His best known achievement, and something that is studied to this day, was
Hawking radiation or black hole evaporation. I only have very crude understanding of it so take this with a sizable pinch of salt.
It starts with the empty space, which isn't really empty because periodically there "pop" into existence virtual particles. Virtual particles are a pair of a particle and it's opposite (antiparticle) particle that could potentially take on any regular particle characteristic. These don't exist for long as they simply annihilate one another other. But if they "popped" into existence so that one particle was just outside of the event horizon of a black hole and its antiparticle just ever so slightly on the inside of the horizon the particle on the inside of the horizon would have a negative energy and fall into the black hole. The one on the outside would radiate away from the black hole with the positive energy which balances out the whole thing. This last particle is what's known as quantum of Hawking radiation.
The reason Hawking radiation is also called black hole evaporation is because the particle that falls into the black hole with the negative energy has an overall effect of decreasing energy of a black hole and consequently its mass.
Although this is a very crude explanation and, as far as I know, Hawking radiation has not been observed in real life, it is based on solid physics and there is little to no doubt of its existence. Similar effect to Hawking radiation has been observed in laboratory settings where analogue black holes have been created. There was an
article earlier this year about a sonic analogue of a black hole created in a lab that was said to have produced Hawking radiation.
A thing that fascinates me about Hawking radiation is that it appears to suggest that the transformation of a virtual particle into a particle with real physical properties radiating away from the black hole would destroy information about its original quantum state violating the
Liouville's theorem, which states that "
the phase-space distribution function is constant along the trajectories of the system." There are some hypotheses as to how this information loss paradox can be solved, including Leonard Susskind and Gerard 't Hooft's holographic principle but the one I find most intriguing is the explanation that in fact
requires information to be lost, proposed by Roger Penrose in a form of conformal cyclic cosmology (CCC) model aimed at explaining the Big Bang.
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