Absolute Zero an unattainable Temperature

[Progenitor A]

So I heard on R4 yesterday

No matter how low temperatues are taken (Temperatures within one Billionth of -273K have been obtained - I wonder what thermometer they used to meaure it?) ,some small amount of heat (or energy) will creep in to stop -273K being reached

This to me sounds entirley reasonable as if -273K wer reached then that would mean that matter ceased to exist wouldn't it? Ther is also teh remnat problem of no-one being ther to measure it!

[alancalverd]

Nothing to do with the nonexistence of matter, but the Carnot equation for the efficiency of a heat engine

E = 1 - Tc/Th where Tc is the temperature of the sink and Th the temperature of the source.

Tc cannot be less than zero (i.e. absolute zero) so as you cool a body (the source) down towards 0K the efficiency of the heat transfer process tends to zero and it requires the expenditure of an infinite amount of energy to achieve the last infinitesimal change.

[Progenitor A]

Mar 8, 2013 22:27:12 GMT 1 alancalverd said:

Nothing to do with the nonexistence of matter, but the Carnot equation for the efficiency of a heat engine

E = 1 - Tc/Th where Tc is the temperature of the sink and Th the temperature of the source.

Tc cannot be less than zero (i.e. absolute zero) so as you cool a body (the source) down towards 0K the efficiency of the heat transfer process tends to zero and it requires the expenditure of an infinite amount of energy to achieve the last infinitesimal change.

According to my sums as Tc approaches zero E approaches 1 - we have an engine with no losses ,so this equation does not tell me very much about the unattainablity of abs zero!

[mrsonde]

This to me sounds entirley reasonable as if -273K wer reached then that would mean that matter ceased to exist wouldn't it?

Yes, I'd agree with that.

Nothing to do with the nonexistence of matter, but the Carnot equation for the efficiency of a heat engine

But not that. "Equations" are not written into the universe, we project them onto it - well, in my long considered metaphysical opinion. That is - the universe is not obeying or acting in accordance with some mathematically defined principle. The principle, insofar as it's correct, is attempting to describe how the universe behaves. Why it behaves in such a way is not given by any equation, whatever mathematicians and certain theoretical physicists like to believe.

[principled]

PA
Combing back the cobwebs of time I seem to remember that the concept of matter disappearing was related to a perfect gas that did not change state and so at 0K had no volume and no thermal energy and thus its temperature could not be measured. As this idea fitted in with the gas laws I used I never bothered to question it further. However, your post caused me to revisit the subject.
I now note that the concept that all molecular movement stops at this temperature is false and that at a quantum level there is still energy as no body can go below its ground state energy. Hmmm.
I also came across this article written in January this year which describes in a way I found difficult to take on board (can anyone explain them better?) that a temperature below 0K is possible!
www.sciencedaily.com/releases/2013/01/130104143516.htm

P

[Progenitor A]

Mar 9, 2013 14:18:16 GMT 1 principled said:

PA
Combing back the cobwebs of time I seem to remember that the concept of matter disappearing was related to a perfect gas that did not change state and so at 0K had no volume and no thermal energy and thus its temperature could not be measured. As this idea fitted in with the gas laws I used I never bothered to question it further. However, your post caused me to revisit the subject.
I now note that the concept that all molecular movement stops at this temperature is false and that at a quantum level there is still energy as no body can go below its ground state energy. Hmmm.
I also came across this article written in January this year which describes in a way I found difficult to take on board (can anyone explain them better?) that a temperature below 0K is possible!
www.sciencedaily.com/releases/2013/01/130104143516.htm

P

Well I read the article P - thanks -and found it difficult

Statements such a sthis do not help really

“The inverted Boltzmann distribution is the hallmark of negative absolute temperature; and this is what we have achieved,” says Ulrich Schneider. “Yet the gas is not colder than zero kelvin, but hotter,” as the physicist explains: “It is even hotter than at any positive temperature – the temperature scale simply does not end at infinity, but jumps to negative values instead.”

[alancalverd]

Mar 9, 2013 9:26:32 GMT 1 Progenitor A said:

According to my sums as Tc approaches zero E approaches 1 - we have an engine with no losses ,so this equation does not tell me very much about the unattainablity of abs zero!

But that presumes that you already have a sink at 0K. The problem is that for any heat engine, as Th approaches Tc the efficiency tends to zero whatever the value of Tc.

[Progenitor A]

Mar 10, 2013 17:00:17 GMT 1 alancalverd said:

But that presumes that you already have a sink at 0K. The problem is that for any heat engine, as Th approaches Tc the efficiency tends to zero whatever the value of Tc.

Quite
And so just how is the equation related to 0K?

[alancalverd]

It means that as you approach 0K (which by definition must be the lowest theoretical value for Tc) you require the expenditure of ever-increasing amounts of energy to extract the next bit of heat from your sample, and the last infinitesimal bit demands infinite energy input to achieve it, so you can't.

There is also the hypothetical question of what would happen if you had a finite heatsink at 0K and you transferred some heat from your sample to it. Then the heatsink would no longer be at 0K so Tc>0 so the efficiency of your transfer engine would go through zero into a negative value.