy ?

 

[Bob_for_short]

Formally T < 0 can be obtained but it corresponds to a special excited state of the system.
Think of a spin subsystem in a magnetic field. The lowest possible energy corresponds to the lowest potential energy determined with the product µB, say, all spin projections down.
Heating such a system means appearing some other projections whose populations depend on T. height temperatures T correspond to fully chaotic spin projections (equally populated). But if you manage to prepare the system in a special state - all spin projections up (fully inverse populations), this state has a very small (zero) entropy and negative temperature: T<0. Anyway this state is above the ground state so it is capable of giving away its energy. In this sense the state with T=0 is the lowest energetic state.

 

[Gerenuk]

Good job, that Borek collected previous longer discussion on that.

I made a comment somewhere in these threads. Basically for most systems (where density of states is a power-law w.r.t. energy) temperature can be related to mean energy as you know. That justifies all energy arguments here.

However, there are enough scientific works dealing with negative temperature (also see Wikipedia). The difference there is that these system do not follow that power law (and they have to have finite energy levels).

So basically, if your temperature measures the "motion of some particles", then you don't get negative temperatures. If however you apply temperature to something which isn't motion, like nuclear spin levels, then you can have negative temperature.

 

[iamthegelo]

That's twice this has been said. It's not correct. At 0K there is the minimum motion.

That was my point...that you can't say there is no motion because it violates the uncertainty principle.

If I had said that there was minimum motion, then my argument wouldn't make sense because then it wouldn't violate the principle.