# How Can Black Holes Ever Collide?

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#### OmegaOm

##### Guest
If I am outside a black-hole and my partner is nearing the event horizon, my partner would be seeing me speed up in time and inversely I would see my partner slow down in time, to the point where my partner seems to freeze in time and never reaching the singularity.
This fact of relativity I heard many times over.
If this is true, how can anything ever get gobbled up by a black-hole? How can Galactic nuclei black holes grow to billions of solar masses if nothing can ever reach the black-hole in our reference frame?

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#### Jimmyboy

##### Guest
very good question omega!!!

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#### Jimmyboy

##### Guest
but they do suggest things speed up near a black hole at the same time, thats how quasars are made, and hawkin radiation.

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#### Jimmyboy

##### Guest
this is only a vague guess, but gravity can slow time and also speed can slow time, so maybe a rotating black hole cancels out the time dilation some how.. im no professor so dont know for sure

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#### OmegaOm

##### Guest
Thanks for commenting Jimmyboy, but I do not think the speed and the gravity around a black hole would cancel out. If anything the effects should multiply. Slow time with gravity and slow time with speed.
This question has been puzzling me for years. I have never gotten a straight answer to this. I am hoping some specialist on relativity will see this and give a good answer.

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#### yevaud

##### Guest
It's not that difficult, really.

What you perceive is the information transmitted to you via light. When a situation such as the one you've stated occurs, light is ******** in it's passage outwards due to the intense gravitation. You will see the person falling inwards slower and slower, but that doesn't actually mean that person is falling inwards slower and slower - only the visual record of his passage.

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#### OmegaOm

##### Guest
vevaud this is not true either. Every particle is effected by time dilation, not just the way we see it. Each clock each person would have would show different times. Also I do not think that light itself would slow down. It would stretch in wave length becoming redder and would be bent to travel the curved space in the area, but would not slow down. Light is always constant. It only slows down when passed through matter because of the many interactions of absorption and reflections of the atoms it passes through.

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#### yevaud

##### Guest
OmegaOm, this is one of the Paradoxes of a black hole.

The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.

In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.

Thus, talking about "speed" is ambiguous, there are multiple definitions of what it might mean. With the modern definition of speed, the speed of light is always constant in SR and in GR. With some other defintions of speed, definitions which are commonly used (though not "modern"), the speed of light is still always constant in SR, but with these alternate defintions, the speed is not necessarily constant in GR.

http://math.ucr.edu/home/baez/physics/R ... light.html

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#### OmegaOm

##### Guest
yevaud, I do not think this is a paradox. From what I understand of the theory, the time dilation is a fact of reality. If the theory claims that nothing can enter a black hole in our frame of reference, then nothing shouldn't enter it. Yet it does. This is not like if I traveled back in time and killed my ancestor, would I not exist?

What you quoted from the link you posted, seemed like an article trying to interpret general relativity in a different way from modern physicists. This was also stated in that article, after the assumption that light varies in a free falling frame.

("This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.")

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#### yevaud

##### Guest
OmegaOm":2xu31ab3 said:
What you quoted from the link you posted, seemed like an article trying to interpret general relativity in a different way from modern physicists. This was also stated in that article, after the assumption that light varies in a free falling frame.

("This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.")
Under certain frames of reference it is apparently valid, though as with you, I do not actually consider that C is a variable quantity. Yet that is the paradox, that is can seem so.

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#### SpaceJunkey

##### Guest
OmegaOm - You are confusing reality with human perception.

Black holes have the ability to bend space and time around them. These two items do very strange things (in our perception) when they are stretched to infinity.

The black holes can merge, but as humans, we don't have the instrumentation to observe or measure this yet.

You are not understanding this paradox, because no one has been able to understand this yet. We simply don't have enough tools to fix this problem.

Things do, in fact, enter black holes. This is observable in looking at disks of superheated gases and debris being sucked into the event horizon.

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#### Retrofire

##### Guest
From our reference frame, anything inside the event horizon effectively IS the singularity. It essentially doesn't matter if a collision occurs or not, because what lies within the event horizon is opaque to any observation, however contributes its mass to that of the singularity.

Even if a star is drawn inside the event horizon and never touches the black hole itself, but instead is crushed into some sort of super dense accretion disk orbiting the singularity, from an exterior perspective, the gravitational influence of the singularity still increases by that stellar mass, and for our purposes may as well have merged.

That said, most of what we think we know of black holes is hypothesis, and its almost certain that what really happens behind that inpenetrable information curtain would hold many surprises for Messrs. Hawking and Einstein.

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#### ramparts

##### Guest
As an observer falling into a black hole approaches the event horizon, they'll cross it without noticing anything unusual (until they get "spaghettified", of course ). It's the faraway observer who'll see unique behavior at the event horizon - mostly, as you said, the infalling observer never crossing it and getting redder and redder (from gravitational redshift, not from frustration!).

So there's nothing prohibiting black holes from colliding. They'll notice nothing unusual about crossing each other's event horizons.

I'd need to look over the math, but when I've calculated this, it's an observer at r=infinity who never sees the infalling observer cross the horizon. My guess is that for a finite, but large, distance from the black hole, one would eventually see the infalling observer disappear. Can anyone else confirm that? One would imagine that an observer at small r from the black hole would see the infalling observer cross the horizon, so generalize between the extremes.

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#### drstein

##### Guest
I think, when matter reaches the event horizont, matter gets to be so unstable so the structure of the atom breaks apart and particles that can pass the event horizon passes true the others just turns into light, as quazars do, they are the brightest stuff in the universe.

!!!!Delema, when you see your pall speed up, wouldnt he get really bright to, becouse I as the falling guy would get more light from the other guy, while the otherguy he might not even see you becouse of lack of light..!!!!

PS:
When we reatch the speed of light i think the same thing happends like near a black hole, and the mass increases too, so maybe we become a black hole directly... Im not sure about that. But this is what i belive!

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#### ramparts

##### Guest
drstein":31bj4jv7 said:
I think, when matter reaches the event horizont, matter gets to be so unstable so the structure of the atom breaks apart and particles that can pass the event horizon passes true the others just turns into light, as quazars do, they are the brightest stuff in the universe.

PS:
When we reatch the speed of light i think the same thing happends like near a black hole, and the mass increases too, so maybe we become a black hole directly... Im not sure about that. But this is what i belive!
To the best of my knowledge there's nothing that suggests atoms break apart at the event horizon. As a matter of fact, as I said above, an observer falling into a black hole won't notice anything especially unusual at the event horizon. The gravity will be strong, but not qualitatively stronger than just outside the horizon. Features of the horizon - like the fact that light can't escape - aren't things that would really affect us. Certainly nothing would turn into light (which is probably impossible in general relativity), and quasars are bright because of the disk of matter surrounding the black hole, nothing to do with the horizon.

Anyway, as for my last post...

I don't have time to run the math all the way through (since I do have actual work to do), but here's the basic idea: the time measured by the infalling observer is the proper time tau, while the time measured by an observer at rest (or at r=infinity, since the black hole spacetime is flat at large distances) is the time t. There's nothing weird about tau, you can calculate it to find out how long the infalling observer thinks it takes him to fall in, but as he approaches the horizon, t goes to infinity, so the observer at rest will never see him enter. In the real world, though, there is no r=infinity, so there are no observers strictly at rest, and the time they measure on the falling guy's clock isn't exactly t.

So the idea that an outside observer doesn't see someone cross the event horizon is an approximation - true to an extent, but after a while it's no longer true.

EDIT: I'm actually not sure about that - someone correct me if I'm wrong The observer's light cones do close up at the Schwarzschild radius so I suppose one would never actually see the crossing, just see the photons eventually stop coming.

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#### Russ_S

##### Guest
Take a look at this wikipedia page for Event Horizon: http://en.wikipedia.org/wiki/Event_horizon. Specifically, look at the section titled, "Interacting with an event horizon". If I understand correctly, it explains that the apparent location of the event horizon depends on how close you are to it, with it always seemingly out of reach--even when you are inside of it (as it appears to a distant observer).
From what I understand, matter falling into a black hole is accelerated to very high speeds and is torn apart into an accretion disk before it reaches the event horizon. Eventually, all of the matter from the accretion disk will get so close to the event horizon (as it appears to a distant observer) that it eventually becomes impossible to observe because the electromagnetic radiation (such as visible light) becomes increasingly red-shifted (stretched out).
As for what happens when black holes collide... who knows? They've done simulations like this one: http://www.newscientist.com/article/dn9012 , but I haven't heard of any black hole collisions occurring within the observable universe since we've been able to detect it.

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##### Guest
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.

Gravity makes them go faster as they're drawn in, and gravity also warps space and time. But to say that the event horizon is the 'singularity', that's just wrong. The event-horizon is simply the point at which the escape velocity is equal to the speed of light. This is the boundary at which no light escapes the black hole. It doesn't mean you will be going the speed of light when you cross it, does it?

That said, even if something were moving at the speed of light, it doesn't you won't see it move at the speed of light! You'll see it careening into the black hole at the speed of light!

That said, any observer away from the black hole will see the first person falling into it because that person is likely not going the speed of light just yet!

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#### ramparts

##### Guest
Russ_S":2dcp6d1d said:
As for what happens when black holes collide... who knows? They've done simulations like this one: http://www.newscientist.com/article/dn9012 , but I haven't heard of any black hole collisions occurring within the observable universe since we've been able to detect it.
We can't detect them, really. The only way we have of detecting black hole collisions is through gravitational waves, and the gravitational wave observatories LIGO and LISA are still in the works. LIGO may have results on a collision within a few years, though, given its readiness and the expected frequency of black hole collisions.

The point is, nothing in the laws of physics suggest that black hole collisions shouldn't be allowed, and as a matter of fact we expect that they've happened frequently, given the existence of supermassive black holes which can't have formed from stellar collapse.

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#### wanderingstar

##### Guest
When your partner closes the event horizon, you as an observer will see the following:

1. Your partner will be traveling very fast, close to the speed of light
2. His heart beat slows down to almost 0 due to the time dilation
3. His size shrinks to almost zero due to space contraction

This is what happens according to relativity. Please note that he is not frozen in space and time, he is still traveling at almost the speed of light. But his aging has stopped, to an observer.

You can effectively "see" him crossing the event horizon. Because when he crosses the event horizon, we will no longer be able to receive any information from him, no light nothing. He simply vanishes from our view. So, we will know that he is inside the event horizon in 3 ways:

1. He suddenly vanishes not only from our view, but from view of all possible detection instruments
2. The black hole's mass increases by your friend's mass
3. The event horizon's surface area (that means radius) increases by a certain amount

If the black hole is large, your friend may survive the entry and may even receive your calls. But he will never be able to call you back...

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#### chuckscherrer

##### Guest
I am not a math major or physicist. I can't explain or describe many things that I read here but I instinctually know what many of you are talking about for some reason. Please bare with me....

I seem to reemember something I heard of from high school math about the Zeno Paradox (I think that's how it's pronounced)....where if you have to go from point A to point B you can never actually get there because of having to traverse half the distance over and over again. In this paradox, you can never actually get to point B. Even thought the math supports this paradox, as it seems to do in others as well, we are still able to get a gallon milk at the corner store when we need to and return safely home afterwards. If relativity (and I barely understand it) means that time is relative to the observer, than there is a paradox and you could never reach the event horizon. But my gut tells me that the universe itself is the observer and it supercedes the individual and thus you are able to go get milk...the same is true for black holes in that they are also able to ignore paradoxes and continue to consume and grow in spite of them. Am I just being foolish or is there any scientific weight to what I'm saying? Please give me feed back as I am struggling to understand this.

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#### budicca

##### Guest
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.

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#### terranoid

##### Guest
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.

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#### ramparts

##### Guest
budicca":31dt41jg said:
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")

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#### Bill_Wright

##### Guest
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

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##### Guest
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 .........

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