A black hole is a region in space-time where the gravity is so strong that nothing can escape; not even light (which is why they appear black). That is why, when Stephen Hawking found a theoretical argument for the existence of a type of black hole radiation, it took the scientific world by storm. This, now experimentally proven(1), phenomenon flipped our understanding of black holes and lead to paradoxes and problems, which remain unsolved.
To understand Hawking radiation we must first understand that there is no such thing as empty space. Even a vacuum is a bath of bubbling particles, spontaneously appearing and then disappearing. This is what the weird world of quantum mechanics tells us. These particles are created just outside the event horizon (the point which, when passed, it is impossible to escape from the black hole). They are created in the form of a particle-antiparticle pair. Normally these two particles would then collide almost instantaneously and destroy each other emitting only a photon, however, when this happens just outside the event horizon, one particle falls into the black hole while the other escapes. In order for energy to be conserved the particle falling into the black hole must have a perceived negative energy. Since negative energy means negative mass (because of Einstein’s famous mass-energy equivalence principle, shown in the formula E=mc2) it therefore follows that the mass of the black hole decreases. This means that if a black hole does not take in any other type of energy it will shrink until it disappears: that is why Hawking radiation is also known as black hole evaporation.
This leads to many different paradoxes, for example, the information paradox. This paradox explains why Hawking radiation seemingly contradicts an assumed tenet of science: that information is never lost. If you take an intricate painting, like the Mona Lisa, and burn it there is no law that states that the painting couldn’t be put back together again from the burnt products. Within the burnt products is exactly the same information as there was before it was burned. However, when we take a black hole this principle seems to disappear.
This is because the Hawking radiation doesn’t come from the material inside the black hole. So when the matter inside the black hole is lost we know that it can’t have been lost through the Hawking radiation. So when a black hole shrinks it would seem as if the information inside the black hole is just disappearing.
Paradoxes like these are extremely fruitful areas of research for physicists. It is Mother Nature’s way of saying ‘You’re missing out on something big’. I wait with anticipation for where this incredibly interesting line of research will take us.
- Castelvecchi, D. 2016. Artificial black hole creates its own version of Hawking radiation. Nature.
3 thoughts on “What is Hawking radiation and what problems has it brought to physics?”
If virtual particles are created in closely coupled pairs as in photon pairs, then there would not necessarily be any energy gain or loss on either side. On the other hand, if the extreme curvature of space-time causes a shift in the vacuum energy ‘equilibrium point’ will time still ‘flow’ or can it stop altogether for both the particle and antiparticle in each pair…in other words both particle and anti-particle are trapped for some measurable length of time inside the black hole without being annhilated?
This has been bugging me for a long time and maybe you can answer it. Take a star which turned into a small black hole at some point, and which is dissolving thanks to Hawking radiation. As it loses mass, at some point it no longer has an infinite gravity, and it has presumably absorbed lots of other stuff during its life. Could it in theory at some point reignite into a star?
Hi. I think I can answer this. To start with, the process of evaporation of a black hole via Hawking radiation takes a LOT of time. And even if a black hole is completely evaporated, it will just be the normal spacetime. I mean, the spacetime curvature that was infinite in the black hole, will become finite again and there will be no trace left. However, a black hole can never reignite into a star. Because, that would require plasma conditions and enough energy to generate nuclear fusion and fuse hydrogen nuclei into helium and so on (nucleosyntheis). When the black hole has already evaporated, who will provide this energy? Of course, under normal conditions, in nebula, a star can be born in that region but that black hole can’t reignite into a star. Because, once it has completely evaporated, it will cease to exist. Hope this helps!