Physicists developed a way to see the elusive “Unruh effect” in the laboratory

Physicists developed a way to see the elusive “Unruh effect” in the laboratory

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Illustration: Karl Gustafson

A team of physicists say they have discovered two properties by accelerating matter that they believe can make a never-before-seen type of radiation visible. The recently described properties mean that observation of the radiation – called the Unruh effect – can take place in a laboratory experiment on tabletops.

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 advances, it may be possible to see the Unruh effect in a laboratory experiment.

In the new research, a team of researchers describes two previously unknown aspects of the quantum field that may mean that the Unruh effect can be observed directly. The first is that the effect can be stimulated, which means that the usually weak effect can be enticed to become 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, named after the physicists who first suggested its existence in the 1970s) is a phenomenon predicted under quantum field theory, which states that a unit (be it a particle or a spaceship)) acceleration in a vacuum will glow – even if that glow wouldnot be visibecame any external observer who does not also accelerate 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 the motion,” said Barbara Šoda, a physicist at the University of Waterloo and lead author of the study, in a video interview with Gizmodo. Just as Hawking radiation is emitted by black holes when their gravity draws in particles, the Unruh effect is emitted by objects as they accelerate in space.

There are a couple of reasons why the Unruh effect has never been directly observed. First, the effect requires a ridiculous amount of linear acceleration to occur; to reach a temperature of 1 kelvin, where the accelerating observer would see a glow, the observer must be accelerateding at 100 quintillion meters per second squared. The glow of the Unruh effect is thermal; if an object accelerates faster, the temperature of the glow is gets hotter.

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

“We want to build a dedicated experiment that can unambiguously detect the Unruh effect, and later provide a platform to study various associated aspects,” said Vivishek Sudhir, a physicist at MIT and co-author of the recent work. “The key adjective here is unambiguous: in a particle accelerator, there are really piles of particles that are accelerated, which means that it becomes very difficult to derive the extremely subtle Unruh effect from the various interactions between particles in a pile.”

“In a way,” Sudhir concluded, “we need to make a more accurate 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, such as those imaged by the Event Horizon telescope.

Hawking radiation is predicted to be emitted by black holes, such as those imaged by the Event Horizon telescope.
Picture: 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 detonating a single atom with photons, the team would lift the particle to a higher energy state, and its acceleration-induced transparency would attenuate the particle to any everyday noise that would obscure the presence of the Unruh effect.

By sticking the particle with a laser, “you will increase the probability of seeing the Unruh effect, and the probability increases with the number of photons you have in the field,” Šoda said. “And that number can be huge, depending on how strong your laser is. ” In other words, because scientists could hit a particle with a quadrillion shotons, they increase the probability that the Unruh effect occurs by 15 orders of magnitude.

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

Of course, these findings do not mean much if the Unruh effect cannot be observed directly in laboratory environments – the researchers’ next step. Exactly when that the experiment will be performed, however, remains to be seen.

More: Laboratory Black Hole shows that Stephen Hawking was right, obviously

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