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
“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
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
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
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.”
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