Nearly 50 Years Ago – Interstellar A Brief History of Time and Certainly the Event Horizon Telescope – PhD William Unruch tried to explain black holes in front of a crowd in a colloquium in Oxford. There were no reference points to compare an object so dense that light could not escape its distorted gravity. So he came up with his own little analogy: Imagine a waterfall and a little fish floating accidentally over its edge, too slow to swim against the water stream. The fish would remain at the bottom of the waterfall forever, never to return home. This is actually what happens to light.
"The universe is not a waterfall passing over the lip, but a black hole is different in other respects, but in some respects they are very similar."
And a few years later, Unruch taught a class in fluid physics. and realizes that mathematics, by this analogy, paints a picture even more like a black hole than he had previously thought. Perhaps analogues, smaller experiments that obey a similar set of black hole physical rules, would mimic other fantastic physical effects found in black holes as well.
For decades, Unruh exhausted ideas only in theory, and by the time he was a professor, he and his post-docs realized that they could make the idea a reality. They could build a black hole-like object in their lab.
Scientists from the 1980s designed and recently constructed an analog of black holes that seek to recreate the strangeness of cosmic time predicted by scientists such as Albert Einstein and Stephen Hawking. Only this year, a team led by physicist Jeff Steinhauer of the Israel Institute of Technology finds the strongest evidence so far of the radiation that Hawking predicts will emerge from the outer rims of black holes, using one such analogue, for example. But the comparison between the analog and the real universe can only go so far.
"The universe is not a waterfall passing over the lip, but the black hole is different in other respects, but in some respects we are very similar," said Unruch, now a professor at the University of British Columbia. But where? Is the resemblance strong enough that the study of analogues can reinforce theories of black hole behavior? "I would say yes," Unru told Gizmodo.
Einstein's theory of general relativity predicts the existence of black holes, objects whose gravity distorts space and time so much that beyond a boundary called event horizon, light cannot escape. But these boundaries are a strain on the theories of physicists, since they obey both the laws of the smallest particles, and quantum mechanics, and the laws of general relativity and its description of gravity – and to date there is no proven hassle-free way for connecting these two theories together. Theorists have come up with several physical phenomena that could occur in these outermost regions, including Hawking radiation, the idea that small fluctuations in energy caused by quantum mechanics on the surface of a black hole can cause black holes to separate particles and the Penrose process, through which turning black holes can inject energy into nearby particles.
But black hole observation experiments such as gravity detectors for gravitational waves LIGO and Virgo and the event horizon telescope certainly cannot resolve event horizons until adolescence. The Big Hadron Collider, the world's largest atomic atomizer, has not yet created a miniature black hole – yes, the physicists are looking for them but no, the mini black holes will not cause damage and will collapse almost immediately, to create them, the Big Hadron Collision will probably need beams that contain far more energy than its current capabilities. Analogues using ultra-cold atoms, lasers, or even running water could at least confirm that certain theoretical processes exist in nature in objects that act as black holes.
In the decades after Hawking debuted his theory of black hole emanation, Unruh used mathematics to extend analogies and to develop the concept of "dumb holes", so-called because they would catch sound, not light. Other physicists have built on the theory and invented their own dull holes. By the 2000s, Unruch and his team, affiliated with the University of British Columbia, civil engineers, were ready to build dull holes in the lab.
These first black hole analogues looked a lot more like waterfalls than interstellar return points. As a PhD student at Unruch at the University of British Columbia, Silky Weinfurtner observed pumps moving down the water and through the barrier representing the black hole. Initially, the scientists hoped to send vibrations, also known as sound, through the water, but the speed of sound in water was 1,500 meters per second – a difficult time to study speed in a small laboratory experiment, Weinfurtner told Gizmodo. Instead, they sent physical waves, such as the waves you would see in the ocean, beyond the barrier.
The team published their first results in 2010. When the waves they produce interact with the barrier, they create pairs of waves on both sides of the barrier, similar to the pairs of particles that Hawking intended to emerge from. both sides of the event horizon, inside and out of the black hole. Other theoretical work by Hawking and Unruch suggests that black holes radiate a "thermal" or "black body" spectrum of wavelengths based solely on their temperatures, which for black holes is directly related to their mass. Weinfurtner's release waves generated a strikingly similar spectrum, and the team claims to have measured "stimulated" Hawking emissions in their analog system.
"The emission of a black hole is one of the most special processes," Weinfurtner tells Gizmodo. Thanks to her experiment, "you can reproduce this process in the laboratory."
More complex dull holes follow; Eventually, Weinfurt continued to lead his own group, now at the University of Nottingham in the United Kingdom, which created an analogue of a black vortex hole made of a draining, rotating fluid. The swirling amplifying waves traveling over the fluid that bounced into it, and the experiment became the first observation of a process called superconversion in the laboratory – an analogy to the Penrose process, in which the rotation of black holes turbocharges particles in the space around them.
The relentless, "perfectionist" physicist cuts everything and chases sonic black holes entirely, alone.
Over the last decade or so, scientists have produced various analogues based on similar concepts. Laser light, when traveling through glass whose properties have been temporarily changed to change the speed light, passes through it, also generates Hawking light particles similar to radiation. But these analogues still lacked some of the quantum characteristics that would drive Hawking radiation into real black holes. Yunro explained that these systems simply become too hot to observe small effects that occur only a small fraction of a degree above absolute zero, a temperature at which things have no heat.
When Unruh first considered observing these quantum effects in an analog system, he did not think that the necessary cold temperatures were possible. But when he told this at a conference in Santa Fe, New Mexico two decades ago, physicist Mark Reisen told him that near-cold temperatures could now be reached in laboratory systems called Bose-Einstein condensates. This year, Steinhauer of the Israel Institute of Technology may have noticed these quantum effects in one of the black hole analogues so far.
Steinhauer tells Gizmodo that he has long been studying the Bose-Einstein condensates, ultra-cold collections of atoms that exhibit quantum mechanical effects on near-macroscopic rocks. He heard about the possibility of creating black holes based on sound waves using Bose-Einstein condensates and began working on them as a side project. After he devised a way to actually build one of these systems, " the inexorable", "the perfectionist " the physicist cut everything and chased the sound black holes entirely, alone.
The Steinhauser system is similar, in concept, to water flowing across a boundary. A small, tube-shaped tens of micrometers contains thousands of rubidium atoms trapped by a laser. An additional laser creates an energy difference – the boundary – that moves through atoms to act as a waterfall; data is recorded from the point of view of the boundary where atoms flow over it. Imagine how a rock that somehow moves back across a river would look more like a waterfall than a river that flows over a stationary rock. Sound travels at two different speeds on both sides of the boundary, from the point of view of the boundary: fast at the top, since atoms are denser and slower moving and slow at the bottom where atoms are less dense and moving at – fast. Phonons, the smallest units of sound, can travel in any direction from above, since the speed of sound is lower than the speed at which the atoms move. On the other hand (the bottom of the waterfall), atoms move faster than sound waves can pass through them – the phonon can never return to the limit and will be stuck in the sound black hole.
Steinhauer presented results in 2014 and 2016, demonstrating hints of ghostly quantum correlations between the respective phonons. This past summer, was now joined by postdoc Juan Ramon Munoz de Nova and students Catherine Golubkov and Victor I. Kolobov, Steinhauer published the result of the experiment with 21 improvements and exceeded 7,400 times. Thermal spectrum, a black hole-like emission whose wavelengths are based solely on the gravity analogue of the system, appeared in the data spontaneously, solely as a result of the tuning of the system, without any inputs generating sound waves, as Hawking predicts, Steinhauer told Gizmodo. They could even extract the analog of "Hawking temperature", the temperature that would determine the nature of Hawking radiation in a true black hole. Scientists today have systems that mimic the laws of black hole physics in their laboratories.
They hope to continue to recreate the theorized features of the black hole in these systems. But without the ability to confirm that black holes in real life actually work this way, what's the use?
Perhaps most importantly, these analogues tell theorists that their work is not entirely unfounded, Uruch said. "This phenomenon has proven so ubiquitous. This happens in so many different situations. "
Some of these scientists are just as interested in using black holes to study flowing liquids and cold atoms as they are to the contrary. "There is a lot to learn about understanding these small fluctuations in fluids and superfluids as they move to the edge," Weinfurt says.
The mathematics behind the theories that seem to govern the distant universe occasionally appear in a highly engineered laboratory. conditions using strange metals or cold atoms.
But some question how close the analogs are to the real black holes. "It is widely discussed if analogues can tell us anything about Space," Daniele Fachio, professor of quantum technology at the University of Glasgow in Scotland, told Gizmodo. Fachio works on his own black hole analogues and agrees that they demonstrate the mathematics behind the theories. "However, my personal opinion is that they cannot tell us whether real black holes also emit Hawking radiation or obey exactly the same behavior we see in the lab."
These conversations go beyond just physics. the black hole. The math behind the theories that seem to govern the distant universe is from time to time cut into highly engineered laboratory conditions using strange metals or cold atoms . Some scientists have even created experiments to simulate the Big Bang using Bose-Einstein condensates. One day, perhaps, these lab experiments will actually reveal new behavior that can then be seen in space. "At the moment, many of the analogies were kind of sweet, proven principles experiments showing that we can do these procedures in our labs," Gretchen Campbell, associate professor and co-director, said to Gizmodo Institute of the University of Maryland. "It will be interesting to see if we can ever get a fresh look at the universe."
But black holes are not waterfalls. It is impossible to say whether technology will ever allow us to understand the nature of true black holes. As to whether the behavior of liquids, light rays, and cold atoms in the lab gives you faith in Hawking's calculations, because of the similar-looking mathematics that governs their behavior, it's up to you. "I would say yes, it gives me more faith," Unruch said, "but it is something that almost every scientist should answer for himself."