It’s unlikely we’ll ever get all thermal motion to stop in an object. But we can get close enough in many experiments that “it’s basically absolute zero for all practical purposes.”

Question: Will it ever be possible to get a temperature down to rnabsolute zero?
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rnWilliam Phillips: Well, that’s an interesting question.  And sadly, the rnanswer isn’t simple.  The simple answer is, no.  But now I’ve got to rnexplain why I’m saying that the answer is no.  And answer is that every rnprocess for cooling either also introduces the possibility that you can rnintroduce some extra energy into the system.  You see, cooling means rntaking energy out, and heating means putting energy in.  But in order torn take energy out, then it turns out that you open the door for energy torn go in. 
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rnTake laser cooling.  Laser cooling takes energy out by having an atom rncoming along and then a photon hits the atoms and slows the atom down, rnbut then that photon has to go someplace.  And when that photon is shot rnout by the atom, the atom recoils and more energy goes in.  so, there’s arn balance between the cooling and the heating and you can try to make rnthat balance work more and more in your favor, but you can never make itrn work 100% cooling and no heating. 
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rnSo that’s one of the reasons why you don’t expect to ever get to rnabsolute zero.  On the other hand, what does it mean to be at absolute rnzero?  It means that all of the thermal motion stops.  Well, I can take rnone atom and I can take as much energy out of it as possible so that rnit’s in what would call the ground state, the lowest possible state of rnenergy.  Is it absolute zero?  Not really because in order to be at rnabsolutely zero, I really have to have a whole bundle of things.  I rncan’t really talk easily about the temperature of a single object.  I rnshould really talk about a whole ensemble.  And if I do that with a rnwhole bunch of atoms, what’s going to happen is, maybe if I’m lucky, rnmaybe 99 percent of them are going to be in the ground state and then rnone percent isn’t.  So, it’s not absolute zero. 
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rnI can’t come up with any procedure that is going to say 100 percent of rnthe time this atom’s going to end up in the ground state.  And that’s rnwhat I would need to be able to claim that I really had gotten down to rnabsolute zero.  But on the other hand, I can get so close to absolute rnzero that for many experiments, it’s basically absolute zero for all rnpractical purposes.  But not for all experiments and we are constantly rnworking on making things colder because for some experiments, it really rnmatters that were not quite there.
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Recorded June 4, 2010
Interviewed by Jessica Liebman
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