Real-life Maxwell’s demon adds fuel to debate about status of the second law

Fight Club had its First Rule (don’t talk about Fight Club). The Transporter enforces Rule Number 1 (never change the deal). And NCIS Special Agent Leroy Jethro Gibbs observes Rule 1 (never mix the suspects together in the same room).

Physics has the second law of thermodynamics.

It’s weird when you think about it. Movies and TV shows always give their prime rule top billing. But the physics rule alleged by Sir Arthur Eddington to hold the “supreme position among the laws of Nature” is only Number 2. Nevertheless, scientists generally consider it the most unbreakable law of all. It is supposedly impervious even to a tiny magical being able to track the paths of single molecules: the hypothetical Maxwell’s demon, which a new paper suggests may be not so hypothetical anymore.

Almost a century and a half ago, the British physicist James Clerk Maxwell invented the demon to illustrate key points about the second law. Although there are various versions of this law, its essence is that heat flows from hot to cold until you get thoroughly mixed lukewarmness. Molecules never separate back into hot and cold (in a closed system absent the input of energy, say to run a refrigerator or air conditioner).

But suppose that you prepared two sealed rooms, connected only by a small window, with fast (hot) molecules in Room A and slow (cold) molecules in Room B. If you open the window, the suspect molecules violate Gibbs’ Rule 1 and mix themselves up. But now, said Maxwell, place a demon at the window to control which molecules pass from one room to the other. If the demon let only fast molecules pass from Room B to A, and only slow molecules from A to B, hot and cold would once again be separated, violating the second law.

You have to assume the window is frictionless, but otherwise Maxwell’s scenario appears to show that the second law isn’t so supreme. Except that no real beings have the power of Maxwell’s demon. Besides, the demon can’t break the second law anyway, as IBM physicists Rolf Landauer and Charles Bennett established decades ago. A demon has to record information about a molecule’s velocity to decide which ones to let through the window. Erasing that information to make room for the next measurement uses up energy, so the demon is just a fancy refrigerator requiring a power source.

Some critics have objected to the Landauer-Bennett explanation. But now the demon itself has spoken, or at least a real-world design of the demon idea has verified the importance of erasing information. In a recent Physical Review Letters, physicists from Germany and Luxembourg describe a model for a Maxwell demon using “quantum dots” (electronic nanodevices that behave a bit like artificial atoms). One of the dots (the demon) senses whether or not an electron is in the other, which serves as a transistor. In such a system the demon dot can drive the electron in the transistor against the voltage, an apparent violation of the second law. But when the information flow in the entire system is taken into account, the second law is preserved.

“To the best of our knowledge, this … establishes for the first time the precise connection between the complete thermodynamic description of a Maxwell demon model and the system it is acting on,” write Philipp Strasberg of the Institute of Theoretical Physics in Berlin and collaborators. “In particular, we have identified the effective level of description of the system where the demon manifests itself solely through an information flow modifying the second law.”

All this does not prove that the second law is unbreakable. It only shows why Maxwell’s demon can’t break it. There are still some renegade physicists out there — the type who would blabber about Fight Club — who believe the second law might be wrong. In fact, dozens of papers have shown up in physics journals in recent years that propose ways around the second law, as Germano D’Abramo of the National Institute for Astrophysics in Rome writes in a recent Studies in History and Philosophy of Modern Physics.

He describes the possibility of devices that could absorb heat from the environment to generate an electric current, a violation of the second law if device and environment start out at the same temperature. But even if such devices actually broke the second law, they wouldn’t solve the energy crisis. “The power output is so minuscule that it is unthinkable to extract usable work from environmental heat,” D’Abramo admits.

Such a second law violation could, though, shed light on the distinction between the original formulations of the second law and its description in terms of the statistics of molecular motion, as developed by Maxwell, Ludwig Boltzmann and J.W. (not L.J.) Gibbs. Gibbs, in fact, proposed a famous paradox whereby mixing molecules in the same room appeared to violate the second law. It doesn’t really, Gibbs said. But when the second law is concerned, it might be a good idea to observe L.J. Gibbs’ Rule 8: Never take anything for granted.

Tom Siegfried is a contributing correspondent. He was editor in chief of Science News from 2007 to 2012 and managing editor from 2014 to 2017.

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