# Black Holes: How Do They Move?

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• Clueless123
In summary, a black hole can move forward if its interior contents are prevented from moving past the event horizon.
Clueless123
The event horizon is an area where the curvature of space is so great that no particles from within the black hole may travel past that point. For analogy, it is as if a highway ramp curved all the way back to its point of origin, an automobile moving forward would just end up going back where it started due to the contour of the environment. Shouldn't the same problem occur for particles inside the interior of a black hole? Of where the gravity/curvature is even greater than the exterior event horizon such that particles would be directed back to their point of origin. How are black holes able to move through space if their interior constituents (regardless if particle, wave, singularity, etc.) are stuck much like what happens at the event horizon, where they are prevented from moving past a certain point? How may black holes move at all?

Clueless123 said:
How may black holes move at all?
Remember your Galilean relativity (or Google for that term if you aren’t already familiar with it). Just as it is equally valid to say that a car is at rest while the road is moving underneath it as that the road is at rest while the car is moving (and someone watching through a telescope from their armchair on Mars would insist that both are moving relative to Mars)…..

A black hole, or anything else for that matter, moving past you is the exact same physical situation as you moving past that something while it remains at rest.

dextercioby, topsquark, Hornbein and 1 other person
Why wouldn't they be able to move? The event horizon is merely the "point of no return". There's nothing there (not more than momentarily as it passes through), so if you're imagining there being matter "stuck" there then that's wrong. Essentially a moving black hole is one with an event horizon that is a more complicated 4d structure than a stationary one.

I'm struggling a bit to see what your problem is, so if mine and @Nugatory's responses don't help you might want to rephrase.

russ_watters and topsquark
Clueless123 said:
How may black holes move at all?
Here's something on binary black holes.

https://en.m.wikipedia.org/wiki/Binary_black_hole

One reason they can move is that they are not bound by the misconceptions engendered by accounts of them in popular science sources.

phinds and topsquark
Clueless123 said:
The event horizon is an area where the curvature of space is so great that no particles from within the black hole may travel past that point. For analogy, it is as if a highway ramp curved all the way back to its point of origin, an automobile moving forward would just end up going back where it started due to the contour of the environment. Shouldn't the same problem occur for particles inside the interior of a black hole? Of where the gravity/curvature is even greater than the exterior event horizon such that particles would be directed back to their point of origin. How are black holes able to move through space if their interior constituents (regardless if particle, wave, singularity, etc.) are stuck much like what happens at the event horizon, where they are prevented from moving past a certain point? How may black holes move at all?
Your analogy is not very good, but in terms of your analogy you can imagine that the highway with the ramp is in fact on a ferry. You can imagine that the moving black hole is like the moving ferry.

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Grinkle, topsquark, malawi_glenn and 1 other person
My question pertains to the internal constituents of the black hole. Not outside the event horizon. The analogy that I used was only to express the degree of curvature, in this case, the extreme curvature in the environment such that any body would be affected in its motion.

So, imagine an electron is inside a black hole, near the event horizon, that is trying to get out. The extreme curvature in the environment prevents the electron from moving forward and past the event horizon. The electron is stuck inside the black hole due to that extreme curvature (much like that highway ramp turning back onto itself).

My question is, wouldn't the same problem exist for the entire interior constituents of the black hole, regardless if particle/wave/singularity/etc., in that they would be prevented in moving forward due to the extreme curvature of the environment?

So, how may black holes move forward if their interior contents are prevented from doing so?

PeroK
Clueless123 said:
My question pertains to the internal constituents of the black hole. Not outside the event horizon. The analogy that I used was only to express the degree of curvature, in this case, the extreme curvature in the environment such that any body would be affected in its motion.

So, imagine an electron is inside a black hole, near the event horizon, that is trying to get out. The extreme curvature in the environment prevents the electron from moving forward and past the event horizon. The electron is stuck inside the black hole due to that extreme curvature (much like that highway ramp turning back onto itself).

My question is, wouldn't the same problem exist for the entire interior constituents of the black hole, regardless if particle/wave/singularity/etc., in that they would be prevented in moving forward due to the extreme curvature of the environment?

So, how may black holes move forward if their interior contents are prevented from doing so?
All your questions come form misunderstanding black holes, which comes from learning only from pop-sci sources. I don't think it is possible to have a meaningful discussion about this at "B" level. For example you keep talking about the extreme curvature near the horizon. What do you mean by that and why do you think the curvature is extreme?

topsquark
Well, look at this way. Does the interior of a black hole is subject to gravity? Sure, that is what's holding it together. Ok, what gravitational effect is experienced by entities? Curvature. Ok, that means that the interior contents of a black hole, regardless if particle/wave/singularity/etc., is experiencing the gravitational curvature effect. At that level of curvature, which is so extreme that it prevents entities from moving forward, how may the interior contents, in whatever form, not be subject to the curvature effects of its own gravity? Again, somewhat similar to the event horizon problem, but for this scenario, looking from the inside out. So, how may black holes move through space if its interior contents are subject to their own gravitational curvature effect which prevents movement due to such extremes?

PeroK
Clueless123 said:
Well, look at this way. Does the interior of a black hole is subject to gravity? Sure, that is what's holding it together. Ok, what gravitational effect is experienced by entities? Curvature. Ok, that means that the interior contents of a black hole, regardless if particle/wave/singularity/etc., is experiencing the gravitational curvature effect. At that level of curvature, which is so extreme that it prevents entities from moving forward, how may the interior contents, in whatever form, not be subject to the curvature effects of its own gravity? Again, somewhat similar to the event horizon problem, but for this scenario, looking from the inside out. So, how may black holes move through space if its interior contents are subject to their own gravitational curvature effect which prevents movement due to such extremes?
A black hole need not be a particularly massive object. They evolve from collapsing stars larger than 3 Sols, so a reasonable black hole might be 3 solar masses. It's not particularly hard to move around a star that's 3 solar masses, so likewise it's no harder to move a 3 solar mass BH.

Gravity tells mass how to move. It doesn't matter what configuration the mass is in. All that matters is the amount and the distance. There's no variable in the formula for gravitational attraction for how squished the mass is.

So a distant star or galaxy curves the spacetime in the vicinity of the black hole, affecting all components of it simultaneously (again, barring tidal effects). The black hole - all components of it - experience a "force" "pulling" it toward the attracting object.

There is no need for any internal jostling of masses as you see it.

Perhaps consider this (very loose) analogy:

Aerogel is one of the lightest substances. It's mostly empty space.
Diamond has a very rigid crystalline structure.

Pick up and move a 1kg cube of aerogel.
Now pick up and move a 1kg cube of diamond (just pretend).

Was the diamond cube harder to move because it is very rigidly structured? Did each atom bang into each other and resist your efforts to drag it around? No. All that matters is the amount of mass in it, not its configuration. (Like I said, it's a loose analogy.)

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haushofer and topsquark
Clueless123 said:
So, how may black holes move through space...
Seems that you did not pay any attention to the very first reply in this thread, right? The one about understanding Galilean relativity?

It is a mistake to think of something moving "through space" - the best that we can do is say that something is moving relative to something else. So your question ("how can a black hole move through space?") is not well-formed. Instead we should ask "how can a black hole be moving relative to me?" and that's easy: if we're in orbit around the black hole, or free-falling towards it, or moving under the influence of our propulsion system anywhere in the universe, the black hole is moving relative to us.

It is essential that you get comfortable with this concept. Otherwise you will find yourself stuck in late 19th century, when the idea that motion could be anything but relative was becoming increasingly untenable.

Once you are through this knothole, we can take on the question that you're really asking: How do we describe what happens to test particles trying to escape from the black hole if we do the math from a perspective in which the black hole is moving and the observer outside is at rest, instead of the more natural other way around? The actual math won't fit in a B-level thread, but we can explain with simpler examples how it would work.

(You should also be aware that pop-sci sources you have been reading will have pushed some serious misconceptions. These won't matter for the particular question in this thread, but you will want to unlearn them eventually if you want to understand black holes. Probably most surprising:
- There is nothing extreme about the curvature at the event horizon, and it's not the curvature at the event horizon that makes it impossible to escape
- The interior of a black hole is 100% vacuum, no mass anywhere.
- The black hole has no center and the volume inside is either (depending on what English words we choose to attach to the math) either "infinite" or "undefined")

vanhees71, topsquark, nasu and 3 others
Clueless123 said:
So, how may black holes move through space if its interior contents are subject to their own gravitational curvature effect which prevents movement due to such extremes?
First, the black hole does not move through space. Instead, the black hole is (a specific shape of) space, so when the black hole moves it means that the space itself moves, so to speak. (See my analogy with the ferry above.)

Second, have in mind that the theory describing it is called general relativity. Relativity means, among other things, that the motion is relative. Namely, motion of the black hole while the outside observer is at rest is equivalent to motion of the outside observer while the black hole is at rest. I guess you don't have problem to understand how can the outside observer move when the black hole is at rest, so your problem is equivalent to the situation which is not a problem.

Third, close to the black hole horizon, the curvature does not need to be extreme, that's one of the misconceptions acquired from popular literature.

vanhees71, topsquark and PeroK
Clueless123 said:
How are black holes able to move through space
How can a pair of pants have its legs twisted around one another?

A pair of black holes orbitting one another and then merging is analogous to a pair of trouser legs which join at the crotch.

vanhees71 and topsquark
Clueless123 said:
At that level of curvature, which is so extreme that it prevents entities from moving forward,
I don't think "curvature...prevents entities from moving forward" is a remotely accurate description of what's going inside a black hole. There is no spatial direction that one cannot move inside a black hole. The problem is that, seen from inside, the event horizon recedes from you at the speed of light, so you can never reach it once you pass through it.

I agree with @martinbn that I don't think there's really a "B" level way to discuss this. The problems with your idea of black holes seem to be that you are envisaging matter that is trapped inside and somehow unable to move, I suspect you are thinking of the singularity as an infinitely dense point at the center of the hole, and your "analogy" for curvature as being something like a highway off ramp. All of these are wrong. Essentially you need to look at black holes four dimensionally. When you do so, a better analogy for the "shape" of an event horizon is a cylinder. In that analogy a moving black hole is a slanted cylinder and orbiting black holes are two cylinders winding around each other as @jbriggs444 suggested.

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topsquark
Thanks for everyone's help understanding all of this.

[Moderator's note: remainder of post deleted as it refers to a now deleted post.]

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Clueless123 said:
My question pertains to the internal constituents of the black hole. Not outside the event horizon. The analogy that I used was only to express the degree of curvature, in this case, the extreme curvature in the environment such that any body would be affected in its motion.

So, imagine an electron is inside a black hole, near the event horizon, that is trying to get out. The extreme curvature in the environment prevents the electron from moving forward and past the event horizon. The electron is stuck inside the black hole due to that extreme curvature (much like that highway ramp turning back onto itself).

My question is, wouldn't the same problem exist for the entire interior constituents of the black hole, regardless if particle/wave/singularity/etc., in that they would be prevented in moving forward due to the extreme curvature of the environment?

So, how may black holes move forward if their interior contents are prevented from doing so?
You are confusing two things. Nothing inside the black hole can achieve escape velocity. So the electron within the Event Horizon is perpetually within the Event Horizon. But the electron can indeed move within that event Horizon. Any of the mass inside can move.

And the Event Horizon, which is not a thing, but a measurement of the balance point between inside and outside, moves with the mass moving.

It seems to me that you are considering the Event Horizon as a thing, that holds a black hole. It isn't a thing. It is the distance from the center of mass where anything needs to move faster than the speed of light to not fall back. That is just a location, not a thing. It is a special location, but not a thing with special properties. It is the black hole that has the unique properties. And like any physical object it can be moving.

votingmachine said:
Any of the mass inside can move
There is no mass inside a black hole; it's vacuum.

votingmachine said:
It isn't a thing. It is the distance
No, it isn't. The event horizon is an outgoing lightlike surface. Neither "thing" nor "distance" nor "location" is an appropriate description of what it is.

vanhees71 and topsquark
PeterDonis said:
There is no mass inside a black hole; it's vacuum.
This surprised me. On this forum, I am used to seeing statements along the lines of 'we don't know anything about mass distribution inside a black hole, in order to model that we need a quantum theory of gravity'.

Am I conflating unrelated things (your post vs my high level understanding) or am I wrong that we can't know there is only vacuum inside a black hole?

I don't have any specific post to reference; if you tell me I'm wrong I'll see if I can dig up any such posts to see where I am going wrong.

Grinkle said:
On this forum, I am used to seeing statements along the lines of 'we don't know anything about mass distribution inside a black hole
Where have you seen such statements? Can you give some specific examples?

Grinkle said:
I don't have any specific post to reference
Then you need to find some. The statement I made is true of any black hole model in GR, and this is the relativity forum.

I certainly meant inside the Event Horizon. I think two different things are being talked about (or so it seems to me): inside the event horizon vs inside the black hole.

votingmachine said:
inside the event horizon vs inside the black hole.
What does "inside the BH" even MEAN if not "inside the EH"?

vanhees71, topsquark and DaveC426913
Grinkle said:
On this forum, I am used to seeing statements along the lines of 'we don't know anything about mass distribution inside a black hole, in order to model that we need a quantum theory of gravity'.
General relativity models that all matter entering a black hole ends up in the singularity in short order. Indeed, eternal black hole models such as Schwarzschild and Kerr are vacuum everywhere, but even in stellar collapse models you will not find stellar remnants floating around in there unless you follow them in right on their heels.

We do expect GR models to go wrong somewhere, true. But we can't answer questions on how black holes behave based on theories we don't have that may not even model gravity as spacetime curvature. The OP is explicitly using spacetime curvature (or some analogy they fondly imagine illustrates it, anyway) and asking in the relativity forum, so answering on the basis of relativistic models of black holes seems reasonable to me.

vanhees71
votingmachine said:
inside the event horizon vs inside the black hole.
I suppose "at the event horizon" could be regarded as inside the black hole while not being inside the event horizon, but otherwise I don't see a distinction between these terms. And that doesn't seem to me to be an important distinction in this case.

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vanhees71 and DaveC426913
PeterDonis said:
Then you need to find some.

I found discussions around the structure of the singularity itself but I conclude I was in error regarding discussions about mass inside the BH not known to all be concentrated at one point (the singularity). Sorry for the noise.

IroAppe

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