In a 1972 lecture at the University of Oxford, a young physicist named William Unruh asked the audience to imagine a fish screaming as it plunges over a waterfall. The water falls so fast in this fictitious cascade that it exceeds the speed of sound at a certain point along the way. After the fish tumbles past this point, the water sweeps its screams downward faster than the sound waves can travel up, and the fish can no longer be heard by its friends in the river above.

Something similar happens, Unruh explained, when you fall into a black hole. As you approach one of these super-dense objects, the fabric of space and time becomes increasingly curved—equivalent to strengthening gravity, according to Albert Einstein’s general theory of relativity. At a point of no return known as the “event horizon,” the space-time curvature becomes so steep that signals can no longer climb to the outside world. Within the event horizon, even light is held captive by the black hole’s gravity, rendering black holes invisible.

In the years following Unruh’s talk, black holes—places where general relativity and quantum mechanics, the two pillars of modern physics, meet and crumble in paradox—rose from obscurity to become leitmotifs in the quest for an all-encompassing theory of “quantum gravity.” Meanwhile, Unruh’s acoustic analogy turned out to work even better than he first thought. In a seminal 1981 paper, he showed that black hole event horizons and sonic horizons in systems like his waterfall—which are now referred to as sonic black holes—can be described by identical equations. Considering their “amazing mathematical similarities,” Unruh, a professor at the University of British Columbia in Vancouver, said recently, “you get the feeling that if you really understand one system, that will give you insight into the other.”

Researchers began plumbing the physics of sonic black holes for clues about actual black holes. And in recent years, they’ve started creating sonic black holes in the lab and devising increasingly sophisticated analogue experiments. This past summer, Jeff Steinhauer of the Technion in Haifa, Israel, reported the ultimate find: the detection of the sonic analogue of Hawking radiation, a hypothetical black hole phenomenon predicted by Stephen Hawking in 1974.

Hawking’s prediction that black holes radiate heat and eventually evaporate completely gives rise to the profound “information paradox,” which asks what happens to information about the stuff that fell into them. Hawking’s calculation suggests that this information is lost, essentially leaking out of the universe when it enters a black hole; in that case, the framework of quantum mechanics, which treats information as the universe’s fundamental, indestructible currency, must be abandoned. But if information is preserved, as most physicists believe, then Hawking’s prediction is wrong, and the task for any theory of quantum gravity is to reveal the flaw in his logic. The information paradox “has sharpened the challenge of what we need to confront to understand quantum gravity,” said Raphael Bousso, a theoretical physicist at the University of California, Berkeley, who was Hawking’s protégé.