Physicists Devised a Way to See Elusive ‘Unruh Effect’ in the Lab

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Illustration: Karl Gustafson

A team of physicists say they’ve discovered two properties of accelerating matter that they believe could make a never-before-seen type of radiation visible. The newly described properties mean that observing the radiation—called the Unruh effect—could happen in a tabletop lab experiment.

The Unruh effect in nature would theoretically require a ridiculous amount of acceleration to be visible, and because it’s only visible from the perspective of the accelerating object in vacuum, it’s essentially impossible to see. But thanks to the recent advances, witnessing the Unruh effect in a lab experiment could be feasible.

In the new research, a team of scientists describe two previously unknown aspects of quantum field that could mean the Unruh effect could be directly observed. The first is that the effect can be stimulated, which means that the ordinarily weak effect could be enticed into becoming more visible under certain conditions. The second phenomenon is that a sufficiently excited accelerating atom can become transparent. The team’s research was published this spring in Physical Review Letters.

The Unruh effect (or the Fulling-Davies-Unruh effect, so-named for the physicists who first proposed its existence in the 1970s) is a phenomenon predicted under quantum field theory, which states that an entity (be it a particle or a spaceship) accelerating in a vacuum will glow—though that glow wouldn’t be visible to any external observer not also accelerating in a vacuum.

“What acceleration-induced transparency means is that it makes the Unruh effect detector transparent to everyday transitions, due to the nature of its motion,” said Barbara Šoda, a physicist at the University of Waterloo and the study’s lead author, in a video call with Gizmodo. Just as Hawking radiation is emitted by black holes as their gravity pulls in particles, the Unruh effect is emitted by objects as they accelerate in space.

There are a couple reasons the Unruh effect has never been observed directly. For one, the effect requires a ludicrous amount of linear acceleration to occur; to reach a temperature of 1 kelvin, at which the accelerating observer would see a glow, the observer would have to be accelerating at 100 quintillion meters per second squared. The glow of the Unruh effect is thermal; if an object is accelerating faster, the temperature of the glow will be warmer.

Previous methods for observing the Unruh effect have been suggested. But this team thinks they have a compelling chance at observing the effect, thanks to their findings about the properties of the quantum field.

“We’d like to build a dedicated experiment that can unambiguously detect the Unruh effect, and later provide a platform for studying various associated aspects,” said Vivishek Sudhir, a physicist at MIT and a co-author of the recent work. “Unambiguous is the key adjective here: in a particle accelerator, it is really bunches of particles that are accelerated, which means that inferring the extremely subtle Unruh effect from amidst the various interactions between particles in a bunch becomes very difficult.”

“In a sense,” Sudhir concluded, “we need to make a more precise measurement of the properties of a well-identified single accelerated particle, which is not what particle accelerators are made for.”

Hawking radiation is predicted to be emitted by black holes, like these two imaged by the Event Horizon Telescope.

Hawking radiation is predicted to be emitted by black holes, like these two imaged by the Event Horizon Telescope.
Image: EHT Collaboration

The essence of their proposed experiment is to stimulate the Unruh effect in a lab setting, using an atom as an Unruh effect detector. By blasting a single atom with photons, the team would lift the particle to a higher energy state, and its acceleration-induced transparency would mute the particle to any everyday noise that would obfuscate the presence of the Unruh effect.

By prodding the particle with a laser, “you’ll increase the probability of seeing the Unruh effect, and the probability is increased by the number of photons that you have in the field,” Šoda said. “And that number can be huge, depending on how strong a laser you have.” In other words, because the researchers could hit a particle with a quadrillion photons, they increase the likelihood of the Unruh effect occurring by 15 orders of magnitude.

Because the Unruh effect is analogous to Hawking radiation in many ways, the researchers believe the two quantum field properties they recently described could possibly be used to stimulate Hawking radiation and imply the existence of gravity-induced transparency. Since Hawking radiation has never been observed, unpacking the Unruh effect could be a step toward better understanding the theorized glow around black holes.

Of course, these findings don’t mean as much if the Unruh effect cannot be directly observed in a laboratory setting—the researchers’ next step. Exactly when that experiment will be conducted, though, remains to be seen.

More: Laboratory Black Hole Shows Stephen Hawking Was Right, Obviously

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