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.
Reference: Research Paper
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