Thread: Absolute Zero Impossible?

Is it actually possible to achieve absolute zero?

From what I understand energy flows out of a system from high energy state to a low energy state. Which means that in order for us to reach absolute zero with basic matter, that last bit of kinetic energy that a molecule or atom for example has, would have to go from that system to a system of lower energy. Which would not work in the case of absolute zero, in order to get to absolute zero we would have to have a lower energy state (lower than absolute zero) available to draw that energy away from that molecule or atom.

Is my thought pattern correct?

Originally Posted by AndresKiani

Is it actually possible to achieve absolute zero?

From what I understand energy flows out of a system from high energy state to a low energy state. Which means that in order for us to reach absolute zero with basic matter, that last bit of kinetic energy that a molecule or atom for example has, would have to go from that system to a system of lower energy. Which would not work in the case of absolute zero, in order to get to absolute zero we would have to have a lower energy state (lower than absolute zero) available to draw that energy away from that molecule or atom.

Is my thought pattern correct?

Yes I think so, though we can get pretty close.

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

Originally Posted by AndresKiani

Is it actually possible to achieve absolute zero?

From what I understand energy flows out of a system from high energy state to a low energy state. Which means that in order for us to reach absolute zero with basic matter, that last bit of kinetic energy that a molecule or atom for example has, would have to go from that system to a system of lower energy. Which would not work in the case of absolute zero, in order to get to absolute zero we would have to have a lower energy state (lower than absolute zero) available to draw that energy away from that molecule or atom.

Is my thought pattern correct?

I think that is a good argument. Based purely on classical thermodynamics, this says you can get as close to 0 as you want, but never achieve it.

As Daecon says, there is also the uncertainty principle which also sets a lower limit on the energy of a system.

Originally Posted by Daecon

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

Yes indeed, zero point energy. However my understanding is this does not prevent attainment of absolute zero, as that is a different thing. Absolute zero is the temperature at which no more energy can be extracted. The residual zero point energy cannot, by definition, be extracted. Therefore it does not contribute to temperature.

(Goes into foetal crouch in anticipation the of arrival of a real physicist)

Originally Posted by Daecon

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

That has nothing to do with the thermal energy state of an atom or molecule.

Yes it does, thermal energy is basically kinetic energy due to motion...

Originally Posted by PhDemon

Yes it does, thermal energy is basically kinetic energy due to motion...

I didn't read the rest of his sentence... I thought he was referring to quantum mechanics, orbital fluctuations.

Quantum fluctuations of molecular motion. Ok, that is a correct statement.

Originally Posted by AndresKiani

Originally Posted by PhDemon

Yes it does, thermal energy is basically kinetic energy due to motion...

I didn't read the rest of his sentence... I thought he was referring to quantum mechanics, orbital fluctuations.

Quantum fluctuations of molecular motion. Ok, that is a correct statement.

Exactly. Don't forget that molecular rotations and vibrations are quantised and the ground state of theses does not imply zero motion.

Re translational motion I think it may be more subtle. As I recall, translational motions are not quantised, and if that's right I think I would not expect there to be a zero point energy. But then the Principle of Indeterminacy may have something to say abut that, I'm not sure. PhDemon may know.

You can "sort of" cool things down below absolute zero. There was a paper out last year describing an experiment that achieved this. It was widely reported and heavily misunderstood in the media. I got that it was not really below absolute zero in the normal sense of thinking about temperature, but don't understand the details well enough to say what's going on exactly.

It is simply a population inversion, the rest is "journalism"...

Originally Posted by exchemist

Re translational motion I think it may be more subtle. As I recall, translational motions are not quantised, and if that's right I think I would not expect there to be a zero point energy. But then the Principle of Indeterminacy may have something to say abut that, I'm not sure. PhDemon may know.

Sorry missed this earlier, my understanding is that translational motion IS quantised but under "normal" conditions the quantum numbers are huge and the energy levels are so close together for all practical purposes (and in the interests of retaining sanity) they can be treated as though they aren't.

For example if we consider the gas molecule as a simple particle in a box the energy of an energy level is:



If we equate this to the translational kinetic energy



at room temperature (298 K) for a reasonable mass of a diatomic molecule (10^-26 kg) in a 1 m³ box the values of the n's are ~10¹⁰, as you know the higher the value of n the closer together the energy levels are and for these values are pretty much a continuum in practice although they are quantised in theory...

CAVEAT: This is a physical chemists interpretation, a real physicist who understands QM better might correct me.

Andres you might also be interested in this: Third law of thermodynamics - Wikipedia, the free encyclopedia

Originally Posted by PhDemon

Originally Posted by exchemist

Re translational motion I think it may be more subtle. As I recall, translational motions are not quantised, and if that's right I think I would not expect there to be a zero point energy. But then the Principle of Indeterminacy may have something to say abut that, I'm not sure. PhDemon may know.

Sorry missed this earlier, my understanding is that translational motion IS quantised but under "normal" conditions the quantum numbers are huge and the energy levels are so close together for all practical purposes (and in the interests of retaining sanity) they can be treated as though they aren't.

For example if we consider translational energy as a simple particle in a box the energy of an energy level is:



If we equate this to the translational kinetic energy



at room temperature (298 K) for a reasonable mass of a diatomic molecule (10^-26 kg) in a 1 m³ box the values of the n's are ~10¹⁰, as you know the higher the value of n the closer together the energy levels are and for these values are pretty much a continuum in practice although they are quantised in theory...

CAVEAT: This is a physical chemists interpretation, a real physicist who understands QM better might correct me.

Yes indeed. I confess I was choosing to neglect the particle in a box quantisation, because I thought this would lead to too small a zero point energy (i.e. the energy of the ground translational state) to be worth bothering with. I thought, though, that I recollected you saying something about the Uncertainty Principle in relation to zero point energy, which might have led to a more significant value for it than that due to the ground state of quantised translational motion. Or are the two approaches (ground state vs. UP) just different ways of arriving at the same result?

Ah, OK, the HUP is related to zero point energy in that it means the particles can't sit still, the ground state must have an uncertainty in position and momentum that satisfies the HUP, this implies non-zero momentum and therefore non-zero kinetic energy.

(Again my take on it a real physicist may correct me).

Originally Posted by PhDemon

Ah, OK, the HUP is related to zero point energy in that it means the particles can't sit still at the bottom of the potential well, the ground state must have an uncertainty in position and momentum that satisfies the HUP, this implies non-zero momentum and therefore non-zero kinetic energy.

(Again my take on it a real physicist may correct me).

Right. That would mean it is effectively the HUP that tells you the ground state is not at zero energy, i.e. at the bottom of the potential well. So indeed the two ways of looking at it are equivalent.

That would mean that in an infinitely large container, the translational zero point energy is zero, since the ground state of the particle-in-a-box system would in such a case be at the bottom of the box. Woudn't it? And more generally, that the amount of zero point translational energy would depend on the degree of confinement of the particle.

(If I am recalling all this stuff correctly - confess I"m too lazy to get Atkins off the shelf at present.)

Originally Posted by exchemist

That would mean that in an infinitely large container, the translational zero point energy is zero, since the ground state of the particle-in-a-box system would in such a case be at the bottom of the box. Woudn't it? And more generally, that the amount of zero point translational energy would depend on the degree of confinement of the particle.

That makes sense to me...

(If I am recalling all this stuff correctly - confess I"m too lazy to get Atkins off the shelf at present.)

Yeah me too, I relying on memories of tutorials with the guy from a while back (mid 1990s)....

Originally Posted by PhDemon

Originally Posted by exchemist

That would mean that in an infinitely large container, the translational zero point energy is zero, since the ground state of the particle-in-a-box system would in such a case be at the bottom of the box. Woudn't it? And more generally, that the amount of zero point translational energy would depend on the degree of confinement of the particle.

That makes sense to me...

(If I am recalling all this stuff correctly - confess I"m too lazy to get Atkins off the shelf at present.)

Yeah me too, I relying on memories of tutorials with the guy from a while back (mid 1990s)....

Just had a Durrh! moment: Atkins would slap me round the head for being thick.

The more you confine the particle, the better you know its position so, as per Uncertainty Principle, the less you know its momentum, i.e. the more indeterminate motion it will have…..and the more zero point energy. And vice versa.

Bingo!

Oh, he never resorted to physical violence he just had a way of looking at you that basically said "dumbass" and using perfectly directed sarcasm on occasion...

Originally Posted by PhDemon

Oh, he never resorted to physical violence he just had a way of looking at you that basically said "dumbass" and using perfectly directed sarcasm on occasion...

Well yes I meant metaphorically. Anyway, self-administered kick up the arse for me, but another connection made……or remade after a 40 year gap…….

p.s. Looking forward to seeing RPW et al at ChCh Chemists' garden party tomorrow. Pity the weather looks rainy but I'm sure it will be fun nonetheless.

Originally Posted by exchemist

p.s. Looking forward to seeing RPW et al at ChCh Chemists' garden party tomorrow. Pity the weather looks rainy but I'm sure it will be fun nonetheless.

Let me know how the old bugger is, I last saw him a few years ago and while he was on good form he was definitely starting to show his age...

Originally Posted by PhDemon

Originally Posted by exchemist

That would mean that in an infinitely large container, the translational zero point energy is zero, since the ground state of the particle-in-a-box system would in such a case be at the bottom of the box. Woudn't it? And more generally, that the amount of zero point translational energy would depend on the degree of confinement of the particle.

That makes sense to me....

Makes sense to me as well. But this would also mean, that a container of any size will have zero heat energy if it has no mass, or radiation. If energy can be stripped from the atom until nothing is left, would it be 0K, or not? Or would is simply be not measurable because you have no point of reference.

But this question arose to me. Does mass increase when the energy of a particle is higher (which means heat energy, potential nuclear energy)? Energy is usually transferred by an electron, positron or a photon, which all have mass. As 2 photons can become an electron & positron. Or do they get their mass from the antineutrino and neutrino they absorb?

If it has no mass or radiation how can it have a heat energy or a temperature? Temperature is a property of stuff, no stuff, no temperature can be defined. As for mass changes when energy is transferred it depends... All you say is the total mass-energy is constant. (and nit pick photons do not have mass).

Originally Posted by PhDemon

Originally Posted by exchemist

p.s. Looking forward to seeing RPW et al at ChCh Chemists' garden party tomorrow. Pity the weather looks rainy but I'm sure it will be fun nonetheless.

Let me know how the old bugger is, I last saw him a few years ago and while he was on good form he was definitely starting to show his age...

He seemed to be pretty lively mentally last night, though physically he is not in great shape. I didn't enquire as to the details. Wears sandals and walks with a stick, takes a lot of pills and is puffy in the face. But he seemed no worse at least than last year and we had a very entertaining evening, complete with various new (to me) Concorde anecdotes. ChCh couldn't give me a room for the night so I ended up at Keble. I can report that the university parks are in good shape this summer - and the Master's Garden at ChCh was sensational. I don't think we properly appreciated these gardens as undergrads: they are something that money just can't buy.

Thanks, good to hear he's still on form as a raconteur even if his health is fading. I lived on Museum Road for 2 years so know the Parks well, a great place to spend Sunday afternoon post pub lunch!

Originally Posted by AndresKiani

Originally Posted by Daecon

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

That has nothing to do with the thermal energy state of an atom or molecule.

Are you sure of that? I don't think that absolute zero can be without absolute non activity. Mind you, I am not saying that Daecon is correct in saying it cannot happen though.

Originally Posted by Mayflow

Originally Posted by AndresKiani

Originally Posted by Daecon

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

That has nothing to do with the thermal energy state of an atom or molecule.

Are you sure of that? I don't think that absolute zero can be without absolute non activity. Mind you, I am not saying that Daecon is correct in saying it cannot happen though.

This was a fluke comment, I didn't read his full sentence and assumed he was talking about the particle charge/the electromagnetic energy of the system.

Originally Posted by AndresKiani

Originally Posted by PhDemon

Yes it does, thermal energy is basically kinetic energy due to motion...

I didn't read the rest of his sentence... I thought he was referring to quantum mechanics, orbital fluctuations.

Quantum fluctuations of molecular motion. Ok, that is a correct statement.

Originally Posted by Mayflow

Originally Posted by AndresKiani

Originally Posted by Daecon

I think there's also some reason to do with quantum uncertainty or fluctuation (or something) that means an atom can never really be completely still, at the quantum level.

That has nothing to do with the thermal energy state of an atom or molecule.

Are you sure of that? I don't think that absolute zero can be without absolute non activity. Mind you, I am not saying that Daecon is correct in saying it cannot happen though.

Not true. Activity is reduced to a minimum, but that minimum still involves the indeterminate motion associated with "zero point energy".

As you will see if you follow the rest of the discussion, there can be various modes of "zero point energy".