Can you think of a feat more incredible than escaping the tug of a black holes gravity? The very thing that is strong enough that light doesn’t escape it?
I cannot, and that makes the idea of Hawking Radiation truly incredible.
What could escape the gravity of a black hole? Not light, not atoms, not any form of matter. You and I most certainly couldn’t. In fact, we would be Spaghettified if we got too close, much like this star recently observed to be wrapped around a supermassive black hole, trailing off into the abyssal depths of intense gravity.
So what could?
In the early 1970’s Stephen Hawking went public with the idea of Hawking Radiation, something inspired by a visit to Russia where two physicists Yakov Zel’dovich and Alexei Starobinsky convinced him that black holes should emit particles.
But what would these particles be?
Hawking put forward that they were virtual particles, made real by the intense warping of spacetime at the edge and within a black hole.
A way to grasp this is to imagine space is filled with annihilating pairs of virtual particles. One has a negative energy and one has a positive. They appear and immediately destroy each other in normal space. But at the edge of a black hole, right at the horizon, things get funky.
Particles that are detectable, real particles opposed to virtual ones, must have a positive energy to exist, hence the pairs that annihilate each other instantaneously, are virtual. However, due the immense gravity and the strain on space and time within a black hole, if a negative virtual particle were to appear inside a black hole, it would in fact become ‘real’, despite it’s negative energy.
How does this happen?
When a virtual pair pops up on the horizon of a black hole, before it can destroy itself, the gravity of the black hole has a chance of whisking away the negative-energy virtual particle and drawing it into it’s warped spacetime. This also allows for the positive virtual particle to escape away and become real (usually becoming a photon with a very low mass).
If black holes absorb these negative-energy particles, their net energy will decrease, and they will slowly, very slowly it must be said, evaporate.
You may have picked up by now that the title of this post is a misnomer, as nothing truly escapes a black hole once it is taken past the horizon. They absorb everything, it’s just that they happen to absorb these negative-energy virtual particles, which over time will decrease their energy and mass, until they too disappear nothingness.
But this is a couple of hundred billion years off and Hawking Radiation itself is at the moment, a theory that has yet to be confirmed by physical observations. That being said, the theory has been put up against many theoretical tests and has passed all so far.
As expected, no evaporating black hole has been observed due to the fact that it would take almost the entire age of the universe for this to happen. There is an idea that tiny, mini-black holes could form and be observed disappearing (CERN is currently hunting for these within its collisions of particles in the Large Hadron Collider), or that there are primordial black holes, formed at the very start of the inflation of the universe that may be evaporating at the moment, we have yet to find one though.
There have been attempts to create analogues to Hawking Radiation on Earth using fluids and sound waves. A study in 2019 made a sonic black hole horizon out of Bose-Einstein condensate (a gas cooled to almost absolute zero). One side of the sonic horizon moved fast enough to break the sound barrier, but the other was not. Instead of photons, the researchers used phonons and found that the phonon on the slow slide could travel away from the horizon whereas the phonon on the other could not.
This isn’t a perfect analogue as it ignores the quantum effects on the particles, however, it is an important stepping stone to proving Hawking Radiation.