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Credit: University of Nottingham
Researchers have developed a new theory for observing a quantum vacuum that could lead to new knowledge about the behavior of black holes.
The Unruh effect combines quantum physics and the theory of relativity. So far it has not been possible to measure or observe it, but now new research from a team led by the University of Nottingham has shed light on how this could be achieved using particles sound. The team’s research was published today in the journal Physical examination letters.
The Unruh effect suggests that if you fly through a quantum vacuum with extreme acceleration, the vacuum no longer looks like a vacuum: instead, it looks like a hot bath filled with particles. This phenomenon is closely related to the Hawking radiation of black holes.
A research team from the Black Hole Laboratory at the University of Nottingham in collaboration with the University of British Columbia and the Vienna University of Technology has shown that instead of studying the empty space in which particles suddenly become visible during acceleration, you can create a two-dimensional cloud of ultra-cold atoms (Bose-Einstein condensate) in which the sound particles, phonons, become audible to an observer accelerated in the silent void of phonons. The sound is not created by the detector, rather it hears what is there just because of the acceleration (an unaccelerated detector would still hear nothing).
The void is full of particles
One of the basic ideas of Albert Einstein’s theory of relativity is that the results of measurements can depend on the state of motion of the observer. How fast does a clock run? 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 how fast the observer is moving. But what about whether a certain area of space is empty or not? Shouldn’t two observers at least agree on this?
No, because what looks like 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, says that for a strongly accelerated observer, 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 causes black holes to evaporate.
“Directly observing the Unruh effect, as William Unruh described it, is completely impossible for us today,” says Dr Sebastian Erne who came from the University of Nottingham to the Atomic Institute at the University of Vienna Technology as an ESQ Fellow. months ago. “You would need a meter accelerated to almost 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 more about this strange effect – by using so-called quantum simulators.
Quantum simulators
“Many laws of quantum physics are universal. We can show that they occur in very different systems. You can use the same formulas to explain completely different quantum systems, ”explains 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 has been particularly useful in understanding black holes, because real black holes are effectively inaccessible,” says Dr. Cisco Gooding of the Black Hole lab. “In contrast, analog black holes can be easily produced here in the lab.”
This is also true for the Unruh effect: if the original version cannot be demonstrated for practical reasons, then another quantum system can be created and examined to see the effect.
Atomic clouds and laser beams
Just as a particle is a “disturbance” in empty space, there are disturbances in cold Bose-Einstein condensate – small irregularities (sound waves) that propagate in waves. As has now been shown, such irregularities should be detectable with special laser beams. Thanks to special tips, the Bose-Einstein condensate is little 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 condensate, this corresponds to the movement of the observer in empty space. If you guide the laser beam in accelerated motion 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 vacuum would perceive a heat bath that is not. not there for the stationary observer. “
“Until now, the Unruh effect was an abstract idea,” explains Prof. Silke Weinfurtner who heads the Black Hole lab at the University of Nottingham, “Many had given up hope of experimental verification. The possibility of incorporating a particle detector into a quantum simulation will give us new perspectives on theoretical models that would otherwise not be experimentally accessible. “
Preliminary planning is already underway to perform a version of the experiment using superfluid helium at the University of Nottingham. “It is possible, but it takes a long time and we have to overcome technical obstacles”, explains Jörg Schmiedmayer. “But that would be a wonderful way to learn more about an important effect that was previously considered virtually unobservable.”
Reference: “Interferometric Unruh Detectors for Bose-Einstein Condensats” by Cisco Gooding, Steffen Biermann, Sebastian Erne, Jorma Louko, William G. Unruh, Joerg Schmiedmayer and Silke Weinfurtner, November 20, 2020, Physical examination letters.
DOI: 10.1103 / PhysRevLett.125.213603
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