In the 19th-century, James Clerk Maxwell came up with a thought experiment that still befuddles physics students (as well as the rest of us) to this day. The great Scottish physicist theorized the existence of a “demon” that seemed able to concentrate the atoms of a gas into a smaller volume without raising their temperature, thus reducing their entropy. That feat seemed to violate the second law of thermodynamics, according to which entropy can never decrease.
What’s the catch?
To find out, get your hands on the March issue of Scientific American, which is now hitting the newsstands. In the article “Demons, Entropy and the Quest for Absolute Zero,” physicist Mark G. Raizen describes how he built a working version of the demon: a one-way gate that only lets atoms go through in one direction only, forcing them accumulate on one side of a container.
Raizen’s is not the first physical realization of a Maxwell’s demon. The molecular machinery of living cells often appears to rely on the related concept of a Brownian ratchet, which has inspired chemists to build molecules that harvest the energy of Brownian motion, in seeming violation of the second law of thermodynamics (see the article on the nanomachines of life and their manmade imitations, which I wrote for Science News three years ago). But the gate Raizen built is particularly elegant in that it is pretty much as efficient as it gets: it sorts atoms using a single photon for each atom.
Raizen used his device to cool a rarefied gas down to temperatures of just millionths of a degree above absolute zero. He started out with a gas that had already been cooled to one one-hundredth of a kelvin (using a device called an atomic coilgun, also described in the text) and placed it in a magnetic trap. He then used his one-way gate to bring the temperature closer to absolute zero by many orders of magnitude.
To see how that works, check out the interactive single-photon cooling animation on SciAm’s web site. It shows how a one-way gate can help cool a gas in two steps: First let the gate concentrate atoms into a smaller volume (but without raising their temperature), then allow them to expand to the original volume (which brings their temperature down).