# B Does time stop at the center of a black hole?

1. Jun 4, 2016

### Benplace

If time does indeed come to a stop at the event horizon, how are we even able to observe it? Wouldn't it cease to exist to someone moving through time the moment time comes to a halt on the event horizon?

2. Jun 4, 2016

### Staff: Mentor

It does not, at least not in the way you probably imagine.
We cannot observe things at or behind the event horizon. That is the point of event horizons.
Does something stop existing if you don't look at it? This gets a bit philosophical, but I don't think it makes sense to say it would stop existing just because you cannot see it any more.

3. Jun 4, 2016

### Orodruin

Staff Emeritus
It does not. The point is that the event horizon is a null surface, which means that there are no observers that are at rest at the event horizon.

4. Jun 4, 2016

### Benplace

Can you explain this a little more, in more laymans terms?

5. Jun 4, 2016

### Benplace

Isn't there white light emanating from the black hole when observed through a telescope? I was thinking I had heard it emanates from the top and bottom of a spiral galaxy in cone shapes.

6. Jun 4, 2016

### rootone

The observer falling through the event horizon does not experience any difference to time passing.
However an observer watching them from outside would see the infaller's watch appearing to move increasingly slower as the horizon is approached.

Quasars are thought to powered by black holes and do emit polar jets, but they are generated outside of the event horizon

7. Jun 4, 2016

### Benplace

Wouldn't time eventually stop once the infaller got close enough?

8. Jun 4, 2016

### Benplace

I had also heard that objects that get close enough to the center of a black hole start behaving in a "Quantum" way? Does that mean objects could exist in two places at the same time?
Sorry for all the questions, I am really interested in physics but I have no formal training whatsoever...

9. Jun 4, 2016

### rootone

We don't know what happens at the center of a black hole.
The math of general relativity produces a 'singularity', which essentially is a nonsense result, an object having infinite density.
It probably does mean quantum effects determine what actually happens, but we don't have a good theoretical model for quantum gravity yet.

10. Jun 4, 2016

### Staff: Mentor

No. There are jets, but those are produced well outside the black hole, from matter that comes close to it and then gets accelerated away by electromagnetic forces.
No, and that concept does not even make sense. Locally, you never notice effects - you can always check your watch and see time ticking it at a rate of 1 second per second.

Note that the event horizon is not the center of a black hole. The event horizon is the "outer" (as much as that makes sense in curved spacetime) part of what is usually called a black hole. Describing the center probably needs a unification of quantum field theory and general relativity, so we don't know how that looks. Not that we can ever observe it...

11. Jun 5, 2016

### sunrah

The issue is Einstein's relativity in which there is no absolute time. Each observer carries with him/her their own senses of duration, e.g. the passage of time. The amount of time measured by an observer depends on their speed and direction relative to the event they are observing. It also depends on their vicinity to sources of gravity like black holes.

Some terminology: EVENT HORIZON: this is an "imaginary" boundary in the region around a black hole. It is imaginary in the sense that it is not made of anything (just spacetime), but it has real physical meaning. Anything crossing this cannot return. Think of it as a point of no return.

The maths works out as follows:
A distant observer watching someone crossing the event horizon will not see them crossing to the other side. Instead they'll seem them stop at the event horizon and stay there forever. As far as the distant observer is concerned it takes an infinite amount of time to cross an event horizon and therefore someone crossing will freeze there forever.
However for the in-falling observer his clock runs quite differently (remember he can also observe himself). In fact, an in-falling observer won't notice anything at all out of the ordinary when crossing an event horizon. He sail through.
This is not paradoxical it's just extreme relativity.

The centre of a black hole does not make sense in general relativity, it's a singularity and we can't say what is really there or what happens there with any kind of certainty. I don't think there really is a singularity inside the black hole, others might disagree.

Your idea of time stopping at the event horizon is not really wrong. A distant observer watching a flashing light approaching the event horizon will see the flashes becoming longer and longer until it reaches the event horizon where it will no longer flash, just stay lit or unlit depending if the light arrives on a flash or not. The distant observer might think that time has stopped for the flashing light. But if he knows something about relativity he'll probably know whats going on.

12. Jun 5, 2016

### Staff: Mentor

Ok so far.

This is not what the distant observer will see. The distant observer will just see flashes spaced farther and farther apart, and the flashes he sees will have been emitted closer and closer to the horizon, but never at it. Once the distant observer sees the last flash emitted before the light reaches the horizon, he sees nothing more at all. A flash emitted right on the horizon stays at the horizon forever; it never gets back out to the distant observer, so that observer never sees it. Flashes emitted inside the horizon eventually hit the singularity.

13. Jun 5, 2016

### sunrah

What is the thinking on the existence of the singularity? It seems completely unphysical, but can't be transformed away like a coordinate singularity. Do we hope that it will disappear with quantum gravity? Thanks

14. Jun 5, 2016

### Staff: Mentor

We expect that it will.

There's an analogy with classical gravitation.
Newton's $F=Gm_1m_2/r^2$ has a singularity at $r=0$ that can't be transformed away. However this singularity doesn't bother us; it's just nature's way of telling us that we cannot always treat gravitating masses as point particles at $r=0$ because at sufficiently small distance scales ($r\lt{6400}$ kilometers or so for the earth) the point-source approximation breaks down.

Likewise, the singularity that appears at $r=0$ in the Schwarzschild solution is (almost certainly) telling us that quantum gravitational effects that are ignored by that solution cannot be ignored in the region sufficiently close to $r=0$, and that when these effects are included there will be no singularity.

15. Jun 5, 2016

### pervect

Staff Emeritus
We can't say anything definite on what really happens at the singularity, except that in Einstein's GR, it's singular. For instance, there are already slightly tweaked variants of GR, such as Einstein-Cartan theory, that make different predictions about what happens at the singularity. While Einstein Cartan theory does suggest one way the singularity COULD be eliminated, it's far from clear if it's actually correct or not.

See for instance the current Wiki on Einstein-Cartan theory,
https://en.wikipedia.org/w/index.php?title=Einstein–Cartan_theory&oldid=713266281

So Einstein-Cartan theory does solve some of the issues with quantum gravity, but not all. It's an active theory of gravity, it's compatible with experiment, it doesn't have the singularity at the center of black holes that GR does, and we really have no idea if the theory is right or not, as the experimental differences from General Relativity are so tiny.

16. Jun 6, 2016

### sunrah

do you mean torsion? this would mean somehow measuring predictions relating to particle spin?
There seem to be so many gravity theories compared to anything else.

17. Jun 7, 2016

### pervect

Staff Emeritus
Yes, allowing torsion to be non-zero is the big difference between Einstein-Cartan theory and GR.