Am I correct in thinking that it would be impossible to cool a particle down to absolute zero because we would then know its position and velocity exactly (i.e., it ain’t moving at all)–which violates the Uncertainty principle?
Yowza! mildly strange typo. Sorry.
I feel all British and junk now … let me go look up the answer in my encyclopaedia …
Does measuring temperature necessarily tell you the particle’s position? Are there measurements that give you both simultaneously? Can you measure a particle’s position exactly without adding any energy to it?
That is correct, toadspittle. If you know that a particle is at absolute zero, then you know that it’s momentum is exactly zero. With no uncertainty in momentum, you have infinite uncertainty in position, so the particle could be anywhere in the Universe.
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I do not know the answer, but in fresman year of high school, there was a kid in my science class who thought the essentially the same thing. He alwaysused to contest it every time the teacher mentioned absolute zero. Sometimes the two of them would argue about absolute zero for nearly 45 minutes, much to the delight of the rest of us who didn’t have to do much work because of it. Needless to say, it did get a bit old after a while.
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Fuuny things happen when you approach abs. zero. The latest thing i have heard is at that point matter will still have some motion but just the lowest motion possible.
Exactly so, k2dave. At absolute zero, there will still be what’s known as zero point motion. At absolute zero, the system will be in its ground state (the quantum state with minumum allowed energy), and the ground state wavefunction still must satisfy the uncertainty principle.
As a matter of practice, of course, cooling something to absolute zero isn’t possible (although we can reach temperatures in the nanoKelvin region, IIRC) because there’s no way to pull all the thermal energy out of the system. But in principle, absolute zero is okay.
(If the world were classical, yes, absolute zero would mean no motion, and hence infinite precision in both the momentum and the position. However, the world is of course not classical, and the uncertainty principle still applies.)
Motivated by Chronos’s post:
If we ever do cool something down to absolute zero, would
that tell us that the universe is infinite in size?
Or to put it another way, does the lower bound on how much
we have cooled something (or observed something to be cool)
give us a lower bound on how big the universe is?
Since this is talking about cold things (and I think that we can agree that at zero degrees Kelvin “cold” loses its relativity), I felt slightly obliged to mention another oddity. This is the Bose-Einstein Condensate. If I am correct, it occurs at about 1/10000 degrees Kelvin, perhaps colder. Anyway, it is the coldest that we have ever gotten a substance (namely Rubidium). The BEC is simply put, a large conglomeration of particles. When they become so cold, the atoms will overlap into a large blob, called a “superatom.” My knowledge is a little rusty on this right now, though.
The temperature to achieve such a condensate would be dependent on what one was using, though suprisingly all have the value of “cold as a well-digger’s ass.” Go figure 
Well, it’s really late at night, and that has allowed me
to slip into wild theoretical physics hypothesis making
mode. I hope you still respect me in the morning.
Carrying on where my last reply left off:
If quantum mechanics postulates a minimum possible quanta of
momentum (I gather this is true from the above posts),
is it possible that this is not constant, but has an inverse
relation to the size of the universe? Perhaps even Planck’s
constant decreases as the size of the universe increases?
Do any existing theories explore this possibility?
i just read something pertaining to this, but i can’t find anything about it for the life of me, and maybe someone else can. it was a new theory about heating something to such extremes to where it made a loop in the temperature scale and became cold, and the theory’s creators believed this was the only way to reach absolute zero, or something like that. it all sounded weird to me, but it was shown to me by a well-respected chemist, so there has to be some validity to it.
This was refered to in another thread recently.
Please read the article, it’s really quite fascinating (and only assumes basic thermodynamics).
I ought to add that the ‘negative temperature’ referenced to above has nothing to do with uncertainty, and very little to do with quantum mechanics. Basically it’s (late) 19th century physics.
We can’t pull enough energy from a system to get it to abs. zero. but can random colisions of atoms (or other particals or energy (but aren’t they the same - that’s another issue)) cause abs. zero in an atom even if it’s just till the next colision. Basically can it be hit in such a way as to fall to is abs. zero ground state by domb luck so to speak?
Chronos gave the ‘physics’ answer. My thermodynamic 2 cents are that you cannot 100% insulate anything from the surroundings which are not at absolute zero. Therefore, some heat transfer will always be taking place.
Anyway, does temperature really apply to 1 atom?
Well, I think some near-absolute zero cooling uses lasers to hold the atoms in place–after all, if you push on something from six different directions, it ain’t going anywhere.
At absolute zero (or at least a few billionths of a degree near it) both the uncertaintu principle and particle/wave duality come into play. As a massive particle (like an atom) loses heat, its motion slows down (Think of heat as an atom vibrating). When the atom reaches absolute zero, its motion stops…and its position can be known with absolute certainty. Therefore, its direction must lose certainty by collapsing into a wave function.
At absolute zero, matter stops being a collection of particles and becomes a wave…Bose/Einstein Condensate.
sigh You people keep on forgetting about zero point motion. When the temperature reaches absolute zero, the system will be in its lowest energy (“ground”) state. When in its ground state, it still shows quantum mechanical behavior; in this case, we see an uncertainty in both position and momentum even in the ground state of the system. In other words, the motion doesn’t stop, because we don’t live in a world where you can just ignore quantum mechanics. These motions are real; when doing accurate computational chemistry, one needs to take the so called “zero point corrections” into account.
I have done a little research and apparently, absolute zero is unachievable as it would violate the third law of thermodynamics.