Physicists discover time can flow both ways in materials

Physicists discover time can flow both ways in materials

Physicists discover time can flow both ways in materials

Time seems to flow in one direction for us. We never see a broken glass reassemble itself on the floor. But for physicists, this is not so obvious. The equations that describe motion work both ways in time.



For example, a video of a swinging pendulum would look the same if you played it backward. We see time as irreversible because of another law of nature, the second law of thermodynamics. This law says that the disorder in a system always increases. If the broken glass reassembled itself, the disorder would decrease.

The same law applies to the aging of materials. But physicists from Darmstadt have found out that this is not the case. They have discovered that the motion of molecules in glass or plastic can be reversed in time if you look at it from a special angle.

The team led by Till Böhmer at the Institute for Condensed Matter Physics at the Technical University of Darmstadt has published its results in Nature Physics.

Physicists discover time can flow both ways in materialsThe Internal clock of glass

Glass or plastic is made of a tangled network of molecules. These molecules are constantly moving and changing their positions. They are always looking for a lower energy state, which affects the material’s properties over time—the glass ages.

But this process can take billions of years for some materials, like window glass. The aging process can be described by “material time.” Think of it like this: the material has a clock that ticks differently from the clock on the wall. The material time ticks faster or slower depending on how fast the molecules in the material rearrange.

This concept was proposed about 50 years ago, but it has been a while since anyone can measure material time. The researchers in Darmstadt, led by Prof. Thomas Blochowicz, have done it for the first time.

A surprising discovery

“It was a huge experimental challenge,” says Böhmer. The tiny movements of the molecules had to be recorded with a very sensitive video camera. “You can’t just watch the molecules jiggle around,” says Blochowicz.

But they did see something. They shone a laser on the glass sample. The molecules in the glass scattered the light. The scattered light beams overlapped and created a random pattern of bright and dark spots on the camera sensor. Using statistical methods, they could calculate how the pattern changed over time. This told them how fast the material’s internal clock was ticking. “This required extremely precise measurements that were only possible with the latest video cameras,” says Blochowicz.

The results were surprising. With the help of researchers from Roskilde University in Denmark, they analyzed the molecular movements regarding material time. They found that these movements were time-reversible. This means they would look the same if the material time ran backward, like the pendulum video.

“However, this does not mean that the aging of materials can be reversed,” stresses Böhmer. Rather, the result shows that the concept of material time is a good way to capture the irreversible part of the aging of the material. Its ticking represents the passage of time for the material itself.

What does It mean?

The researchers also found that only some things moving in the material contribute to aging. Only the movements that are related to the material time matter. To illustrate this, they used a metaphor: children playing around in the back seat of a car do not affect its motion.

The Darmstadt researchers believe this finding applies to all disordered materials, not just glass or plastic. They tested this hypothesis by examining two classes of material and simulating a model material on a computer. They got the same results in all cases.

The physicists’ success is just the beginning. “This leaves us with a mountain of unanswered questions,” says Blochowicz. For example, they want to know how the reversibility of material time is related to the reversibility of the physical laws of nature or how different materials have different internal clocks.

The researchers are eager to explore more, so we can expect more exciting discoveries.

Source: Interesting Engineering

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Physicists discover time can flow both ways in materials

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