How Can Black Holes Ever Collide?

[budicca]

For the person falling into the into the black hole, time slows down as they move faster and get closer to the black hole. But even though time slows down, it doesn't mean their SPEED slows down.

I realize that I'm not a genius when it comes to this stuff and I may be behind the times but I seem to recall something about Einstein saying that everything in the universe moves at the speed of light, including us. And the speed of light is finite. The way I understand it is while we are moving through space at any given rate, we are also moving through time. So the faster you move through space the slower you move through time and vice versa. I guess it's proportional. We are always moving through spacetime at the speed of light, it's just a matter of HOW you are moving through spacetime. As gravity warps space and time things are still moving at the speed of light.

Now I have a sort of abstract thought: Given our imperfect definition of speed, which requires movement through space AND time and the speed of light is the fast you can go and time stops at the speed of light (because that's equilibrium), does anything really "exist" at the speed of light. Food for thought I guess.

[terranoid]

I remember an article here that seems to be somewhat related to the topic and could complicate thing even more. It was about a scientist who challenged the idea of an event horizon around singularities. By applying a particular interpretation of Einsteins formulas he could show that it would take an enormous amount of time for an event horizon to form. He pointed out that even if a star in the early time of the universe collapsed into a singularity, the relativistic effects in its vicinity would slow down the formation of a event horizon so much that now there still would basically be none. It seems to be the same problem. If the time in the vicinity of a singularity comes close to a stand still, so does the formation of the event horizon, at least from the perspective of a outside/distant observer. He further theorized that if super massive black holes at the centers of galaxies do actually have an event horizon, they must have formed by the big bang at the beginning of the universe.

[ramparts]

For the person falling into the into the black hole, time slows down as they move faster and get closer to the black hole. But even though time slows down, it doesn't mean their SPEED slows down.

I realize that I'm not a genius when it comes to this stuff and I may be behind the times but I seem to recall something about Einstein saying that everything in the universe moves at the speed of light, including us. And the speed of light is finite. The way I understand it is while we are moving through space at any given rate, we are also moving through time. So the faster you move through space the slower you move through time and vice versa. I guess it's proportional. We are always moving through spacetime at the speed of light, it's just a matter of HOW you are moving through spacetime. As gravity warps space and time things are still moving at the speed of light.

That's exactly right. The sum of your velocities through space and time (technically the Pythagorean sum) is equal to c, the speed of light. Speeding up in space means that your speed in time accordingly drops, to maintain the total of c.

Now I have a sort of abstract thought: Given our imperfect definition of speed, which requires movement through space AND time and the speed of light is the fast you can go and time stops at the speed of light (because that's equilibrium), does anything really "exist" at the speed of light. Food for thought I guess.

Well, it's a bit hard to imagine, but a photon (light particle) doesn't actually experience "time". Of course, we see them move through space, and perceive them as changing location in time, but a hypothetical clock travelling at the speed of light wouldn't measure it. If you think this would be very weird to experience if you travelled at the speed of light, you're exactly right - luckily, the laws of physics pretty well forbid us from doing that (if you're curious, I recommend you read up on the difference between time and "proper time")

[Bill_Wright]

1) failure to yield the right of way
2) perfect timing

My guess is that the vast majority of near collisions are just that and the end result is the less massive singularity gets tossed from the galaxy. The real fireworks would be a collision of their accretion disks. As a Six Sigma Black Belt and 50+ year amateur astronomer I would guess that we have not observed enough collisions to make a good estimate on the results.

-- Bill

[bunnysinghbadshah]

see omega I dont know surely what happens but I think- as black holes are actually the gravity hole hole of a red giant ( extremely big stars, red in colour with an expansion), they can also pull other black holes but htey would not collide........... instead the more powerful, bigger and that one having more mass ( as mass enhances gravitational pull) will devour the other one................. this event may occur explosively or even unnoticed also..................... see we cant know whats happening in other galaxies thus we can only imagine it .........

[ramparts]

see omega I dont know surely what happens but I think- as black holes are actually the gravity hole hole of a red giant ( extremely big stars, red in colour with an expansion), they can also pull other black holes but htey would not collide........... instead the more powerful, bigger and that one having more mass ( as mass enhances gravitational pull) will devour the other one................. this event may occur explosively or even unnoticed also..................... see we cant know whats happening in other galaxies thus we can only imagine it .........

Well, that is more or less what happens, but a larger black hole pulling in a smaller one and "devouring" it is the same thing as black holes colliding

[ashish_india]

There is a full reason that black holes can collide with each other. Black holes have lot of gravitional pull
and when we are comparing two black holes, one will have larger pull than other.
It cant be equal.In that case, the black
hole with large gravitional pull can attract black hole with small pull, and then they may certainly collide

[ttaczak]

Great question, gets to the very root of relativity theory.

Asking what's 'inside' of a black hole isn't the right question. Nobody has come up with the physics for it (yet) since even the basic rules of physics break down at the event horizon because of general relativity --- both space and time get turned inside out, and even unknown 'rules' might come into play. When you're talking about black holes, it's only what we can observe that matters. But it's all an illusion, and so far when you take the 'known rules' into account, everything has been consistent.

There was a great book written by a Russian scientist in the 70's, called 'Frozen Star', talking about the very same thing. The gist of the book was that nothing can EVER fall through the event horizon from an outsider's perspective. Anything falling toward the event horizon just fades from view, time-dilated, so the energy of any EMR gets red-shifted to infinitely low levels.

So getting back to your original question, there's no doubt that black holes CAN collide, however infrequently, but from our POV, we'll just 'see' what is already dictated by our own 'known rules.

[erdrickswincicki]

I would suggest that blackholes perform in the same manner
as whirlpools. Since water uses the gravity formula from the
earth. Space might function the same. So your answer may be
what happens if two whirlpools collide.

[OmegaOm]

Thankyou everyone for all your posts. I guess from what I read here, that there is no answer to this question. It is like jimmyboy said, a paradox of some sort. I however do not believe in paradoxes. I can only assume that Eienstiens theories on relativity are incomplete, just like Newtons were. No knowledge can ever be known %100. We can only get closer and closer to the truth. Just like matter trying to approach the speed of light. It can never quite get there.
Science is suppose to modify theories to fit observations. Observations show that black holes most likely exist, and that they can have galactic masses. If this is true then the time dilation equation needs to be modified.

[EgNagRah]

I think about the holographic universe when I think of black holes. Maybe they are the projectors of the "god particle" being looked for...an underlying method of intersection (in terms of holographic projection).

[armorsmith]

I've asked this very question before and from what I've been able to figure out, the singularity is a myth to make the math easier.

As you approach the speed of light, time appears to slow down and the apparent length of an object shrinks. By the time you hit the event horizon, your apparent length has approached an infinite zero, even the individual atoms have been ripped apart by tidal forces, time has effectively stopped and everything is "plated" into an infinitely thin layer one Planck length inside the event horizon.

If anyone has a better understanding, God knows I'd love to hear it.

[ramparts]

I've asked this very question before and from what I've been able to figure out, the singularity is a myth to make the math easier.

As you approach the speed of light, time appears to slow down and the apparent length of an object shrinks. By the time you hit the event horizon, your apparent length has approached an infinite zero, even the individual atoms have been ripped apart by tidal forces, time has effectively stopped and everything is "plated" into an infinitely thin layer one Planck length inside the event horizon.

If anyone has a better understanding, God knows I'd love to hear it.

Understanding, no

There is one singularity that is a "myth" - the event horizon. It looks like a singularity in the equations, but playing a bit with the equations shows that it's just an issue with our coordinates, kind of like asking what's the longitude at the north pole If we use coordinates to describe the Earth's surface with latitude and longitude, these coordinates work everywhere except the poles. That's a coordinate singularity, too.

So there's the answer to your question (well, comment ) - the event horizon is only a coordinate singularity, and just as nothing funny happens to you at the north pole (except getting a bit chilly), you actually don't notice a thing upon crossing the horizon. What other people outside the black hole see, well, that's a different story.

The "center" of the black hole is a perfectly real singularity, at least as far as general relativity is concerned. But that's certainly an incomplete description. Most of the things you talk about ("spaghettification" and so forth) happen closer to that point.

[derekmcd]

The "center" of the black hole is a perfectly real singularity, at least as far as general relativity is concerned. But that's certainly an incomplete description. Most of the things you talk about ("spaghettification" and so forth) happen closer to that point.

Not necessarily. This might hold true for supermassive black holes. However, spaghetification can begin to occur outside the event horizon of stellar sized black holes. I have no doubt you are familiar with the gravitational inverse square law of 1/r^2. A stellar sized black hole will have an event horizon close enough to its singularity that the gravitational gradient between your head and feet is sufficiently strong enough to make you worthy of Ragu.

[ramparts]

Oh, absolutely. That's why I spoke in sufficiently vague generalities - "most of the things"

[ttaczak]

My point (no pun intended), based on the 'known' laws of relativity, is that the singularity and the event horizon are one and the same. The only reason we 'see' a diameter for a black hole is that space and time are warped, so it's only an apparent size, just an illusion. I still believe that it's a singular point with a mass (as measured from our POV) that can grow infinitely. When it reaches infinity, that will be the 'Big Crunch', and we may or may not end up in the next 'Brahma year', one of the beautiful theories in the Hindu Theory of World Cycles.

[Mee_n_Mac]

My point (no pun intended), based on the 'known' laws of relativity, is that the singularity and the event horizon are one and the same. The only reason we 'see' a diameter for a black hole is that space and time are warped, so it's only an apparent size, just an illusion. I still believe that it's a singular point with a mass (as measured from our POV) that can grow infinitely. When it reaches infinity, that will be the 'Big Crunch', and we may or may not end up in the next 'Brahma year', one of the beautiful theories in the Hindu Theory of World Cycles.

I don't think so. The singularity and the event horizon are 2 different things. The singularity is, by definition, a true point having no radius. While I personally doubt the real world will allow a true singularity it's still going to be mucho smaller in radius that the event horizon (assuming the BH has one).

[joycejohnson]

The derived math, in my opinion, is only as good as our knowledge base. Since no one has experienced a black hole (no one that I'm aware of anyway), then everything we've observed and translated to math are still just speculation. I think that our understanding of blackholes (and for that matter, the universe) is like a bunch of blind men touching different parts of a big elephant.

[ramparts]

My point (no pun intended), based on the 'known' laws of relativity, is that the singularity and the event horizon are one and the same. The only reason we 'see' a diameter for a black hole is that space and time are warped, so it's only an apparent size, just an illusion.

That is untrue. Have a look at the Schwarzschild metric (on Wikipedia or something); you don't need to understand all of the math, just a basic knowledge of algebra should explain it - there is one term in the equation that suggests both singularities. It looks like 1 divided by (1-2GM/r), where G is Newton's constant, M is the mass of the black hole, and r is your distance from the black hole. Clearly, when r is equal to 0, that term will get infinitely large, because you can't divide by 0. r=0 means a distance of 0 from the black hole's center - in other words, the bona fide singularity (if the universe doesn't allow a singularity, which is very possible, it would be because of quantum mechanical reasons about which we don't currently know enough).

But the other way to get a divide-by-0 error is to set 2GM/r equal to 1, so that (1-2GM/r), the denominator of the whole thing, is equal to zero. This suggests a singularity at r=2GM, the so-called Schwarzschild radius of the black hole. That's the location of the event horizon. There are ways to tell that that isn't a *real*, physical singularity but rather, as I said above, only a coordinate singularity. But, the math in answering that question is considerably more complex so I think I'll leave it at that Anyway, the math clearly shows two distinct points for the event horizon and the singularity, so there's nothing suggesting they're the same thing.

The derived math, in my opinion, is only as good as our knowledge base. Since no one has experienced a black hole (no one that I'm aware of anyway), then everything we've observed and translated to math are still just speculation. I think that our understanding of blackholes (and for that matter, the universe) is like a bunch of blind men touching different parts of a big elephant.

That's also untrue - or at least, deeply misleading (misled?). No, we haven't been able to experiment upon a black hole, but they're a uniform prediction of things (like general relativity) that are very well-tested, and it would take a lot of work (nothing we've been able to come up with, incidentally) to make them not be there. In fact, the Schwarzschild metric, which describes black holes, also describes other spherically symmetric systems - so, to good accuracy, it describes the gravitational influence of the Sun, and we've been able to test that very accurately (e.g., the precession of Mercury's perihelion). So we have very good reason to trust the conclusions of GR, including the existence of black holes.

(Incidentally, with a fair number of purported black holes, we've been able to measure, in astrophysical systems, areas so small and containing so much mass, without emitting light, that the laws of physics as we know them wouldn't allow there to be anything but a black hole. No, we haven't experimented on a black hole, and likely never will, but the corroborating evidence is pretty damn impressive ).