Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: black hole physics

Learn more physics!

Most recent answer: 08/13/2013
Physicists say that black holes break all physical laws at their center (singularity). However I think such a statement is a misunderstanding of Relativity. Relativity states that nothing can travel faster than c. Inside the event horizon, light and everything else is trapped inside the black hole forever since the escape velocity is greater than c once inside the black hole. So it makes perfect sense to say that once something crosses the event horizon, it left our universe for good. Since it is no longer in our universe, different physical laws must be at work inside the singularity. But we can never study those physical laws because no information on these can come back out of the black hole. So a singularity doesn't break all the laws but is rather subject to some other laws, perhaps to the same laws that existed @ t=0...
- Anonymous
Rather than address your statement directly, I'll update the description of black hole physics. It's been known from the start that the relativistic time-dilation effects prevent a black hole from quite ever full forming, although it can get very close. The quantum treatment of black holes showed that in an empty enough space they do not last forever but rather evaporate, very slowly. These ideas have been integrated in modern descriptions. The matter falling toward a black hole spreads out in a sheet very close to the horizon. The best current guess, for what it's worth, is that the sheet consists of random-looking states of hypothetical strings. On a very long time scale the sheet evaporates, without ever quite forming a black hole. From our perspective outside, the basic laws are never violated.

Of course, from the point of view of whoever is falling in, after an ordinary finite length of time they cross the horizon. As they do, they see the last of our outside world,  looking into the "future" of a narrower and narrower cone of sight. What happens to them afterwards from their point of view? You're right to be asking questions. As for how we can fit their point of view and ours into some unified concept of the world, we leave that headache as an exercise for the reader.

Mike W.

(published on 06/13/2011)

Follow-Up #1: can gravity escape a black hole?

Since gravity travels at the speed of light how can it escape a black hole and affect other objects?
- Don (age 60)

That's a nice question. If somehow a black hole could appear without any gravitational bending of spacetime associated with it, we'd have trouble explaining what could happen. However, that's not what happens, or what can happen. As the black hole gradually forms, the gravitational effects from that mass already extend out throughout space. In other words, gravity doesn't have to escape the black hole and travel elsewhere, it already is elsewhere before the black hole forms. Incidentally, as we mention above, it actually takes the black hole infinitely long to fully form, so we actually never quite get to that stage.

Mike W.

"Our Theory of Gravitation is as good as perfect.... Nothing can act but where it is...; only, WHERE is it?"   Thomas Carlyle. 1831

(published on 08/13/2013)

Follow-up on this answer.