Forbes If the fluid’s speed exceeds the wave’s speed, then waves cannot travel upstream. It’s like being onboard one supersonic plane leading another: you can’t hear the engine noise of the second. Only for black holes, it’s the light that can’t escape, rather than the sound. by Sabine Hossenfelder
'This analogy doesn’t only work for surface waves, but also for sound waves in flowing gases. If you push gas through a narrow channel, thereby increasing its speed so much that it exceeds the speed of sound, you create an acoustic horizon. No sound can cross the acoustic horizon because the gas is flowing too fast.
'Sound-traps of this type have been coined “dumb holes” by Bill Unruh, who pioneered the idea that gravity can be mimicked by fluids in the mid 1980s. Since then, this field of “analogue gravity” has flourished. Physicists have found many other systems where waves travel like in strong gravitational fields, and they devised ways to simulate not only black holes, but also rapidly expanding spaces like that of the early universe. And all this can now be done in the laboratory just by observing how perturbations travel in fluids or gases.
'Physicists would like to know, for example, what happens nearby black holes or close (in time) to the big bang. This is most interesting when the waves also have quantum properties, in which case particles – known as phonons – are associated with the waves. For the purpose of studying quantum behavior, however, water will not suffice.
'Whether the black hole radiation is entangled across the horizon is a pressing question, for the fate of information falling into a black hole depends on it. If the particles are entangled and remain entangled, one of them must eventually fall into the singularity where it gets destroyed. This destruction leaves its partner in an ambiguous state: information has been erased. But such information erasure is forbidden in quantum mechanics, which poses a huge conundrum: physicists don’t know how to make quantum theory and gravity work together.'
'In a new experiment now, Jeff Steinhauer from the Israel Institute of Technology measured the entanglement of the Hawking radiation in an analogue black hole; his results are available on the arxiv.'