‘How heat passes through vacuum decoded’

LOS ANGELES: Researchers have shown that heat energy can leap across ultrasmall distances in complete vacuum due to a weird phenomenon in quantum physics, an advance that may lead to better computer chip designs.
The researchers, including those from the University of California (UC) Berkeley in the US, said while vacuum generally prevents heat transfer, a phenomenon called Casimir interaction at sub-atomic scales can cause heat energy to leap across a few hundred nanometres — about the width of a single human hair — in complete vacuum.
They explained that heat is usually conducted in a solid through the vibrations of atoms or molecules, or so-called phonons, but in a vacuum, they said, there is no physical medium.
The study, published in the journal Nature, revealed that phonons can be transferred across a vacuum by invisible sub-atomic scale fluctuations.
“So, for many years, textbooks told us that phonons cannot travel through a vacuum,” said study co-author Xiang Zhang from UC Berkeley.
Zhang’s and his team placed two gold-coated membranes made of the compound silicon nitride, a few hundred nanometres apart, inside a vacuum chamber.
When they heated up one of the membranes, the other one also warmed up — even though there was nothing connecting the two membranes, and negligible light energy passed between them.
“This discovery of a new mechanism of heat transfer opens up unprecedented opportunities for thermal management at the nanoscale, which is important for high-speed computation and data storage,” said Hao-Kun Li, co-first author of the study, also from UC Berkeley.
They explained that molecular vibrations across a vacuum can be accomplished because, according to quantum mechanics, there is no such thing as truly empty space.
“Even if you have empty space — no matter, no light — quantum mechanics says it cannot be truly empty. There are still some quantum field fluctuations in a vacuum,” said the other first author of the study King Yan Fong from UC Berkeley.
“These fluctuations give rise to a force that connects two objects, which is called the Casimir interaction. So, when one object heats up and starts shaking and oscillating, that motion can actually be transmitted to the other object across the vacuum because of these quantum fluctuations,” Fong explained.
To carry out the study, the researchers had to carefully select the size and design of the silicon nitride membranes, and they could transfer the heat energy only over a few hundred nanometres of vacuum.
They said this distance was far enough that other possible modes of heat transfer such as convection and radiation were negligible.
According to the researchers, even if the Casimir interaction is only significant on very short length scales, it may have profound implications for the design of computer chips, and other nanoscale electronic components where heat dissipation is key. (AGENCIES)