Imagine you have two particle-antiparticle pairs just outside the event horizon: for pair one, the antiparticle falls in and the particle escapes, while for pair two, the particle falls in and the antiparticle escapes. The escaping particle from pair one and antiparticle from pair two interact, producing two photons (which is what you need to conserve both energy and momentum), which can escape as Hawking radiation with real, positive energy.

But that energy isn’t free! Where did it come from? It must be subtracted from the mass of the black hole, something that can happen thanks to the infalling virtual particles from the original “in” part of the “out-in” pair and the “in-out” pair, respectively. So in the end, we have escaping radiation and a lower mass for the black hole!

Image credit: Adam Apollo.

Although the only way to get the exact answer is to do the quantum field theory calculations in strongly curved space, this picture I’ve outlined for you is very, very close to what actually happens. The subtle difference is that the radiation emitted is blackbody and continuous, something you wouldn’t know from the picture I painted, above. What’s also amazing is that the rate of energy loss (encoded in the temperature of the hole) is faster around lower mass black holes, since the curvature of space actually is more intense around event horizons for small black holes!

It would take a whopping ~10^67 years for a black hole the mass of the Sun to evaporate, and around ~10^100 years for the largest black holes in the Universe. That may be far longer than the age of the Universe, but it’s still not forever. Although black holes may live longer than any other object known in the Universe, even they have their limits, and now you know how come!