[36grit]

Are there, or is it possible that clouds or regions in space can drop hundreds of degrees below absolut zero?

[iNow]

By definition, there is no such thing as "below absolute zero." Temperature is a measure of the movement of atoms, and at absolute zero there is no movement whatsoever. Essentially, time stops at this temperature. We can get very close to it, but not all the way to it... and most certainly not below it.

[imatfaal]

iNow, on 20 December 2011 - 06:54 AM, said:

By definition, there is no such thing as "below absolute zero." Temperature is a measure of the movement of atoms, and at absolute zero there is no movement whatsoever. Essentially, time stops at this temperature. We can get very close to it, but not all the way to it... and most certainly not below it.

There are quantum fluctuations - the zero point energy of the ground state

[iNow]

imatfaal, on 20 December 2011 - 12:04 PM, said:

There are quantum fluctuations - the zero point energy of the ground state

How do you know? Absolute zero is not itself achievable... Merely something very close to it.

[imatfaal]

iNow, on 20 December 2011 - 02:55 PM, said:

How do you know? Absolute zero is not itself achievable... Merely something very close to it.

it is part of the basic theory of quantum mechanics that even at absolute zero a system retains an energy of . this was initially found as a limiting case as hydrogen was cooled to very close to abs zero. it also ties in with the Heisenberg uncertainty principle. we can also measure the zero point energy of the vacuum through casimir effect experiments. it is possible that the whole theory breaks down at absolute zero - but our models using qm have so far been shown to be remarkably accurate in amazingly disparate situations, so I will side with qm at present.

[timo]

imatfaal, on 20 December 2011 - 03:39 PM, said:

It is part of the basic theory of quantum mechanics that even at absolute zero a system retains an energy of .

I do not think the statement about "ground state" energy means much more than to highlight an unexpected effect - that the energy of the ground states of the classical and the respective QM system (given the same Hamiltonian) differ. You can -to my knowledge- not measure absolute energy levels - only energy differences. And you also cannot pull a switch to transform a non-QM system into a QM system. To me, ground state energy seems like one of the more overrated and misunderstood concepts in quantum mechanics.

Quote

This [ground state energy] was initially found as a limiting case as hydrogen was cooled to very close to abs zero.

That's rather interesting considering that I just claimed you couldn't measure absolute energies. Are you sure they didn't measure the different distributions of energy levels above the ground state, instead?

I also don't really see how you want to make "quantum fluctuations" (what exactly is that supposed to be, anyways?) fit into the picture of a system being in its ground state. The standard examples of "basic quantum mechanics" have a ground state in which the probability density is constant as a function of time (which in fact directly follows for all systems with a unique ground state and a time-independent Hamiltonian).

(btw.: no offense meant by being disruptive; most of the physics related posts in here are even below the level of even bothering to reply)

edit: on 2nd tough, you do not even need a unique ground state

This post has been edited by timo: 20 December 2011 - 06:21 PM

[iNow]

imatfaal, on 20 December 2011 - 03:39 PM, said:

it is possible that the whole theory breaks down at absolute zero - but...

But... nothing.

We largely agree, imatfaal, and I'm hardly going against the well established principles of QM, so I find it odd that you would insinuate otherwise. We both you and I share an acceptance of the validity of the field of QM, but I'm merely pointing out to you that absolute zero is not itself achievable so you cannot make the absolutist assertions you continue to make.

You may as well be discussing what happens to an object with mass while traveling at c. It's not a realistic situation, so you can speculate, and pontificate, and extrapolate all you want, but you're still putting forth assertions that are largely a matter of opinion and personal bias.

[swansont]

I don't know why my earlier post didn't show up, but I'll try to recreate it here:

Temperature is an equilibrium condition, but if you apply it to some non-equilibrium examples you can get a negative temperature. One example is the population inversion in a laser. But the system is actually quite hot, in the sense that energy will flow away from that system as it approaches equilibrium.

http://en.wikipedia....ive_temperature

——

Time does not stop at absolute zero. Center-of-mass motion stops, since that's what temperature correlates to/with, but time is not affected. The definition of the second is for a system at absolute zero.

[iNow]

swansont, on 20 December 2011 - 05:38 PM, said:

Time does not stop at absolute zero. Center-of-mass motion stops, since that's what temperature correlates to/with, but time is not affected. The definition of the second is for a system at absolute zero.

I don't distrust what you're saying, but this seems contrary to what I've read. There is no motion at absolute zero, and we cannot achieve it. We can get extraordinarily close to it, but not all the way... Or, so I thought.

[A Tripolation]

This post might interest you, 36grit.

This post has been edited by A Tripolation: 20 December 2011 - 05:42 PM

[swansont]

As far as QM goes, remember that thermodynamics is a classical theory. There is a zero point energy, but the value of the frequency is dependent on the container, i.e. the potential well, which you can make arbitrarily large, so the frequency tends to zero.

[iNow]

But even if your container is the size of the universe, you will still never truly reach absolute zero, correct?

[A Tripolation]

iNow, on 20 December 2011 - 05:39 PM, said:

I don't distrust what you're saying, but this seems contrary to what I've read. There is no motion at absolute zero, and we cannot achieve it. We can get extraordinarily close to it, but not all the way... Or, so I thought.

I believe you're under some misconceptions about how time is measured with respect to certain systems.

This is a good and quick read. It's accurate with the physics I've studied thus far.

http://www.newton.de...00/phy00423.htm

[swansont]

iNow, on 20 December 2011 - 05:39 PM, said:

I don't distrust what you're saying, but this seems contrary to what I've read. There is no motion at absolute zero, and we cannot achieve it. We can get extraordinarily close to it, but not all the way... Or, so I thought.

I agree we can't achieve absolute zero. But why would time stop?

Temperature is a measure of the COM motion, which is what would cease (classically) at T=0. It puts all atomic/molecular systems into their ground state; the distribution of excited states is related to temperature which is why a population inversion gives you a negative value. But the systems don't collapse or anything.

[iNow]

swansont, on 20 December 2011 - 06:01 PM, said:

I agree we can't achieve absolute zero. But why would time stop?

It's something I read in a book by John Gribbon about 15 years ago called, "In Search of Schrodinger's Cat." I guess I'll need to reacquaint myself with the field to address some of these minor misconceptions.

A Tripolation, on 20 December 2011 - 05:56 PM, said:

This is a good and quick read. It's accurate with the physics I've studied thus far.

http://www.newton.de...00/phy00423.htm

Thanks, man. Helpful.

I interpreted absolute zero to mean no motion, and hence time didn't exist at that point since it can only be defined by relative motion.

Where it seems I was wrong is that there is still motion even at absolute zero, it's just that the system can no longer release energy to the surrounding environment.

This makes sense since the system is still likely part of a larger system which is itself moving relative to something else.

[ajb]

What happens in supersymmetric systems with unbroken supersymmetry? The ground state energy is zero, this is a consequence of the algebra. Such a system would have zero kinetic energy at absolute zero, just by the definitions. I am not really sure what this means for quantum statistical mechanics and similar.

[DrRocket]

iNow, on 20 December 2011 - 05:39 PM, said:

I don't distrust what you're saying, but this seems contrary to what I've read. There is no motion at absolute zero, and we cannot achieve it. We can get extraordinarily close to it, but not all the way... Or, so I thought.

Time does not stop ever, including at absolute zero. Time has nothing to do with the issue at hand.

Classically one might expect all motion to cease at absolute zero, but the atomic world is quantum mechanical, not classical.

The fact that absolute zero is not achievable in a finite number of thermodynamic steps has nothing to do with the fact that there is still motion at absolute zero. The inability to reach absolute zero in a finitte number of steps is the Third Law of Thermodynamics.

Absolute zero is simply a ground state in quantum mechanic,s and the Pauli exclusion prohibits all fermionic particles from occupying the same state, even at absolute zero.

[swansont]

iNow, on 20 December 2011 - 06:42 PM, said:

I interpreted absolute zero to mean no motion, and hence time didn't exist at that point since it can only be defined by relative motion.

That's not really the case, either. Motion is a fuzzier concept at the QM level, and even if one stuck to the classical notion, not being able to measure time doesn't mean it ceases to exist.

[imatfaal]

ok - first can I apologize for sending this thread in a completely tangential direction - but the posts above made it worthwhile.

iNow - I cannot explain my position better or from the same position of knowledge than the posts above and the thread on Newton linked by Trip. I also find the position of arguing about the parameters of a fundamentally impossible state fascinating - reminds me of a thread by ydoaps (i will post link when I find). looking back I should have made the differentiation between classical definitions and qm more explicit.

Timo - I don't think I can do your questions justice, they are beyond me, I could grope for answers and bluster but I won't. I will try and formulate a decent reply, but it won't be quick, and it might not be cogent either.