More on the demons of thermodynamics

Robertson replies: Daniel Sheehan, Garret Moddel, and James Lee draw attention to an interesting book, Challenges to the Second Law of Thermodynamics: Theory and Experiment, which provides a survey of recent work that throws doubt on the inviolability of the second law. Do those avenues resurrect Maxwell’s demon? Do they conjure new ones?

Of course, no scientific law is immutably beyond reproach, no matter how many famous physicists have sworn by it. But in considering the avenues suggested in the book, it can be helpful to scrutinize what we mean by the “second law.” ¹ If the second law is that entropy—of which there are many forms—cannot decrease, then it can surely be violated; the Boltzmann entropy decreases in macroscopically indeterministic processes. But if the second law is taken to be that no engine is more efficient than a Carnot engine, then at least the previous example is not necessarily a violation.

As David Wallace said in a talk at the University of Cambridge in November 2015, the distinction is between whether we can find ingenious and cunning devices or whether we can solve the energy crisis! The faith in the implausibility of the latter—that we will not find a perpetual motion machine of the second kind—is what those aggrandizing thermodynamics attest to. I meant to have captured that distinction between different kinds of “violations” of the second law in my discussion of “deft illusionists” versus “true magicians.” But that may have sounded dismissive; to be clear, important insights are revealed by studying the cases that Sheehan, Moddel, and Lee highlight, especially with respect to how the macroscopic domain may differ from the mesoscopic and microscopic ones. And if there were a true magician, then that would be welcome news in the current energy crisis.

Similarly, whether the spin-echo experiment counts as a violation depends on precisely what we mean by the second law—no one is expecting to create a greater-than-Carnot efficiency engine out of that scenario. The point is merely that a system “retracing its steps” may have seemed nigh on impossible to Josef Loschmidt, but the spin-echo case provides a nice illustration of its feasibility. I am sympathetic to Jean Jeener ’s view that the spin echo is not a case of increasing and then decreasing entropy, but then again, if we just look at the unitary dynamics, then entropy is neither increasing nor decreasing.

One feature absent from my original article—as rightly emphasized by Harvey Leff and Andrew Rex —is the connection between thermal physics and information forged by Maxwell’s demon. That is often the lynchpin or starting place for those interested in quantum thermodynamics (referenced as “quantum steampunk” in my article) and, as Leff and Rex emphasize, stochastic thermodynamics. That brings up a question though: If information is central to thermodynamics, does that raise the specter of anthropocentrism?