The direction of time seems pretty obvious; it goes from the past towards the future, though the reason why that is the case is unclear. This arrow of time has been linked to entropy, the measurement of the disorder of a system. Over time, in an isolated system, entropy always increases. This process is irreversible. It applies to the aging everything experiences, or an egg rolling off the counter and breaking apart. But under a certain perspective, there are materials that defy this behavior. And they are very common: glasses and plastics.
These materials consist of tangles of molecules, usually in a random distribution. Even as solids, the constituent molecules are moving, though it’s an incredibly slow process. The molecules always look for the most favorable energetic state, and this process changes the properties of the material over time. For the glass in a window, it would take billions of years.
To look at this process from the point of view of the material, researchers use the “material time” – the internal clock ticking inside the substance in question. This depends on how quickly the molecules within the material reorganize, meaning you can have material that has a very long material time. Still, globally the arrow of time points toward the future. Everything ages.
Measurements of material time are far from easy. Researchers created a setup to study the molecular movement in a sample of glass, and statistical methods were used to establish the fluctuations over time. On analyzing the results, the team discovered that these molecular fluctuations are time-reversable. This means that they would look the same looking forward or backward in time.
“However, this does not mean that the aging of materials can be reversed,” lead author Till Böhmer, from the Technical University of Darmstadt, said in a statement.
So, the molecular small movements don’t affect the aging of the whole system. They shake and shimmy without affecting the material time. As far as that time arrow is concerned, in glasses and plastic, the molecular changes could be going backward or forward. The direction of the arrow of time from the molecular point of view is irrelevant. But overall, the glass still ages.
“This leaves us with a mountain of unanswered questions,” added co-author Prof. Thomas Blochowicz.
Does the reversibility come from physical laws that are reversible? How is the material time different in different materials? Does this finding apply to all disordered materials as the team suspects? Those are the questions that future research must now determine the answers to.
The study is published in Nature Physics.