Theoretical physics is full of weird and wonderful concepts: wormholes, quantum foam and multiverses, just to name a few. The problem is that although such things easily emerge from theorists' equations, they are practically impossible to create and test in a laboratory setting. But for one such “untestable” theory, an experimental setup might be just on the horizon.
Researchers at the Massachusetts Institute of Technology and the University of Waterloo in Ontario say they've found a way to test the Unruh effect, a bizarre phenomenon predicted to arise from objects moving through empty space. If scientists are able to observe the effect, the feat could confirm some long-held assumptions about the physics of black holes. Their proposal was published in Physical Review Letters in April 2022.
If you could observe the Unruh effect in person, it might look a bit like jumping to hyperspace in the Millennium Falcon—a sudden rush of light bathing your view of an otherwise black void. As an object accelerates in a vacuum, it becomes swaddled in a warm cloak of glowing particles. The faster the acceleration, the warmer the glow. “That's enormously strange” because a vacuum is supposed to be empty by definition, explains quantum physicist Vivishek Sudhir of M.I.T., one of the study's co-authors. “You know, where did this come from?”
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Where it comes from has to do with the fact that so-called empty space is not exactly empty at all but rather suffused by overlapping energetic quantum fields. Fluctuations in these fields can give rise to photons, electrons, and other particles and can be sparked by an accelerating body. In essence, an object speeding through a field-soaked vacuum picks up a fraction of the fields' energy, which is subsequently reemitted as Unruh radiation.
The effect takes its name from the theoretical physicist William G. Unruh, who described his eponymous phenomenon in 1976. But two other researchers—mathematician Stephen A. Fulling and physicist Paul Davies—worked out the formula independently within three years of Unruh (in 1973 and 1975, respectively).
“I remember it vividly,” says Davies, who is now a Regents Professor at Arizona State University. “I did the calculations sitting at my wife's dressing table because I didn't have a desk or an office.”
A year later Davies met Unruh at a conference where Unruh was giving a lecture about his recent breakthrough. Davies was surprised to hear Unruh describe a phenomenon very similar to what had emerged from his own dressing-table calculations. “And so we got together in the bar afterward,” Davies recalls. The two quickly struck up a collaboration that continued for several years.
Davies, Fulling and Unruh all approached their work from a purely theoretical standpoint; they never expected anyone to design a real-world experiment around it. As technology advances, however, ideas that were once relegated to the world of theory, such as gravitational waves and the Higgs boson, can come within reach of actual observation. And observing the Unruh effect, it turns out, could help cement another far-out physics concept.
“The reason people are working on the Unruh effect is not because they think that accelerated observers are so important,” says Christoph Adami, a professor of physics, astronomy, molecular genetics and microbiology at Michigan State University, who was not involved in the research. “They are working on this because of the direct link to black hole physics.”
Basically, the Unruh effect is the flip side of a far more famous physics phenomenon: Hawking radiation, named for the late physicist Stephen Hawking, who theorized that an almost imperceptible halo of light should leak from black holes as they slowly evaporate.
In the case of Hawking radiation, that warm, fuzzy effect is essentially a result of particles being pulled into a black hole by gravity. But for the Unruh effect, it's a matter of acceleration—which is, per Einstein's equivalence principle, gravity's mathematical equal.
Imagine you are standing in an elevator. With a jolt, the car rushes up to the next floor, and for a moment you feel yourself pulled toward the floor. From your viewpoint, “that is essentially indistinguishable from Earth's gravity suddenly being turned up,” Sudhir says.
The same holds true, he says, from a math perspective. “It's as simple as that: there is an equivalence between gravity and acceleration,” Sudhir adds.
Despite its theoretical prominence, scientists have yet to observe the Unruh effect. (They haven't managed to see Hawking radiation, either.) That's because the Unruh effect has long been considered extraordinarily difficult to test experimentally. Under most circumstances, researchers would need to subject an object to ludicrous accelerations—upward of 25 quintillion times the force of Earth's gravity—to produce a measurable emission. Alternatively, more accessible accelerations might be used—but in that case, the probability of generating a detectable effect would be so low that such an experiment would need to run continuously for billions of years to yield a useful result. Sudhir and his co-authors, however, believe that they have found a loophole.
By grabbing hold of a single electron in a vacuum with a magnetic field, then accelerating it through a carefully configured bath of photons, the researchers realized that they could “stimulate” the particle, artificially bumping it up to a higher energy state. This added energy multiplies the effect of acceleration, which means that, using the electron itself as a sensor, researchers could pick out Unruh radiation surrounding the particle without having to apply so many g-forces (or having to wait for eons).
Unfortunately, an energy-boosting photon bath also adds background “noise” by amplifying other quantum-field effects in the vacuum. “That's exactly what we don't want to happen,” Sudhir says. But by carefully controlling the trajectory of the electron, the experimenters should be able to nullify this potential interference—a process Sudhir likens to throwing an invisibility cloak over the particle.
And unlike the kit required for most other cutting-edge particle physics experiments, such as the giant superconducting magnets and sprawling beamlines of the Large Hadron Collider at CERN near Geneva, the researchers say their Unruh effect simulation could be set up in most university labs. “It doesn't have to be some huge experiment,” says paper co-author Barbara Šoda, a postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Ontario. In fact, Sudhir and his Ph.D. students began work on designs for a version meant to actually be built.
Adami sees the new research as an elegant synthesis of several different disciplines, including classical physics, atomic physics and quantum-field theory. “I think this paper is correct,” he says. But much like the Unruh effect itself, “to some extent, it's clear that this calculation has been done before.”
For Davies, the potential to test the effect could open up exciting new doors for both theoretical and applied physics, further validating nigh-unobservable phenomena predicted by theorists while expanding the tool kit experimentalists can use to interrogate nature. “The thing about physics that makes it such a successful discipline is that experiment and theory very much go hand in hand,” he says. “The two are in lockstep.” Testing the Unruh effect promises to be a pinnacle achievement for both.