UH Hilo’s Binder Resolves Old Physics Paradox

Date: Tuesday, September 12, 2023
Contact: Alyson Kakugawa-Leong, (808) 932-7669

For Immediate Release

A University of Hawaiʻi at Hilo physics professor has published a surprising way to resolve an old physics paradox in the September 2023 issue of International Journal of Theoretical Physics, a Springer-Nature journal.

Dr. Philippe Binder’s “The Reversibility Paradox: Role of the Velocity Reversal Step” focuses on re-examining the reversibility paradox, which is related to the Second Law of Thermodynamics.

“Back when scientists started contemplating the existence of atoms, several thought experiments were proposed to challenge or refine the underlying physics,” Binder noted. “Among the most famous of these are (James Clerk) Maxwell’s demons and the reversibility paradox, both proposed in the mid-1800s.

“The consensus has been that the first of these was fully explained in the 1980s by Charles Bennett of IBM. The reversibility paradox has continued to be the object of some debate, with several possible resolutions still being discussed.”

Central to the paradox is the concept of entropy, a property of a physical system that measures the configurations of atoms consistent with what humans can see or measure. This quantity is loosely and somewhat incorrectly called a “measure of disorder,” Binder said. In a closed system – one that does not interact with its environment – entropy tends to increase. This is known as the Second Law of Thermodynamics.

The paradox relies on the notion of reversibility, demonstrated when a simulation of atoms moving and colliding either in forward or reverse motion will appear to be physically correct.

“The paradox, dating from the 1860s, starts with a low-entropy physical system, which is allowed to spontaneously relax to higher entropy, as is expected,” Binder said. “If the velocities of all particles are reversed, the system’s entropy now starts to naturally decrease. Hence, normal behavior plus a simple velocity reversal leads to forbidden Second-Law behavior, wherein lies the paradox.”

The problem, Binder proposed, involves the moment when the particle velocities are reversed.

“The system, say, a gas, must come in brief contact with its environment. In order to reverse the velocities, some agent (conscious or inert) must learn the positions and velocities of all particles and temporarily store them in memory,” he explained. “If the measurement is irreversible, the entropy of the measuring device must increase; if it is reversible, there is also an entropy cost for returning the measuring device to its original, ‘ready to measure’ state. The latter case is covered by an idea called Landauer’s principle.

“The bottom line,” Binder added, “is that at least as much entropy is generated through the velocity reversal process as is later lost by the gas finding its way back to its initial, low-entropy state. Thus, the Second Law is safe.

“What I have done is solve a 160-year-old problem with a 60-year-old tool,” Binder continued. “In this sense it is not revolutionary physics, but it should certainly generate some discussion among those interested in the foundations of physics.”

Binder’s article is accessible at: https://rdcu.be/dlBuJ.

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