Nearly 50 years ago—before Interstellar, A Brief History of Time, and certainly the Event Horizon Telescope—postdoctoral researcher William Unruh was attempting to explain black holes to a crowd at an Oxford University colloquium. There were no reference points with which to compare an object so dense that light cannot escape its warped gravity. So he devised his own little analogy: Imagine a waterfall, and a little fish accidentally swimming over its edge, too slow to swim against the water’s flow. The fish would forever be stuck at the bottom of the waterfall, never to return to its home. That’s basically what was happening to the light.
A few years later, Unruh was teaching a physics class on the behaviour of fluids, and realised the mathematics of this analogy painted a picture even more similar to a black hole than he’d previously considered. Perhaps analogs, smaller experiments that obey a similar set of physical rules to the black holes, would mimic other fantastical physical effects found in black holes as well.
For decades, Unruh flushed out the ideas in theory only, and by the time he was a professor himself, he and his postdocs realised they could make the idea a reality. They could build a black hole-like object in their lab.
Scientists since the 1980s have designed, and more recently constructed, a slew of black hole analogs that attempt to recreate the weirdness of spacetime predicted by scientists like Albert Einstein and Stephen Hawking. Just this year, a team led by physicist Jeff Steinhauer at the Technion-Israel Institute of Technology discovered the strongest evidence yet of the radiation that Hawking predicted would emanate from black holes’ outer rims using one such analogue, for example. But the comparison between the analogue and the real universe can only go so far.
“The universe isn’t a waterfall flowing over a lip, and a black hole is different in other aspects, but in some aspects they’re very similar,” said Unruh, now a professor at the University of British Columbia. But how far? Is the similarity strong enough that studying the analogues can bolster theories about black hole behaviour? “I would say yes,” Unruh told Gizmodo.
Einstein’s theory of general relativity predicts the existence of black holes, objects whose gravity warps space and time so much that beyond a boundary called the event horizon, light can’t escape. But these boundaries are a strain on physicists’ theories, since they obey both the laws of the tiniest particles, quantum mechanics, and the laws of general relativity and its description of gravity—and as of today, there is no proven seamless way to tie these two theories together. Theorists have devised a few physical phenomena that might occur in these extreme regions, including Hawking radiation, the idea that tiny fluctuations of energy caused by quantum mechanics on the black hole’s surface can cause black holes to emit particles, and the Penrose process, by which spinning black holes might inject energy into nearby particles.
But black hole-observing experiments like the LIGO and Virgo gravitational wave detectors and the Event Horizon Telescope certainly can’t resolve event horizons all the way down to the teeny quantum scale. The Large Hadron Collider, the world’s largest atom smasher, has yet to produce a miniature black hole—yes, physicists are looking for them, but no, mini black holes would not cause any harm and would disintegrate almost immediately. To create them, the Large Hadron Collider would probably need beams containing far more energy than its present capabilities. Analogs using ultra-cold atoms, lasers, or even flowing water could at least verify that certain theorised processes exist in nature in objects that act like black holes.
In the decades after Hawking debuted his theory of black hole radiation, Unruh used mathematics to expand the analogies and develop the concept of “dumb holes,” so called because they would trap sound rather than light. Other physicists built on the theory and devised dumb holes of their own. By the 2000s, Unruh and his team, joined by University of British Columbia civil engineers, were ready to construct dumb holes in the lab.
These first black hole analogs looked a lot more like waterfalls than interstellar points-of-no-return. As a postdoctoral researcher under Unruh at the University of British Columbia, Silkie Weinfurtner oversaw pumps moving water down a flume and across a barrier representing the black hole. The scientists initially hoped to send vibrations, also known as sound, through the water, but the speed of sound in water is 1,500 meters per second—a difficult speed to study in a small lab experiment, Weinfurtner told Gizmodo. Instead, they sent physical waves, like waves you’d see in the ocean, across the barrier.
The team published their first results in 2010. When the waves they produced interacted with the barrier, they created pairs of waves on either side of the barrier, similar to the pairs of particles that Hawking predicted would appear on either side of the event horizon, inside and outside of the black hole. Hawking and Unruh’s other theoretical work together implies that black holes emit a “thermal” or “blackbody” spectrum of wavelengths based solely on their temperatures, which for black holes is directly related to their mass. The waves from Weinfurtner’s flume generated a strikingly similar spectrum, and the team claimed to have measured “stimulated” Hawking emission in their analogue system.
“Black hole radiation is one of the perhaps most peculiar processes,” Weinfurtner told Gizmodo. Thanks to her experiment, “you can reproduce this process in the lab.”
More complex dumb holes followed; Weinfurtner eventually went on to lead her own group, now at the University of Nottingham in the United Kingdom, which devised a black hole analogue from a vortex produced by a draining, rotating fluid. The vortex amplified waves travelling over the liquid that bounced into it, and the experiment became a first observation of a process called superradiance in the lab—an analogy to the Penrose process, where spinning black holes turbocharge the particles in the space around them.
In the past decade or so, scientists have produced a variety of analogs based on similar concepts. Laser light, when it travels through a glass whose properties have been temporarily altered to change the speed light travels through it, also generates Hawking radiation-like light particles. But these analogs still lacked some of the quantum features that would govern Hawking radiation in real black holes. Unruh explained that these systems simply run too hot to observe tiny effects that occur at just a small fraction of a degree above absolute zero, the temperature at which things have no heat.
When Unruh first considered observing these quantum effects in an analogue system, he didn’t think it was possible to access the required cold temperatures. But when he told this to a conference in Santa Fe, New Mexico two decades ago, physicist Mark Raizen told him that nearly cold enough temperatures could already be accessed in laboratory systems called Bose-Einstein condensates. And this year, Steinhauer at the Technion–Israel Institute of Technology may have spotted these quantum effects in one of the most black hole-like analogs yet.
Steinhauer told Gizmodo he’d long been studying Bose-Einstein condensates, ultra-cold collections of atoms that demonstrate quantum mechanical effects on nearly macroscopic scales. He’d heard about the possibility of creating sound wave-based black holes using Bose-Einstein condensates and began working on them as a side project. Once he figured out a way to actually build one of these systems, the “relentless,” “perfectionist” physicist dropped everything and pursued sonic black holes full time, alone.
Steinhauer’s system is similar, in concept, to water flowing over a boundary. A small, tube-shaped region tens of micrometres across contains thousands of rubidium atoms trapped by a laser. An additional laser creates an energy difference—the boundary—that moves through the atoms to act as the waterfall; data is recorded from the point of view of the boundary where the atoms are flowing over it. Imagine how a cliff somehow moving backward through a river would look as much like a waterfall as a river flowing over a stationary cliff.
Sound travels at two different speeds on either side of the boundary, from the boundary’s perspective: quickly on top, since the atoms are denser and slower-moving, and slowly on the bottom, where the atoms are less dense and faster-moving. Phonons, the tiniest units of sound, can travel in either direction on top, since the speed of sound is slower than the speed the atoms are moving. On the other side (the bottom of the waterfall), the atoms are moving faster than a sound wave could travel through them—a phonon could never return to the boundary and would be stuck in the sonic black hole.
Steinhauer presented results in 2014 and 2016, demonstrating hints of the spooky quantum correlations between corresponding phonons. This past summer, now joined by postdoc Juan Ramón Muñoz de Nova and students Katrine Golubkov and Victor I. Kolobov, Steinhauer published the result of the experiment with 21 improvements and ran over 7,400 times. A thermal spectrum, the black hole-like emission whose wavelengths were based solely on the system’s analogue to gravity, appeared in the data, spontaneously, solely as a result of the system setup without any sound wave-generating inputs, the way Hawking envisioned, Steinhauer told Gizmodo. They could even extract the analogue to the “Hawking temperature,” the temperature that would determine the nature of the Hawking radiation in a real black hole.
Today, scientists have systems that mimic the laws of black hole physics in their labs. They hope to continue recreating theorised black hole peculiarities in these systems. But without the ability to confirm that real-life black holes actually act this way, what’s the use?
Perhaps most importantly, these analogs tell theorists that their work isn’t entirely off-base, Unruh said. “This phenomenon has turned out to be so ubiquitous. It occurs in so many varied situations.”
Some of these scientists are just as interested in using black holes to study flowing fluids and cold atoms as they are the reverse. “There’s much to be learned to understand these small fluctuations in fluids and superfluids when they’re driven to the edge,” said Weinfurtner.
But some question how close the analogs get to real black holes. “It is widely debated if the analogs can tell us something about the cosmos,” Daniele Faccio, professor in Quantum Technologies at the University of Glasgow in Scotland, told Gizmodo. Faccio works on his own black hole analogs and agrees that they demonstrate the mathematics behind the theories are correct. “However, my personal opinion is that they cannot tell us if real black holes also emit Hawking radiation or obey the exact same behaviour that we see in the lab.”
These conversations go far beyond just black hole physics. The mathematics behind the theories that appear to govern the distant universe occasionally crop up in highly engineered lab conditions using strange metals or cold atoms. Some scientists have even devised experiments to mimic the Big Bang using Bose-Einstein condensates. One day, perhaps these laboratory experiments will actually reveal a new behaviour that then can be seen in space.
“Right now, a lot of the analogies have been sort of cute, proof-of-principle experiments showing that we can do these procedures in our laboratories,” Gretchen Campbell, adjunct professor and co-director of the Joint Quantum Institute of the University of Maryland, told Gizmodo. “It will be interesting to see if we can ever provide new insight into the universe.”
But black holes aren’t waterfalls. It’s impossible to say whether our technology will ever allow us to understand the nature of real black holes. As for whether the behaviours of fluids, beams of light, and cold atoms in the lab give you faith in Hawking’s calculations because of the similar-looking maths that governs their behaviour, that’s up to you. “I would say yes, it does give me more faith,” Unruh said, “but it’s something that almost every scientist would have to answer for themselves.”