Researchers have developed a new theory for observing a quantum vacuum that could lead to new insights

Researchers have developed a new theory for observing a quantum vacuum that could lead to new insights into the behavior of black holes.

The Unruh effect combines quantum physics and the theory of relativity. It has not been possible to measure or observe it so far, but now new research by a team led by the University of Nottingham has shed light on how this could be achieved using sound particles. The team’s research was published today in the journal Physical Review Letters.

The Unruh effect suggests that if you fly through a quantum vacuum with extreme acceleration, the vacuum no longer looks like a vacuum – rather, it looks like a hot bath full of particles. This phenomenon is closely related to Hawking radiation from black holes.

A research team from the Black Hole Laboratory of the University of Nottingham in collaboration with the University of British Columbia and the University of Technology Vienna has shown that instead of studying empty space where particles suddenly become visible during acceleration , it is possible to create a two-dimensional cloud of ultra-cold atoms (Bose-Einstein condensate) in which the sound particles, the phonons, become audible to an accelerated observer in the silent phonon vacuum. The sound is not created by the detector, rather it is listening to what’s there just because of the acceleration (an unaccelerated detector wouldn’t hear anything anyway).

The vacuum is full of particles

One of the basic ideas of Albert Einstein’s theory of relativity is: the measurement results can depend on the state of motion of the observer. How fast does a clock tick? How long does an object last? What is the wavelength of a ray of light? There is no universal answer to this, the result is relative: it depends on the speed with which the observer moves. But what about the question of whether a certain area of ​​space is empty or not? Shouldn’t two observers at least agree on this?

No, because what appears to be a perfect vacuum to one observer can be a turbulent swarm of particles and radiation to the other. The Unruh effect, discovered in 1976 by William Unruh, states that for a strongly accelerated observer the vacuum has a temperature. This is due to so-called virtual particles, which are also responsible for other important effects, such as Hawking radiation, which evaporates black holes.

“Observing the Unruh effect directly, as William Unruh described it, is completely impossible for us today,” explains Dr Sebastian Erne, who came from the University of Nottingham to the Atomic Institute of the Vienna University of Technology as an ESQ Fellow. a few months ago. “You would need a metering device accelerated almost to the speed of light in a microsecond to see even a small Unruh effect, we can’t do that.” However, there is another way to learn about this strange effect: to use so-called quantum simulators.

Quantum simulators

“Many laws of quantum physics are universal. They can be shown to occur in very different systems. You can use the same formulas to explain completely different quantum systems, “says Jörg Schmiedmayer of the Vienna University of Technology. This means that you can often learn something important about a particular quantum system by studying a different quantum system.”

“Simulating one system with another was particularly useful for understanding black holes, since real black holes are actually inaccessible,” points out Dr. Cisco Gooding of the Black Hole lab. “Conversely, analog black holes can easily be produced right here in the lab.”

This also applies to the Unruh effect: if the original version cannot be proven for practical reasons, then another quantum system can be created and examined to see the effect there.

Atomic clouds and laser beams

Just as a particle is a “disturbance” in empty space, there are disturbances in the cold Bose-Einstein condensation – small irregularities (sound waves) that spread out in waves. As has now been shown, such irregularities should be detectable with special laser beams. Using special precautions, the Bose-Einstein condensate is minimally disturbed by the measurement, despite the interaction with the laser light.

Jörg Schmiedmayer explains: “If you move the laser beam, so that the point of illumination moves on the Bose-Einstein condensation, this corresponds to the observer moving through empty space. If you guide the accelerated moving laser beam over the atomic cloud, you should be able to detect disturbances that are not seen in the stationary case, just as an accelerated observer in a vacuum would perceive a heat bath that is not there for the stationary observer. “

‘Until now, the Unruh effect was an abstract idea,’ says Professor Silke Weinfurtner who heads the Black Hole laboratory at the University of Nottingham. ‘Many had given up hope of experimental verification. The possibility of incorporating a particle detector in a quantum simulation will give us new insights into theoretical models that would otherwise not be accessible experimentally. “

Preliminary planning is already underway to run a version of the experiment using superfluid helium at the University of Nottingham. “It is possible, but it takes a long time and there are technical obstacles to overcome,” explains Jörg Schmiedmayer. “But it would be a wonderful way to learn about an important effect that was previously thought to be virtually undetectable.”