Ask Ethan #57: How will Black Holes die? (Synopsis)

“Sit down before fact as a little child, be prepared to give up every preconceived notion, follow humbly wherever and to whatever abysses nature leads, or you shall learn nothing.” -T.H. Huxley

It's said that nothing lasts forever, and as far as we can tell, this is true. Every living thing that has ever lived will die; every star that's ever burned fuel will run out of it; galaxies will eventually be destroyed as their stars are thrown out from gravitational interaction, and even black holes will eventually decay.

This last one takes perhaps a bit longer than the others, but destruction is no less assured for them. The question, of course, is how, exactly, does this happen? The popular picture -- of particle/antiparticle pairs created outside the event horizon, with one falling in and the other escaping -- is quite flawed, and not the whole story.

Image credit: Oracle Thinkquest, via Image credit: Oracle Thinkquest, via

What's wrong with it? And what happens instead? Find out on this week's Ask Ethan!


More like this

Dear Ethan,
Everyone (and you;) says that eventually black holes will evaporate due to Hawking radiation. Well what does evaporate mean? Does it mean that the black hole gets smaller and smaller until pffft the last bit of energy comes out and then there's nothing? That seems strange, because at some point I'd expect the mass-energy to fall below the threshold required for a black hole and then suddenly something, (what? particles, light?) would appear in normal space-time. If that happens, could remnants of black holes eventually drift toward each other, combine, and form new black holes? That would slow down the drift toward nothingness;)

This all assumes that the "big rip" you discussed in Ask Ethan #53 doesn't happen, right? What happens if it does? Is there dark energy inside the event horizon? Could it expand space-time faster than c, and thus transport matter out of the event horizon and back into the observable universe? Well, what's left of it, I guess.

The virtual particle analogy has a problem as I see it. If if really was a particle/anitparticle pair I would expect the energy of the resulting photons to be comparable to the rest energy of typical fundamental particles, i.e. we would be getting out gamma-rays, and even after severe red-shifts due to (barely) escaping the gravity well, wouldn't they still have high energy. Let black body photons at those sorts of supercold temperatures would be dominated by long wavelength radio waves. Or is the redshift effect much greater than I imagine). Also I suspect the wavelength of this radiation is not small when compared to a characteristic distance over which the curvature of spacetime is large. Presumably that can be accounted for in the equations?

By Omega Centauri (not verified) on 12 Oct 2014 #permalink

I file this nonsense in the same place as string theory.

By JackWayne (not verified) on 13 Oct 2014 #permalink

So just doing some quick math, the peak in the spectral energy density of Hawking radiation for a 10M_sun black hole is VLF radio. And the peak wavelength only gets longer as the hole gets heavier. Is this fact and the minuscule power the reasons why Hawking radiation in astrophysical blackholes may never be observed?