Einstein's 100-year-old Theory Bolstered by New Discovery

New research about the transfer of heat suggests that in thermal insulators, heat is conveyed by atomic vibrations and by random hopping of energy from atom to atom. Jill Hemman and Adam Malin/Oak Ridge National Laboratory, US Dept. of Energy

New findings from researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL) have supported a theory first proposed by Albert Einstein in 1911 that explains how heat moves through solids.

In a study published in the journal Science, the scientists examined a class of materials known as thermal insulators, which block the transfer of heat—a fundamental natural process.

"We saw evidence for what Einstein first proposed in 1911—that heat energy hops randomly from atom to atom in thermal insulators," Lucas Lindsay, a materials scientist at ORNL, said in a statement. "The hopping is in addition to the normal heat flow through the collective vibration of atoms."

In materials that conduct heat easily this random energy hopping is not very noticeable, the researchers said. However, it can be seen in those that are less able to transmit heat.

The findings will help scientists to better understand how heat travels in thermal insulators. This could lead to the development of new materials that recover wasted heat or prevent heat transmission. These materials have the potential to drastically reduce energy costs and carbon emissions, the ORNL team said.

To make their discovery, the researchers used advanced vibration-sensing equipment and supercomputers to detect the motion of atoms and simulate how heat moves through a crystal made from the chemical element thallium—a thermal insulator.

They found that the vibration of atoms in the crystal—which are ordered in a lattice—did not contain enough energy to transmit much heat. However, they still observed evidence of heat transfer which was not accounted for in their predictions.

"Our predictions were two times lower than we observed from our experiments. We were initially baffled," Lindsay said. "This led to the observation that another heat transfer mechanism must be at play."

This "other mechanism" is the heat hopping described by Einstein.

At present, this process may only be detectable in highly insulating materials, Lindsay said, but it may also be present in other crystalline solids, which will create new opportunities for managing heat.