Are black holes black?

I always thought they were, but heard a guy on the radio the other day casting doubt on this. What’s the dope?

While I’m not an astrophysicist, my understanding is that black holes give off radiation (Hawking Radiation) and have accrestion discs. Apparently a reasonably sized black hole doesn’t give off an appreciable amount of radiation, but they can be spotted by the accretion disc if they have one. So any black hole massing a couple solar masses or more (which is most of them, I think) will be pretty much black (radiating at less than the cosmic micro-wave background) but any mass being pulled into it will give off visible radiation.

They’re called “black” holes because once light passes it’s event horizon it can’t escape, meaning it doesn’t reflect any light, meaning we can’t see them.

Part of the confusion is that “black hole” is used in two different ways simultaneously: for the singularity at the center and for the entire space surrounding it up to the event horizon. We can’t see the singularity, it’s too small. (Either point size or planck-length size depending on whose theories you listen to.) We can’t see the space surrounding it either: as said, no light can escape from it. We can see the effects it has on the space just outside of the event horizon, though, and it’s from these effects that we infer that a black hole must be the cause.

That black holes radiate energy and eventually will evaporate, Hawking’s theory, is kind of a red herring, so to speak. The radiation occurs from quantum effects at the event horizon. We only see the half of the radiation that is on the outside, not the part that stays inside the horizon.

If somebody has a new theory about this, kevegan, (and somebody has a new theory about black holes every day of the year), we’re going to need more info to comment on it.

When we detect a black hole, what we’re actually detecting is not the hole itself, but the stuff falling into it. NASA is just now releasing some results from the Chandra X-ray satellite, one of whose primary uses is looking at that infalling stuff, so what you heard on the radio is probably something in connection with that. Things generally release energy when they fall, and there’s an awful lot of energy to be released when you’re falling into a black hole, so black hole accretion disks can be very bright indeed. But once the infalling matter passes the event horizon, you can’t see it any more.

There is also a phenomenon called Hawking radiation, whereby the hole itself gives off radiation, and hence could be said to glow, even without anything falling into it. But for normal-sized black holes (the mass of a star, or larger), this effect is absolutely miniscule: It behaves as though it has a temperature of about a millionth of a degree above absolute zero. Theoretically, a smaller black hole would be hotter and brighter, but none such have ever been observed, and it’s not even agreed whether there are any in the Universe.

But even if you had a very small black hole, such that the Hawking radiation was significant and it was glowing brightly, it’d still be accurate to say that it’s black. Black objects will, in general, give off light when heated, with the color and other properties of the light depending only on the temperature of the object. Many objects, such as stove burners and stars, can be reasonably approximated as black bodies, so that, for instance, when your stove burner is red-hot, it’s at about the same temperature as the surface of a red star. But while “black” is a pretty good approximation for most objects, for a black hole, it’s exact: A black hole is about as black as you can conceivably get. The Hawking radiation from a black hole is an exact blackbody spectrum.

It’s like, how much more black could they be? And the answer is, “None.”

None . . . more black.