Time's inexorable march might well wait for no one, but a new experiment by researchers at the Technical University of Darmstadt in Germany and Roskilde University in Denmark shows how in some materials it might occasionally shuffle.
An investigation into the way substances like glass age has uncovered the first physical evidence of a material-based measure of time being reversible.
For the most part the laws of physics care little about time's arrow. Flip an equation describing the movement of an object and you can easily calculate where it started. We describe such laws as time reversible.
While many individual laws thumb their nose at time, the fate of our Universe as a whole is dictated by the inevitable pull of chaos. In one direction, freshly laid eggs. In the other, omelets. No amount of calculating will get you a nice, round egg again.
In scientific terms, we might say time is a feature of the second law of thermodynamics – the tendency for isolated systems to be more disordered than they were in the past in ways that can't be easily rewound.
What decides whether a material system is time reversible or driven by entropy is hard to say. We can easily imagine the rusting of an old car, the weathering of a statue, or steady decay of a beached ship, yet materials like glass can slowly change in ways that have nothing to do with external corrosive forces.
Consisting of liquid-like jumbles of particles, non-crystalline substances that include various polymers and amorphous solids like glass relax into a theoretically stable state according to their own entropy-driven clock. Think of it as a kind of special theory of relativity based not on gravity or acceleration, but the steady reconfiguration of assorted molecules thermodynamically falling into place.
Physicists describe this measure of aging as material time. While the concept has been around since the early 1970s, its interpretation in what's known as the Tool–Narayanaswamy formalism has never been experimentally measured.
There's a good reason for this. Glass ages slowly in ways that can't be captured simply by watching it closely.
"It was a huge experimental challenge," says lead author Till Böhmer, a condensed matter physicist from the Technical University of Darmstadt.
The team made clever use of a highly sensitive video camera to record scattered laser light, which, upon hitting a glass sample, formed interference patterns that could be statistically interpreted as fluctuations that conveyed a sense of material time within three different glass-forming substances.
Rather than being locked on a path towards equilibrium, they found evidence of time being reversible on a molecular level, varying as particles pushed and pulled one another into new arrangements. Rewind the entire process, it becomes impossible to tell whether the movie is being played forward or in reverse.
"However, this does not mean that the aging of materials can be reversed," says Böhmer.
As a whole, the system is destined to settle into a state as determined by entropy.
Yet the tiny swings of the molecular pendulums don't contribute to this process, tick-tocking back and forth, seemingly without a thought to the tides of time that surround them.
This research was published in Nature Physics.