<p>For an outside observer, everything moves slower the closer it is to a black hole's event horizon, even photons, due to the black hole's time dilation.</p>
<p>We also know that when two black holes collide, they are first orbiting each other for some time, and then they fuse in the fraction of a second (if they are small. The bigger they are, the longer the event takes).</p>
<p>But it has also been found rogue black holes that are travelling through cosmos. So even if it is extremely rare, it should also be possible for a black hole to collide with another black hole in the same way a bowlingball smashes into another bowlingball.</p>
<p>Does the merging of the two black holes happens just as fast in such an incident?</p>
<p>If it is, then we have this scenario:</p>
<p>An asteroid is travelling towards a black hole. On the other side of the black hole there is another, but smaller, black hole that is approaching too. The asteroid and the smaller black hole has about the same diameter and are travelling at the same speed. Both enter the gravity well of the larger black hole simultaneously, but from opposite directions.</p>
<p>The closer the asteroid comes the black hole's event horizon, the slower it moves for the outside observer. But what about the smaller black hole? Will that too seem to move slower the closer it comes its larger sibling's event horizon, or will it simply continue its path as if nothing slowed it down and then merge?</p>

CC BY-SA 4.0
2022-04-05T22:41:53.680Z
What do you mean by "The asteroid and the smaller black hole has about the same size"? Please [edit] your post to clarify. If by "size" you mean *mass*, then the small BH's event horizon will be microscopic. If you mean *diameter* of the small BH's event horizon, its mass will be billions of times that of the asteroid – but how would you be able to observe it? In any case, both asteroid and small BH follow the same laws of physics when in a gravity well.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2022-04-06T00:12:13.523Z
@Chappo The speed of light is always *c* for a local observer, but it can change for a distant observer. Please see [How can a black hole reduce the speed of light?](https://physics.stackexchange.com/q/145110/123208) & https://physics.stackexchange.com/q/77227/123208 & https://en.wikipedia.org/wiki/Shapiro_time_delay
PM 2Ring
CC BY-SA 4.0
2022-04-06T01:08:10.057Z
If we can observe the smaller black hole is irrelevant, so is its mass. The question was about the time dilation as an object gets closer to the event horizon. The asteroid will move slower for an observer, but when the smaller black hole is approaching the event horizon, then what? Will it move just as slow as the asteroid? Colliding black holes doesn't use millions of years to merge, after all.
Devon
CC BY-SA 4.0
2022-04-07T07:14:29.733Z
@PM2Ring thnx for the link, and the explanation makes sense. I'll delete my comment, but I think the links in yours add value for visitors to this question, even without the context of my comment.
Chappo Hasn't Forgotten
<black-hole><time-dilation><event-horizon><merger>
Time dilation if an asteroid and a small black hole entered a larger black hole?
Devon
# Time dilation if an asteroid and a small black hole entered a larger black hole?

(Asked by: username_0 on 2022-04-05)

For an outside observer, everything moves slower the closer it is to a black hole's event horizon, even photons, due to the black hole's time dilation.

We also know that when two black holes collide, they are first orbiting each other for some time, and then they fuse in the fraction of a second (if they are small. The bigger they are, the longer the event takes).

But it has also been found rogue black holes that are travelling through cosmos. So even if it is extremely rare, it should also be possible for a black hole to collide with another black hole in the same way a bowlingball smashes into another bowlingball.

Does the merging of the two black holes happens just as fast in such an incident?

If it is, then we have this scenario:

An asteroid is travelling towards a black hole. On the other side of the black hole there is another, but smaller, black hole that is approaching too. The asteroid and the smaller black hole has about the same diameter and are travelling at the same speed. Both enter the gravity well of the larger black hole simultaneously, but from opposite directions.

The closer the asteroid comes the black hole's event horizon, the slower it moves for the outside observer. But what about the smaller black hole? Will that too seem to move slower the closer it comes its larger sibling's event horizon, or will it simply continue its path as if nothing slowed it down and then merge?

**Tags:** black-hole, time-dilation, event-horizon, merger

> **Comments:**
> **username_1**: What do you mean by "The asteroid and the smaller black hole has about the same size"? Please [edit] your post to clarify. If by "size" you mean *mass*, then the small BH's event horizon will be microscopic. If you mean *diameter* of the small BH's event horizon, its mass will be billions of times that of the asteroid – but how would you be able to observe it? In any case, both asteroid and small BH follow the same laws of physics when in a gravity well.  
> **username_2**: @Chappo The speed of light is always *c* for a local observer, but it can change for a distant observer. Please see [How can a black hole reduce the speed of light?](https://physics.stackexchange.com/q/145110/123208) & https://physics.stackexchange.com/q/77227/123208 & https://en.wikipedia.org/wiki/Shapiro_time_delay  
> **username_0**: If we can observe the smaller black hole is irrelevant, so is its mass. The question was about the time dilation as an object gets closer to the event horizon. The asteroid will move slower for an observer, but when the smaller black hole is approaching the event horizon, then what? Will it move just as slow as the asteroid? Colliding black holes doesn't use millions of years to merge, after all.  
> **username_1**: @PM2Ring thnx for the link, and the explanation makes sense. I'll delete my comment, but I think the links in yours add value for visitors to this question, even without the context of my comment.  

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