In the experiment, Steinhauer studied how particles behave on the edge of his “black hole” – the equivalent of an event horizon, which is essentially the “point of no return” in spacetime, beyond which events can not affect an outside observer of an analogue black hole. But an acoustical model of a black hole – no matter how well-designed – simply isn’t a black hole.
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To measure Hawking radiation, he pinged the fluid with a short laser pulse.
Proposed by physicist Stephen Hawking back in 1974, Hawking radiation describes the small amounts of high-energy radiation that could theoretically escape the gravitational pull of a black hole.
“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”, Steinhauer told Business Insider.
However, confirming Hawking’s theory has proven extremely hard given that the radiation he was describing is incredibly weak, leading scientists to believe that it was essentially undetectable.
Steinhauer says that black-hole analogues might help to solve some of the dilemmas that the phenomenon poses for other theories, including one called the black-hole information paradox, and perhaps point the way to uniting quantum mechanics with a theory of gravity. And other scientists have expressed both admiration for and caution toward the results.
Pointing out another inherent shortcoming of an analogue black hole – which does not evaporate – Leonard Susskind, a theoretical physicist at Stanford University in California, said: “I don’t believe it will illuminate the so-called information paradox”.
He has been working in his hand-assembled lab, complete with lasers and dozens of mirrors, lenses, and magnetic coils to simulate a black hole, since 2009.
Steinhauer observed that a particle inside the analogue black hole is entangled with its counterpart outside, just as Hawking originally predicted for a real black hole. What’s more, they were even entangled, meaning that, when one phonon fell into the black hole, the one on the outside still retained all the information of that particle.
His starting point was that the randomness of quantum theory ruled out the existence of true nothingness.
The energy fluctuations that give rise to entangled photons in space did so in Steinhauer’s Bose-Einstein condensate too, but as entangled phonons.
Under normal circumstances, they are created as particle-antiparticle pairs which destroy each other in a sort of quantum physics duel of opposites. Was he right about black holes?
This Hawking radiation, while very weak, would ultimately mean that black holes very slowly wither and die, losing energy to this process.
Earlier this year, Hawking published a “solution” to the information paradox, which hasn’t convinced everyone in the field, but basically suggests that black holes might actually have a halo of “soft hair” surrounding them, which are capable of storing information, so it’s not lost altogether.
Theodore Jacobson, a theoretical physicist at the University of Maryland, is hesitant to celebrate Steinhauer’s detection of quantum Hawking radiation. He imagined a medium that experienced accelerated motion, such as water approaching a waterfall.
It was packets of sound waves, called “phonons”, that played the part of entangled particles on the fringe of a black hole.
He found that pairs of phonons – particles of sound – appear spontaneously in the void at the event horizon.
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They are made by cooling atoms of the element rubidium to just above absolute zero, where they form an exotic, fluid form of matter. They did so in a Bose Einstein condensate (BEC), a special, quantum state of matter in which many very cold atoms behave like a single atom. The partners should separate from each other, with one partner on the supersonic side of the horizon and the other forming Hawking radiation. Atoms that don’t make it over the step potential move at subsonic speeds, and are interpreted as being outside of the event horizon. These latest results might not be definitive proof that Hawking radiation seeps out from black holes, but we’ll have to just keep chipping away at the secrets of the cosmos.
