Physicists devised a way to see the elusive ‘Unruh effect’ in the lab

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Illustration: Karl Gustafson

A team of physicists says they 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 lab experiment on a table.

The Unruh effect in nature would theoretically require a ridiculous amount of acceleration to be visibleand because it is only visible from the perspective of the accelerating object in vacuum, it is essentially impossible to see. But thanks to recent developments, it would be possible to witness the Unruh effect in a lab experiment.

In the new study, a team of scientists describes two previously unknown aspects of the quantum field that could mean that the Unruh effect can be observed directly. The first is that the effect can be stimulated, meaning that the normally weak effect can be tempted to become more visible under certain circumstances. The second phenomenon is that a sufficiently excited accelerating atom can become transparent. The team’s research was: published in Physical Review Letters this spring.

The Unruh effect (or the Fulling-Davies-Unruh effect, so named after the physicists who first proposed its existence in the 1970s) is a phenomenon predicted by quantum field theory, which states that an entity (whether a particle or a spaceship) ) accelerating in a vacuum will glow – although that glow would’not to be seenbe able to any outside 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,” Barbara Šoda, a physicist at the University of Waterloo and the study’s lead author, said in a video call. with Gizmodo. Just as Hawking radiation is emitted by black holes as their gravity pulls particles towards them, the Unruh effect is emitted by objects as they accelerate in space.

There are a few reasons why the Unruh effect has never been directly observed. First, the effect requires a ridiculous amount of linear acceleration; to reach a temperature of 1 kelvin, at which the accelerating observer would see a glow, the observer should be acceleratednear 100 trillion meters per second squared. The glow of the Unruh effect is thermal; if an object accelerates faster, the temperature of the glow is will be warmer.

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

“We want to build a specific 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. “Unambiguously, the key adjective here is: In a particle accelerator, it’s really beams of particles that are being accelerated, meaning it becomes very difficult to infer the extremely subtle Unruh effect amid the various interactions between particles in a beam.”

“In a sense,” Sudhir concluded, “we need to more accurately measure the properties of a well-identified singularly accelerated particle, and that’s not what particle accelerators are made for.”

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

Hawking radiation is predicted to be emitted from black holes like these two captured by the Event Horizon Telescope.
Image: EHT collaboration

The essence of their proposed experiment is to stimulate the Unruh effect in a laboratory setting, using an atom as an Unruh effect detector. By bombarding a single atom with photons, the team would elevate the particle to a higher energy state, and the acceleration-induced transparency would muffle the particle to all mundane noise that would obscure the presence of the Unruh effect.

By piercing the particle with a laser, “you increase the chances of seeing the Unruh effect, and the chances increase with the number of photons 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 probability of the Unruh effect occurring by 15 orders of magnitude.

Because the Unruh effect is analogous to Hawking radiation in many ways, the researchers think the two quantum field properties they recently described could potentially be used to boost Hawking radiation and imply the existence of gravity-induced transparency. Since Hawking radiation has never been observed, extracting the Unruh effect could be a step towards better understanding of the theoretical glow around black holes.

Of course, these findings don’t mean much if the Unruh effect can’t be observed directly in a lab setting — the researchers’ next step. exactly when? that experiment will be conducted, however, remains to be seen.

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