Photo credit: University of Nottingham
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 suggests that when you fly through a quantum vacuum with extreme acceleration, the vacuum no longer looks like a vacuum, but rather like a warm bath full of particles. This phenomenon is closely related to Hawking radiation from black holes.
A research team from the University of Nottingham’s Black Hole Laboratory, in collaboration with the University of British Columbia and the Vienna University of Technology, has shown that instead of examining the empty space in which particles suddenly become visible when accelerating, you create a two-dimensional cloud ultracold atoms (Bose-Einstein condensate), in which sound particles, phonons, can be heard by an accelerated observer in the silent phonon vacuum. The sound is not produced by the detector, it hears what is only there because of the acceleration (a detector that was not accelerated would still not hear anything).
The vacuum is full of particles
One of the basic ideas of Albert Einstein’s theory of relativity is that measurement results can depend on the motion status of the observer. How fast does a clock tick? How long is an object? 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 the question of whether or not a particular room is empty? Shouldn’t at least two observers agree on this?
No – because what looks like a perfect vacuum to one observer can be a turbulent swarm of particles and radiation to another. The Unruh effect, discovered in 1976 by William Unruh, states that the vacuum has a temperature for a strongly accelerated observer. This is due to so-called virtual particles, which are also responsible for other important effects, such as B. Hawking radiation, which causes evaporation from black holes.
“It is completely impossible for us today to observe the Unruh effect, as William Unruh described it,” 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 measuring device that is accelerated to almost the speed of light within a microsecond to see even a tiny balance-wheel effect – we can’t do that.” However, there is another way to learn about this strange effect: the use of so-called quantum simulators.
Quantum simulators
“Many laws of quantum physics are universal. It can be shown that they occur in very different systems. The same formulas can be used to explain completely different quantum systems, ”says Jörg Schmiedmayer from the Vienna University of Technology. “This means that you can often learn something important about a particular quantum system by studying another quantum system.”
“Simulating one system with another was particularly useful in understanding black holes, as real black holes are practically inaccessible,” emphasizes Dr. Cisco Gooding from the Black Hole Laboratory. “In contrast to this, analog black holes can easily be created here in the laboratory.”
This also applies to the Unruh effect: if the original version cannot be demonstrated for practical reasons, 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 condensate – small irregularities (sound waves) that propagate in waves. As has now been shown, such irregularities should be detectable with special laser beams. With special tricks, the Bose-Einstein condensate is only 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 over the Bose-Einstein condensate, this corresponds to the viewer moving through the empty space. If you guide the laser beam across the atomic cloud in accelerated motion, you should be able to detect disturbances that cannot be seen in the stationary case – just as an accelerated observer in a vacuum would perceive a heat bath that is not present for the atomic cloud is a stationary observer. ”
“Up until now, the balance wheel effect was an abstract idea,” says Professor Silke Weinfurtner, head of the Black Hole Laboratory at the University of Nottingham. “Many had given up hope of experimental verification. The possibility of including a particle detector in a quantum simulation gives us new insights into theoretical models that are otherwise not experimentally accessible. ”
Preliminary plans to conduct a version of the superfluid helium experiment at the University of Nottingham are underway. “It is possible, but very time-consuming and there are technical hurdles that we have 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.”
Reference: “Interferometric balance detectors for Bose-Einstein condensates” by Cisco Gooding, Steffen Biermann, Sebastian Erne, Jorma Louko, William G. Unruh, Jörg Schmiedmayer and Silke Weinfurtner, November 20, 2020, Physical Examination Letters.
DOI: 10.1103 / PhysRevLett.125.213603
