(Illustration from Encyclopedia Londinensis, 1810-29.)

A quick quote from a PBS two-part documentary first broadcast in 2008 and based on the “definitive” book on cold, Tom Shachtman's Absolute Zero and the Conquest of Cold. The scientist quoted is Seth Lloyd, of MIT:

Getting to absolute zero is tough. Nobody's actually been there at absolute 0.000000, with an infinite number of zeros. That last little tiny bit of heat becomes harder and harder to get out. And, in particular, the timescales for getting it out get longer and longer and longer, the smaller and smaller the amounts of energy involved. So eventually, if you're talking about extracting an amount of energy that's sufficiently small, it would indeed take the age of the universe to do it. Also you, actually you'd need an apparatus the size of the universe to do it, but that's another story.

Full program transcript here.