An ongoing debate between Nobel Laureate Frank Wilczek and physicist Patrick Bruno involves the fate of the first perpetual motion machine that scientists are taking seriously in several centuries.

Every school-going kid is taught that friction is a necessary evil. There’s another necessary evil hiding in plain sight that they aren’t taught about: the law of conservation of energy. Sure, violating this law would lead to catastrophic ergonomic consequences in this universe, but it’s also the law standing in the way of perpetual motion machines.

Perpetual motion machines have efficiencies tending toward infinity. This means that for a specific input, the output is infinite. It’s like a contraption that, if set in motion once, it cannot be stopped. They feature in different forms – at least in human imagination. Unfortunately, practical implementations have been ceaselessly thwarted by the law of conservation of energy (except perhaps Johann Bessler’s Orffyreus’s Wheels from 1717, the physics behind which no one knew).

A new kind of perpetuum mobile

On February 12, 2012, physicist and Nobel Laureate Frank Wilczek suggested a strange new form of perpetual motion machines, but without any implication of the law of conservation of energy being violated. His recourse was in the redefinition of what are called ground states.

To understand ground states, let’s draw on energy conservation again: If anything is going to keep rotating, it must radiate some energy. Eventually, for a given input, all the energy will have been radiated and the output will drop to zero, stopping the machine. In this scenario, the machine will have come from a higher energy state to a lower energy state. The lowest energy state a machine can be in is called its ground state.

In this state, it's presumed that the machine is stationary - in time and space - and can perform no work. If it is do some work, then some energy has to be provided to it, and over time, it will do work, radiate the input energy to its environment, and return to its ground state.

However, for a perpetual motion machine to hold its ground, it must continue moving even in its ground state. Frank Wilczek proposed that there could exist quantum mechanical systems, which he called 'time crystals', that were in their ground state, and also exhibited periodic motion in that state: rotating or vibrating, etc. Since machines in their ground state are 'frozen' in space and time, any sort of periodic motion would violate the time symmetry.

Let's test this thing

Needless to say, the physics community was stunned. There were ripples and whatnot, but some people quickly gathered their wits and said, “Hey, let’s test this thing.” This is when things got more interesting.

As of now, there has been one big proposal to test the idea, and one extensive response arguing against the idea. The proposal came from Tongcang Li, et al, on June 21, 2012, from UCal Berkeley, and described a system of trapped beryllium ions that could be set in rotation at a temperature of one-billionth of a kelvin. On May 4, 2013, there was a suggestion to extend this proposal in the form of a prototypical idea for a device that could measure and manipulate time crystals using lasers.

While that pans out, consider the arguments posed by Patrick Bruno, of the European Synchrotron Radiation Facility. His first comment was submitted to arXiv on October 15, 2012. His second paper came in response to Wilczek's reply (published March 11, 2013), and was submitted to arXiv on June 26, 2013.

The principal argument against the idea of time crystals was simple: that they weren’t in their ground state. Bruno contended that the error lay in the way Wilczek had conceptualised his time crystals. In response, Wilczek acknowledged Bruno's argument, but denied that the idea of time crystals were, or could be, invalidated. Because Wilczek's concept had been rejected, the implied and existing problem is this: In theory, can the general idea of a time crystal be conclusively validated or invalidated?

As physicist Jakub Zakrzewski told APS, "I’m not convinced time crystals exist. I’m not convinced they don’t exist. It is controversial, but [the possibility] is open."

Finishing the debate

On November 20 last year, Bruno had commented on why the UCal team's idea of the device was flawed. Subsequently, the research group published a reply, too. In the June-27 paper, Bruno claims to present a general solution, a "no go" - the conclusive invalidation. The math that accompanies this debate, and the parallel one with Wilczek, is far advanced, but if you’re still inclined: Wilczek’s first paper, Bruno’s comment, Wilczek’s response, and Bruno's latest.

In fact, Bruno hopes that the matter will soon be laid to rest: "Unless someone could prove there is an error in my theorem, I believe that this work should put an end to the time-crystals hypothesis," he said.

This blog post was edited on July 4, 2013 (Thursday), to reflect some changes as well as clarify some confusion and distortions that arose through the author's presentation of the concept. The content has been accordingly modified.