Quantum jitter lets heat travel across a vacuum

For the first time, scientists have measured the heat transferred by the quantum effervescence of empty space.

Two tiny, vibrating membranes reached the same temperature despite being separated by a vacuum, physicists report in the Dec. 12 Nature. The result is the first experimental demonstration of a predicted but elusive type of heat transfer.

Normally, a vacuum prevents most types of heat transfer — that helps a vacuum-sealed thermos keep coffee piping hot. But “quantum mechanics gives you a new way for heat to go through” a vacuum, says coauthor King Yan Fong, a physicist who worked on the study while at the University of California, Berkeley. For distances on the scale of nanometers, heat can be transferred through a vacuum via quantum fluctuations, a kind of churning of transient particles and fields that occurs even in empty space (SN: 11/13/16).

Made of gold-coated silicon nitride, the two membranes each measured about 300 micrometers across. The researchers cooled one membrane and heated the other, to a temperature difference of about 25 degrees Celsius. That heat translated into a drumheadlike motion of the membranes — the warmer the membrane, the more vigorously it vibrated. When the membranes were brought within a few hundred nanometers of one another, separated by nothing but empty space, their temperatures equalized, indicating that heat had transferred between them.

“It’s super exciting,” says quantum optics researcher Sofia Ribeiro of Durham University in England, who was not involved with the study. Scientists have been working to develop tiny machines that take advantage of quirks of thermodynamics on quantum scales (SN: 3/8/16). The new study could be fodder for that effort. “This opens … a huge platform that’s going to be very interesting to explore,” she says.

Heat typically travels through three main pathways: conduction, convection and radiation. Conduction describes heat transfer via direct contact of materials, whereas convection is heat transfer arising from motions of gases or liquids, like hot air rising. Those two don’t apply for empty space. But radiation — heat transfer via electromagnetic waves — can occur across a vacuum, as in the sun warming the Earth. Now, the researchers say they’ve experimentally shown another mechanism by which heat can make it across a vacuum, though the effect is significant over only very small distances.

The new variety of heat transfer is a result of the Casimir effect, which describes how quantum fluctuations produce an attractive force between two surfaces separated by a vacuum (SN: 3/2/15). In quantum mechanics, empty space can never be truly empty: Transient electromagnetic waves are constantly blipping into and out of existence. Those waves, although virtual, can exert real forces on materials. In the space between the surfaces, electromagnetic waves can occur only with particular wavelengths. But waves of any size can fit outside, and that excess of exterior waves creates an inward pressure. In the experiment, the two membranes influence one another by way of that force — the jiggling of the hotter object jolts the colder one, for example — equalizing their temperatures.

“It’s a very neat experiment,” says physicist John Pendry of Imperial College London, who was not involved with the research.

This new type of heat transfer could be harnessed to improve performance of nanoscale devices. “Heat is a huge issue in nanotechnology,” Pendry says. The performance of the tiny circuits found in cell phones and other electronics is limited by how fast the device can dissipate heat.

Pendry hopes to see future such experiments geared more toward real-life applications of the effect, though he acknowledges that’s too much to ask for the first demonstration. “That’s being greedy,” he says. “You don’t get all the candy at the same time.”