Hacker News Reader

Comment by metalliqaz | Hacker News Reader

Please correct my understanding as I am not a scientist, but why do you assert that Hawking radiation "escapes" or "comes out of" a black hole? Nothing crosses the event horizon. Isn't it more correct to say that Hawking radiation is produced by the extreme warping of the spacetime very near the event horizon, and the black hole shrinks to conserve the energy? Black holes can also shrink as gravitational waves are produced, but we don't say that the waves "escape" the black hole.

View on HN

Strangely, it is _not_ the instrinsic curvature of spacetime that produces Hawking radiation. There's a variation on Hawking radiation -- called Unruh radiation -- experienced by accelerated observers in flat spacetime. Any two observers will agree on the value of a field (e.g. the electric field) at a point, but an accelerated observer will experience empty spacetime as a _thermal bath of particles_, with a temperature proportional to the acceleration. Relatively accelerated observers don't agree on what the vacuum is.

My understanding is that Hawking radiation is where:

1. a particle/anti-particle pair is created at the event horizon;

2. the particle is on the outside edge of the event horizon, so "escapes" the black hole;

3. the anti-particle is on the inside edge of the horizon, so decreases the size of the black hole due to particle/anti-particle annihilation (with a corresponding particle on the inside of the black hole).

This is often used as an explanation in pop-sci materials, but it isn't particularly accurate, IIRC.

For example, Hawking radiation is always photons, with the wavelength corresponding to the size of the black hole. That isn't explained purely by virtual particles.

Also, for that explanation to work, it would always have to be an anti-matter particle that is selected to cross the event horizon, and I don't see how that would be the case.

Minor quibbles: Hawking radiation has a blackbody spectrum, so we get a variety of wavelengths, with a peak determined by the size of the hole. The outgoing particles should eventually include any kind of particle that can be created in the Standard Model, not just photons and gravitons.

Reply on news.ycombinator.com