How Low Can We Go?

Question: Will it ever be possible to get a temperature down to \r\nabsolute zero?
\r\n

\r\nWilliam Phillips: Well, that’s an interesting question.  And sadly, the \r\nanswer isn’t simple.  The simple answer is, no.  But now I’ve got to \r\nexplain why I’m saying that the answer is no.  And answer is that every \r\nprocess for cooling either also introduces the possibility that you can \r\nintroduce some extra energy into the system.  You see, cooling means \r\ntaking energy out, and heating means putting energy in.  But in order to\r\n take energy out, then it turns out that you open the door for energy to\r\n go in. 
\r\n
\r\nTake laser cooling.  Laser cooling takes energy out by having an atom \r\ncoming along and then a photon hits the atoms and slows the atom down, \r\nbut then that photon has to go someplace.  And when that photon is shot \r\nout by the atom, the atom recoils and more energy goes in.  so, there’s a\r\n balance between the cooling and the heating and you can try to make \r\nthat balance work more and more in your favor, but you can never make it\r\n work 100% cooling and no heating. 
\r\n
\r\nSo that’s one of the reasons why you don’t expect to ever get to \r\nabsolute zero.  On the other hand, what does it mean to be at absolute \r\nzero?  It means that all of the thermal motion stops.  Well, I can take \r\none atom and I can take as much energy out of it as possible so that \r\nit’s in what would call the ground state, the lowest possible state of \r\nenergy.  Is it absolute zero?  Not really because in order to be at \r\nabsolutely zero, I really have to have a whole bundle of things.  I \r\ncan’t really talk easily about the temperature of a single object.  I \r\nshould really talk about a whole ensemble.  And if I do that with a \r\nwhole bunch of atoms, what’s going to happen is, maybe if I’m lucky, \r\nmaybe 99 percent of them are going to be in the ground state and then \r\none percent isn’t.  So, it’s not absolute zero. 
\r\n
\r\nI can’t come up with any procedure that is going to say 100 percent of \r\nthe time this atom’s going to end up in the ground state.  And that’s \r\nwhat I would need to be able to claim that I really had gotten down to \r\nabsolute zero.  But on the other hand, I can get so close to absolute \r\nzero that for many experiments, it’s basically absolute zero for all \r\npractical purposes.  But not for all experiments and we are constantly \r\nworking on making things colder because for some experiments, it really \r\nmatters that were not quite there.
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Recorded June 4, 2010
Interviewed by Jessica Liebman

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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."

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