Figure 2. Maxwell’s demon is a hypothetical being that can observe individual molecules in a gas-filled box with a partition in the middle separating chambers A and B. If the demon sees a fast-moving gas molecule, it opens a trapdoor in the partition to let fast-moving molecules into chamber B while leaving slow-moving ones behind. Repeating that action would allow the buildup of a temperature difference between the two sides of the partition. A heat engine could use that temperature difference to perform work, which would contradict the second law of thermodynamics.
Topics: Chemistry, History, Materials Science, Quantum Mechanics, Thermodynamics
Thermodynamics is a strange theory. Although it is fundamental to our understanding of the world, it differs dramatically from other physical theories. For that reason, it has been termed the “village witch” of physics.1 Some of the many oddities of thermodynamics are the bizarre philosophical implications of classical statistical mechanics. Well before relativity theory and quantum mechanics brought the paradoxes of modern physics into the public eye, Ludwig Boltzmann, James Clerk Maxwell, and other pioneers of statistical mechanics wrestled with several thought experiments, or demons, that threatened to undermine thermodynamics.
Despite valiant efforts, Maxwell and Boltzmann were unable to completely vanquish the demons besetting the village witch of physics—largely because they were limited to the classical perspective. Today, experimental and theoretical developments in quantum foundations have granted present-day researchers and philosophers greater insights into thermodynamics and statistical mechanics. They allow us to perform a “quantum exorcism” on the demons haunting thermodynamics and banish them once and for all.
Loschmidt’s demon and time reversibility
Boltzmann, a founder of statistical mechanics and thermodynamics, was fascinated by one of the latter field’s seeming paradoxes: How does the irreversible behavior demonstrated by a system reaching thermodynamic equilibrium, such as a cup of coffee cooling down or a gas spreading out, arise from the underlying time-reversible classical mechanics?2 That equilibrating behavior only happens in one direction of time: If you watch a video of a wine glass smashing, you know immediately whether the video was in rewind or not. In contrast, the underlying classical or quantum mechanics are time-reversible: If you were to see a video of lots of billiard balls colliding, you wouldn’t necessarily know whether the video was in rewind or not. Throughout his career, Boltzmann pursued a range of strategies to explain irreversible equilibrating behavior from the underlying reversible dynamics. Boltzmann’s friend Josef Loschmidt famously objected to those attempts. He argued that the underlying classical mechanics allow for the possibility that the momenta are reversed, which would lead to the gas retracing its steps and “anti-equilibrating” to the earlier, lower-entropy state. Boltzmann challenged Loschmidt to try to reverse the momenta, but Loschmidt was unable to do so. Nevertheless, we can envision a demon that could. After all, it is just a matter of practical impossibility—not physical impossibility—that we can’t reach into a box of gas and reverse each molecule’s trajectory.
Technological developments since Loschmidt’s death in 1895 have expanded the horizons of what is practically possible (see figure 1). Although it seemed impossible during his lifetime, Loschmidt’s vision of reversing the momenta was realized by Erwin Hahn in 1950 in the spin-echo experiment, in which atomic spins that have dephased and become disordered are taken back to their earlier state by an RF pulse. If it is practically possible to reverse the momenta, what does that imply about equilibration? Is Loschmidt’s demon triumphant?
The demons haunting thermodynamics, Katie Robertson, Physics Today