There's an obscure science fiction short story by William Morrison (a.k.a., Joseph Samachson), called "A Feast of Demons," in which a scientist creates a hardy little band of so-called "Maxwell's demons," capable of changing the temperature of various objects, and even reversing or accelerating the aging process in humans. (Naturally the demons run amok and weak havoc on an unsuspecting civilization. Otherwise there would be no plot!) The story is included in a collection edited by Ken Kesey, The Demon Box; in fact, all the stories explore similar themes, based on one of the most famous physics thought experiments of the 20th century (second only to the infamous Schroedinger's cat), devised by a Scottish physicist named James Clerk Maxwell.
It all started with that pesky second law of thermodynamics, a.k.a., "thermogoddamnics" to those who fight a losing battle against entropy — i.e., everyone, whether they realize it or not. That's the one that says, basically, not only can you not have a closed system that puts out more energy than you consume, but you're always going to lose a little bit of energy in the energy conversion process. We're talking about converting potential energy into kinetic energy. One of the neat things about thermodynamics is that if you can create a large enough differential — for example, a big difference in temperature between, say, two compartments — you've got yourself a handy energy source to tap into should the need arise.
Refrigerators work on this simple concept, known as the Carnot cycle. Gas (usually ammonia) is pressurized in a chamber, said pressure causes that gas to heat up, this heat is then dissipated by coils on the back of the appliance, and the gas condenses into a liquid. It's still highly pressurized, sufficiently so that the liquid flows through a hole to a second low-pressure chamber. That abrupt change in pressure makes the liquid ammonia boil and vaporize into a gas again, also dropping its temperature — thereby keeping your perishable foodstuffs nicely chilled. The cold gas gets sucked back into the first chamber, and the entire cycle repeats ad infinitum — or at least as long as the appliance is plugged in. That's always the catch, you see. The refrigerator is not a truly "closed system": it gets a constant influx of energy from the wall outlet that enables it to operate continuously. Left on its own, without that crucial influx, and the interior would cease to be nicely chilled, and all the food therein would perish.
So that's the second law of thermodynamics, and frankly, it's pretty unyielding. But while it can't be broken, perhaps it can be bent by a cunning infusion of energy that escapes detection by all but the most perceptive eye. James Clerk Maxwell proposed the most famous evasion of thermodynamics back in 1871, dubbed "Maxwell's Demon." Maxwell was one of those kids who liked to know how things worked, taking things apart and trying to put them back together again — one assumes not always successfully, which must have been quite trying for his parents. He ended up earning a degree in mathematics and taking a chair in natural philosophy at King's College in London, where he formed his famed equations for electromagnetism that are still in use today.
But he was equally fascinated by thermodynamics, notably the fact that heat cannot flow from a colder to a hotter body. And one day Maxwell had an idea: what if hot gas molecules merely had a high probability of moving toward regions of lower temperature? He envisioned an imaginary, tiny creature who could wring order out of disorder to produce energy by making heat flow from a cold compartment to a hot one, creating that all-important temperature difference. The imp guards a hypothetical pinhole in a wall separating two compartments of a container filled with gas — similar to the two chambers in a refrigerator — and can open and close a shutter that covers the hole whenever it wishes.
Now, the gas molecules in both compartments will be pretty disordered, with roughly the same average speed and temperature (at least at the outset), so there's very little energy available for what physicists call "work": technically, it's defined as the force over a given distance (W=fd), and it means that you'll spend the same amount of energy carrying a heavy load over a short distance, as you will carrying a feather over a very long distance. But I digress. It Maxwell's thought experiment, the atoms start out in a state of thermodynamic equilibrium. But they're still jiggling around all the time, as atoms are wont to do, so over time, there are small fluctuations as some molecules will start moving more slowly or more quickly than others, balance will soon be restored, since the excess heat will be transferred from hotter to colder molecules until they are all once again in equilibrium.
Ah, but then Maxwell's little demon interferes. Whenever it spots a molecule moving a bit faster in the right compartment and start to move towards the pinhole, he opens the shutter just for a moment so it can pass through to the left side. It does the same for slower molecules on the left side, letting them pass to the right compartment. So what happens as time passes? The molecules in the left compartment get progressively hotter, while those on the right side get colder. The creature creates a temperature difference, and once you have that, well, it's a trivial matter to harness that difference for work. Entropy has been outwitted — or so it would seem. (You can embrace your inner science imp and play a nifty online game of Maxwell's Demon here.)
Maxwell was too clever by half: in reality, his thought expression was a trick question. Maxwell himself supplied two reasons why his clever little demon couldn't exist in the physical world. First, it's statistically impossible to sort and separate billions of individual molecules by speed or temperature; Nature just doesn't do this. You can't throw a glass of water into the sea and expect to get back the exact same glass of water, right down to the last single molecule.
Okay, perhaps hypothetically you might be able to do this, provided you knew the exact speeds and positions of each and every molecule (at the quantum level, of course, this is an impossibility thanks to the Uncertainty Principle). But you'd have to expend a huge amount of energy to collect that detailed information, far more than the energy you'd get out of the system once you'd succeeded in creating the crucial temperature difference. And that's the catch. (There is always a catch. Energy is never "free.") Just like the refrigerator, Maxwell's mischievous little imp also requires energy to operate. There is no such thing as a perfect heat engine; you'll always lose some heat in the process. That's the bane of every researcher striving to develop alternative energy sources, and they have to be cost-competitive as well as energy-efficient.
That hasn't kept physicists from playing around with the concept of Maxwell's Demon experimentally in the ensuing 130+ years, and it's been a busy year in this area so far. First, a January 31, 2008, article in Physics World described a nifty manmade molecular machine created by another Scotsman, David Leigh, and his colleagues at the University of Edinburgh. Most biological processes involve driving chemical systems away from thermal equilibrium, so Leigh devised a chemical "information ratchet" that performs much the same role as Maxwell's hypothetical demon: creating a temperature difference out of thermal equilibrium, thereby seemingly "reversing" entropy. To quote from the article:
"To perform the feat, they use 'rotaxane,' an assembly of molecules comprising a dumbbell-shaped axle on which a ring can slide, hindered only by a gate located part way along. By shining light on rotaxane, the ring absorbs photons and transfers energy to the gate, which then temporarily changes shape to let the ring pass. Once the ring has passed, however, it cannot transmit energy back to the gate, and is therefore stuck — or ratcheted — in place."
It still requires an extra influx of energy to operate the chemical "ratchet," according to Leigh, but nonetheless, it's definitely another step towards the practical realization of manmade molecular machines similar to those found in Nature.
Then, in the March 7 Physical Review, a paper appeared by Mark Raizen and Gabriel Price of the University of Texas at Austin, describing their experiments with a laser-based cooling trap combined with a magnetic trap, divided by a barrier beam. The setup is a little complicated– you can read the details here — but in essence, one laser beam serves as the "barrier" while another excites certain atoms of specific frequencies. The end result is a "sorting" of atoms, such that eventually all the atoms end up on one side of the barrier.
Raizen and Price first conceived of the device in 2005 as a means of cooling gases to very low temperatures, perhaps even just a few degrees above absolute zero. Laser cooling has been around in some form or another since the mid-1980s, when Stanford physicist Steven Chu first wove a "web" out of infrared laser beams to create what he called "optical molasses." The beams keep bombarding the atoms with a steady stream of photons — a bit like hail constantly hitting you in the face — tuned to specific wavelengths so that they will only be absorbed if they collide head-on with an atom. This causes the atoms to slow/cool down.
Laser cooling was combined with evaporative cooling in the 1990s to produce the world's first Bose-Einstein condensate (BEC), an exotic state of matter first predicted by Albert Einstein and the Indian physicist Satyendra Bose in the 1920s. Get atoms cold enough, they reasoned, to a few billionths of a degree above absolute zero, and they will be packed so densely that they'll coordinate themselves like one big "superatom." It took 70 years, but by gum, physicists succeeded. The work earned Carl Wieman, Eric Cornell, and Wolfgang Ketterle the Nobel Prize in Physics in 2001, and the honor was justly deserved. But BECs to date have only been achievable with specific kinds of gases, like rubidium or cesium. That's why Raizen and Prize's method was so intriguing. Not only would this enable physicists to study even more exotic states of matter, it might also give us new types of atomic clocks (which currently use cesium atoms, mostly, for keeping time).
And now a paper has just appeared in the June 20 Physical Review Letters describing another innovation on the laser-based Maxwellian demon concept, this one devised by Daniel Steck of the University of Oregon in Eugene. It's another laser barrier set-up in which the beam lets atoms pass through only in one direction, such that they all eventually end up on a single side, chilled to extremely low temperatures. Steck created a "box" out of laser light/electromagnetic fields, and then added two parallel lasers that together serve as the "trapdoor." The beam on the right is the barrier, and the one of the left is the "demon," responsible for the "sorting." In concept, it's similar to Raizen and Price's method and the secret, according to Science News' Davide Castelvecchi, is a subtle one:
"Like all lasers, Steck's pumping beam is an orderly arrangement of photons, all traveling in the same direction. And a photon increases the energy level of a rubidium atom by scattering off of it. 'But the scattered photon goes in a random direction,' Steck observes. So while the atoms get a little more order in their lives, the pumping laser ends up with a little less."
In other words: it's a tradeoff. I told you energy is never free. Note, also, that all of these experiments emphasize that they cool the atoms to fractions of a degree above absolute zero (technically defined as minus 495 degrees Fahrenheit), without ever actually reaching that fundamental limit. That's the unofficial "third law" of thermodynamics. All atoms vibrate to some degree. How fast they vibrate depends on heat: the hotter they are, the faster they vibrate, the colder they are, the more slowly they vibrate. But they never cease to move entirely and exist in a state of absolute rest, so to the best of our knowledge to date, absolute zero is impossible to achieve.
Physicists devised this handy mantra summing up the basics of thermodynamics: you can't win, you can't break even, and you can't get out of the game. Now that is truly demonic.