For a long time, it was assumed that the gravitational pull of a black hole was total — that nothing, not even light, could escape the clutches of its event horizon. Then, in 1974, an up-and-coming physicist named Stephen Hawking made the bold suggestion that thanks to the peculiarities of quantum physics, black holes should actually emit a tiny amount of electromagnetic radiation (known as Hawking Radiation) and slowly shrink over time. Now, an Israeli physicist named Jeff Steinhauer claims to have proven Hawking's theory — by creating an acoustic black hole in the lab.
Steinhauer's black hole is, of course, just a shadow of the real thing. It builds on the proposals of a physicist named Bill Unruh, who suggested in the '80s that scientists could recreate the physics of a black hole using other substances — for example, a swimmer on the edge of waterfall unable to move fast enough to escape the pull of the water, is acting, in a broad sense, similar to particle of light being sucked into a black hole.
Steinhauer recreated a black hole using sound waves instead of particles of light
Instead of using water or light as its medium, though Steinhauer used sound waves. His experiment involves cooling a cloud of rubidium atoms to just above absolute zero (the lowest temperature theoretically possible), at which point they enter a state known as a Bose-Einstein condensate. Using lasers, Steinhauer creates a lip in the cloud like the edge of a waterfall — with atoms moving slowly on one side, before pouring over the edge faster than the speed of sound.
In the same way that Hawking predicted that particles of light might escape an event horizon, Steinhauer observed phonons (particles representing individual packets of sound) escaping the pull of his rubidium waterfall. The full results of this experiment were published this week in the journal Nature Physics.
Crucially, Steinhauer observed that escaping particles seemed to be entangled with particles pulled over the edge — meaning that their physical properties matched. This so-called quantum entanglement is one of the key attributes of Hawking radiation, as it's thought that the radiation is created when a particle and anti-particle pair pop into existence on the edge of a black hole (this happens all the time in quantum mechanics). The event horizon splits the pair like a knife, causing one particle to be sucked inside — while the other escapes as Hawking radiation.
The results could help us understand the information paradox
Steinhauer's results are important not just because they support Hawking's theory, but because they may also help us understand something called the information paradox. This is the apparent contradiction between a commonly assumed cornerstone of physics (that physical information cannot be completely lost) and our current theories on black holes (if they're slowly shrinking as Hawking suggests, then information is being lost in the process). So, the more proof we have that Hawking radiation exists, the more pressure there is reconcile the information paradox.
As Steinhauer told Business Insider: "The reason people care about black holes and Hawking radiation is not to learn about the black holes themselves so much as to test the new laws of physics [...] Verifying that Hawking radiation really occurs is a good step toward trying to figure out what the new laws of physics are."
However, physicists aren't totally sure that Steinhauer's experiment does confirm the existence of Hawking radiation. After all, a simulation of a black hole — no matter how accurate — is still just a simulation. "This experiment, if all statements hold, is really amazing," theoretical and experimental physicist Silke Weinfurtner told Nature, adding: "It doesn’t prove that Hawking radiation exists around astrophysical black holes." Unruh, the physicist who first proposed this sort of experimental confirmation, echoed these claims, telling the New Scientist that the work needed independent confirmation. "Big results needs solid proof," said Unruh. "In any case, I regard this as a very beautiful experiment, one that people have thought of doing for 10 years now, but he is the first to do so."