Hawking radiation, which is particles predicted by Stephen Hawking that are emitted by black holes, are at the center of huge debate among physicists. Unfortunately, we’ve only recently been able to take a black hole’s picture, let alone study the quantum particles it might be giving off. So for clues on how real black holes behave, scientists are creating stand-ins in a lab and taking their temperature. What could be analogous to a black hole, you ask? Well, black holes curve space-time so much that the fastest thing in the universe, light, cannot escape.
Once it crosses what’s known as the event horizon, it can’t come back. So instead of making something light can’t escape, what if we made a medium another wave cannot escape? Like, sound. A fluid moving at supersonic speeds could do just that. Amazingly in 1981, it was shown that the exact same equations that describe event horizons can also be used to describe sonic horizons in a system like that. The math even predicted vibrations called phonons, which can act as the sonic equivalent of Hawking radiation under the right circumstances. In empty space, virtual particles are popping into existence all the time, and when they meet, they immediately annihilate each other again. When these virtual particles form straddling the cusp of an event horizon, however, one of them gets sucked into the black hole, while the other escapes.
In the same way, quantum units of sound called phonons can arise in fluids. In normal circumstances, these tiny vibrations will meet and cancel each other out, but if one phonon forms where the fluid is moving slower than the speed of sound and it’s opposite forms where the fluid is supersonic, they should be separated and made permanent. It took until 2009 for scientists to actually make one of these sonic black holes. They supercooled rubidium atoms until they formed a Bose-Einstein condensate and got them flowing. By zapping the moving fluid partway along its path with a laser, that section of fluid was accelerated to supersonic speeds, creating a sonic event horizon. Sure enough, the scientists observed entangled phonons that were consistent with sonic
Hawking radiation. Since that experiment scientists have got even more crafty with their black hole analogs, creating multiple sonic horizons that would bounce phonons back and forth, amplifying them and making them easier to detect. And in 2019, they finally measured the temperature of the photons, which could prove Hawking right on a controversial prediction about his radiation. You heard me, the quantum sounds also gave off a bit of heat, about 0.35 billionths of a kelvin. Hawking predicted his radiation would also have a temperature, but those predictions are a huge sticking point for quantum mechanics. If Hawking radiation is thermal, it would be a random spread of energies. That would mean it carries no information about what it once was before falling into the black hole. But quantum mechanics treats information as indestructible and says that the past state of the universe can always be determined if you rewind from the present. Either Hawking is wrong or we need to rethink quantum mechanics.
Unfortunately for quantum mechanics, the fact that these photons have a temperature consistent with Hawking’s calculations points to Hawking being right. That is, of course, assuming that a sonic black hole is a perfect stand-in for a black hole, and even the researchers behind this experiment admit it may very well not be. Until we can take the temperature of a real black hole or come up with a theory of quantum gravity that combines gravity with quantum mechanics, Stephen Hawking’s prediction will remain unresolved. Thanks for watching be sure to subscribe to learn all the weird ways we’re trying to untangle the mysteries of black holes, like how they might be like fuzz balls. Pun absolutely intended. Maren spins a yarn in this video here. The disagreement between Hawking’s predictions and quantum mechanics’ rules about information is known as the information paradox.
That’s all for me, see you next time on Seeker!