A new test of water ripples supports the idea of quantum heat in a vacuum

This in-the-lab analog of the Unruh effect matches expectations for accelerated motion in space

empty space

HEAT WAVE  Accelerating at a high rate could make empty space seem hot, according to an idea called the Unruh effect. Scientists may have spotted signs of an analogous phenomenon in ripples of water.


Empty space might feel hot to a traveler zipping through at a rapidly increasing clip — or so some physicists predict. And a new experiment provides a hint that they might be right.

That idea, known as the Unruh effect, seems to be supported by an analogous effect that appears in a tank of rippling water. Patterns in the waves, when analyzed as if seen by an accelerating observer, appear to re-create the expected signature of the effect, researchers report September 7 at arXiv.org. If it holds up to further scrutiny, the result would mark the first time a version of the Unruh effect has been spotted.

It’s a counterintuitive concept: To an observer moving at a constant velocity, a perfect vacuum would be frigidly cold. But someone accelerating through that empty space might work up quite a sweat. “The Unruh effect is basically saying that if you are accelerated enough in the vacuum, you can burn to death,” says theoretical physicist George Matsas of São Paulo State University in Brazil.

Dreamt up in the 1970s, the Unruh effect — named after one of its discoverers, physicist William Unruh, now at the University of British Columbia in Vancouver — hinges on the fact that in quantum physics, empty space is never truly empty. Instead, chaotic swirls of particles and antiparticles are constantly being created and annihilated. For anyone moving at a constant velocity, those particles are transient, or “virtual.” Such virtual particles appear to violate conservation of energy for a brief period of time — a quirk made possible by Heisenberg’s uncertainty principle. But for an accelerating observer, those particles would act like real, persistent particles with well-defined energies, and producing a temperature that increases with greater acceleration.

But measuring the Unruh effect is a daunting task: Observing a temperature of 1 kelvin (1 degree Celsius above absolute zero) would require an acceleration 10 billion billion times that generated by gravity on Earth’s surface. That requirement means the Unruh effect is unconfirmed, leaving some scientists skeptical about whether it is real.

So physicist Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, and colleagues set out to observe an analogous effect that wouldn’t require such extreme acceleration. A “classical” version of the Unruh effect — minus the quantum mechanics — occurs in waves on the surface of water, the researchers report.

By vibrating a channel of water, the scientists created waves reminiscent of the frothing vacuum that begets particles and antiparticles. A video camera recorded the height of the resulting ripples. The researchers traced the waves’ height along the trajectory an accelerating observer would have taken, which produced a pattern analogous to a temperature: When broken up according to their frequencies, the distribution of ripples matched the frequencies of the radiation that would be associated with the temperature predicted by the Unruh effect.

“This is a legitimate analog and the results are quite convincing,” says Matsas, who was not involved with the research.  

Study coauthor Leonhardt declined to comment on the work, as it has yet to be peer-reviewed.

The Unruh effect is closely related to Hawking radiation, extremely faint radiation emitted by a black hole at a temperature determined by its mass.  Scientists have previously created black hole‒like states in water, and thereby demonstrated something akin to Hawking radiation (SN: 12/18/10, p. 28). Both Hawking radiation and the Unruh effect are intriguing to physicists as these phenomena connect quantum mechanics and gravity, between which lies a poorly understood netherworld of physics.

“It’s a great idea to look for the Unruh effect in analogs,” says Silke Weinfurtner of the University of Nottingham in England. But, she notes, the researchers “haven’t really accelerated the observer.” Instead, they looked only at what a hypothetical observer might detect, and deduced that such an observer would feel a pattern of ripples analogous to the temperature predicted due to the Unruh effect.

Likewise, other physicists are waiting for further confirmation before getting too excited. “It is a very interesting paper,” says Germain Rousseaux of the University of Poitiers in France. Still, he says, “this is a first step; I would be cautious with the conclusion.”

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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