Black hole movement with no time......?

In summary, the conversation discusses the concept of time in relation to black holes and how they are able to move or orbit despite experiencing infinitely slow time at their singularity. It is clarified that time stops for objects falling into the black hole, but for the black hole itself, time does not stop. Gravity is also discussed, with the understanding that it is the geometry of spacetime rather than a force. The misunderstandings about black holes and the structure of the singularity are addressed, with the suggestion to read further resources for a better understanding.
  • #1
TL;DR Summary
If a black hole experiences infinitely slow time (IE time stops) due to their singularity, how are they able to move/orbit?

If supermassives occupy the centre of most/all galaxies. How are the galaxies moving if the black holes can't?
If a black hole experiences infinitely slow time (IE time stops) due to their singularity, how are they able to move/orbit?

If supermassives occupy the centre of most/all galaxies. How are the galaxies moving if the black holes can't?
 
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  • #2
All your questions are based on a fundamental misunderstanding of black holes.
 
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  • #3
Psychonuclear said:
If a black hole experiences infinitely slow time

It doesn't.

Psychonuclear said:
How are the galaxies moving if the black holes can't?

They can.
 
  • #4
Time "appears" to stop for us "observing from the outside." Time for anything falling into it carries on as normal as far as that falling object is concerned. If time actually "stopped" for anything falling in, how would it continue to fall in?
 
  • #5
MikeeMiracle said:
Time "appears" to stop for us "observing from the outside."

This is not correct for the black hole itself. It is only correct (with an appropriate and carefully chosen meaning for "time appears to stop") for objects free-falling into the hole, as viewed from outside.
 
  • #6
PeterDonis said:
This is not correct for the black hole itself. It is only correct (with an appropriate and carefully chosen meaning for "time appears to stop") for objects free-falling into the hole, as viewed from outside.

My point exactly. Of course I am aware of the observation that to an outsider time persists. However, the experience for the observer at the singularity itself, there is insufficient time for anything to occur and zero time for any force to act on it.

Would it be fair to deduce that:
- Gravity still appears to affect the singularity (they still gravitation-ally respond to other masses)
- There is not time to affect the singularity
- Therefore the force of gravity must operate extraneous of time?
 
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  • #7
Psychonuclear said:
Would it be fair to deduce that:
- Gravity still appears to affect the singularity (they still gravitation-ally respond to other masses)
- There is not time to affect the singularity
- Therefore the force of gravity must operate extraneous of time?
It would be better to say that:
1)The singuarity (at the "centre" of a black hole) is the portion of spacetime where the theory of General Relativity (GR) breaks down.
2) There is no "force" of gravity in GR. Gravity is the geometry of spacetime.
3) The event horizon is not a singularity.

Note that GR is the theory of spacetime and its geometry. To ask whether anything in GR operates outside time is meaningless.
 
  • #8
While it's true that there's a singularity - that doesn't mean there's "no time" - a singularity simply means our models break down. Apparently, whatever does happen there doesn't cause it to get ... stuck ... in time, or whatever.

And it wouldn't sense sense anyway. What is "stuck"? What would it mean for a BH to "not be able to move"?
It would require some sort of absolute substrate to which it could be stationary with respect to. And GR tells us there's no such thing.

The fact that the physics of a BH is unknown to us doesn't change the fact that it's still really just a lot of mass in one place. Gravitationally, a BH of 20 Solar masses behaves exactly like any other object of 20 solar masses - it's just more compact.

The sun can move around; so can Canis Majoris - the biggest star known. If you piled a bunch of stars all in one place, that wouldn't change that fact that they're still "just" mass. And mass has inertia. It continues to move based on the inertia of the mass that fell into it, and it is still responsive to the larger gravitational field in which it is immersed.
 
  • #9
Psychonuclear said:
However, the experience for the observer at the singularity itself

There is no observer at the singularity. The singularity is, first, not part of the spacetime; it is a limit that can be approached but never reached. And second, it is a moment in time, not a place in space; observers can't be "at" it.

Psychonuclear said:
Gravity still appears to affect the singularity

Meaningless. See above.

Psychonuclear said:
There is not time to affect the singularity

Meaningless. See above.

Psychonuclear said:
Therefore the force of gravity must operate extraneous of time?

Meaningless; gravity is not a force in GR and "extraneous of time" makes no sense.
 
  • #11
Psychonuclear said:
However, the experience for the observer at the singularity itself, there is insufficient time for anything to occur and zero time for any force to act on it.
You appear to have an incorrect idea of the structure of a black hole. The "time stops" thing happens at the event horizon, not the singularity. "Time stops" is also not accurate - the Schwarzschild time coordinate breaks down at that point, but an observer passing that point notices nothing unusual about time. Indeed, for a sufficiently large black hole, you can fall through the horizon with no ill effect - you would not be torn apart until some time later.

There is a singularity "inside" a black hole, but it isn't a place. It's a moment in time, which can be seen by noticing that ##r## is timelike inside the event horizon. You cannot, therefore, describe an observer at the singularity, any more than you can describe an observer "at" 3pm on Tuesday.

Most of your problems come from taking Schwarzschild coordinates too literally. It's a bit like taking latitude and longitude literally. Imagine approaching the north pole - forward is north, right? So when you get to the pole itself you can't go further forward because there's no more north? No - you've just picked a bad set of coordinates to use for the physical situation you are talking about. Use different ones. Kruskal-Szekeres coordinates allow you to describe horizon crossing in finite time, and you can see that the "time stops" thing is a misunderstanding.
 

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