Can anything escape from a black hole? Aren’t black holes the ones which swallow everything and spit nothing? Well, Hawking in 1970s discovered that particles can leak out of a black hole making it to evaporate eventually.
The vacuum is not empty but is full of particle-antiparticle pairs spontaneously popping into existence. When such a particle pair pop up in the vicinity of an event horizon (the boundary of a black hole), one particle falls into the black hole while the other escapes. The one that falls into the black hole makes it hot and causes the emission of blackbody radiation known as Hawking radiation.
Hawking radiation depends on the mass of the black hole. Low mass black holes are hotter and can emit radiations hotter than massive ones. These radiations are very faint to be detected.
Sonic Black holes
William Unruh in 1981 showed that equations on event horizon can be applied to sonic horizons too. Sonic horizon is the boundary beyond which phonons, quanta of sound, cannot escape.
Even though it is fairly impossible to create an event horizon, physicist Jeff Steinhauer and his group created a sonic horizon using accelerated Bose-Einstein condensate. When a phonon pair is produced near the sonic horizon, one phonon falls into the region where fluid flow is greater than the speed of sound and the other escapes through the region where fluid flow is less than the speed of sound.
“It’s like if you were trying to swim against a current that
was going faster than you could swim, you’d feel like you were
going forward, but you were really going back. And that’s
analogous to a photon in a black hole trying to get out of the
black hole but being pulled by gravity the wrong way”, said Steinhauer.
The phonon that falls into a sonic black hole heats it up. The results of this experiment show that Hawking radiation emitted from the sonic black hole is blackbody radiation and can be described by a single temperature as Hawking argued.
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