# Can another blackhole pull you out of the event horizon of a blackhole

1. Aug 31, 2014

### maa105

say you fell beyond the event horizon of a black hole can another black hole passing close enough pull you out?

2. Aug 31, 2014

### mathman

No. Most likely the two black holes would merge to form a bigger one.

3. Aug 31, 2014

### maa105

I understand that fact, but im interested if say it wasnt a head on collision where the other black hole passes by close enough where its gravitational pull pulls you out of the other black hole's grip and not pull you into it's own. is that scenario possible?

4. Aug 31, 2014

### Staff: Mentor

No. Once you are inside the event horizon of a black hole, nothing can get you out.

5. Sep 1, 2014

### Chronos

Black hole event horizons can merge, but, confinement inside an event horizon is the gift that keeps on giving.

6. Sep 8, 2014

### Algr

Does this mean that if two event horizons intersected, it would be impossible to prevent the black holes from merging?

It seems to me that two event horizons would repel each other. You can't escape the horizon, but could the horizon be pushed past you, thus releasing you?

7. Sep 8, 2014

### fobos

In my understanding, If black holes are objects with a huge gravitational pull, then both will try to pull each other; in the end the larger one will actually succeed; in the same way that would actually happen with any other large objects in space with different masses.

That means that looking with a slow motion camera I will actually see what is after the event horizon coming out from the first black hole and entering into the second one ?? I have difficulty understanding merging when it comes to black holes.

8. Sep 8, 2014

### phinds

yes

No. Why would they repel?

First off, the EH of a black hole is not a physical thing, it's just a DISTANCE. The distance from the singularity below which light cannot escape the gravitational pull of the singularity. EH's don't attract or repel. What is involved is gravitational attraction. There is no repulsion to be found anywhere in the scenario of two BH's coming close and then merging.

9. Sep 8, 2014

### Staff: Mentor

Two event horizons intersecting *is* the black holes merging. (Actually, even that's not quite right: if you have a spacetime with two black holes merging, then there is only one event horizon, but it's shaped like a pair of trousers rather than a cylinder, so to speak--if you imagine time being vertical and two of the three space dimensions being horizontal. The single EH has two "branches" when you follow it into the past.)

An event horizon isn't a thing that can get "pushed". It's the boundary of a region of spacetime that can't send light signals out to infinity. In other words, it's part of the spacetime geometry, not a "thing" that gets added to the spacetime geometry. "Pushing" the EH would be like "pushing" the Earth's equator to occupy some other place on the Earth's surface--the idea makes no sense.

10. Sep 8, 2014

### Staff: Mentor

Actually, the best way to think of the geometric relationship between the EH and the singularity is not as a distance, but as a time--the singularity is a certain amount of time to the future of the EH.

11. Sep 8, 2014

### Algr

Event horizons would repel each other because they would tip an object's light cone in opposite directions, thus canceling out the effect of their gravity. It's rather like how you would be weightless at the center of the Earth.

12. Sep 9, 2014

### phinds

Fair enough. I DO tend to not understand that time-like vs space-like business inside an EH.

13. Sep 9, 2014

### Staff: Mentor

The claim here is not correct, but it does raise a good question: given that the light cones *are* tipped in opposite directions by two black holes that are falling towards each other, what do the light cones look like when the horizons merge?

To answer this, it's important to bear in mind that there are actually two types of horizons: apparent horizons and absolute horizons. The apparent horizon is the point at which outgoing light no longer moves outward--it just stays at the same radius. The absolute horizon is the boundary of the black hole, i.e., the boundary of the region that can't send light signals to future null infinity.

For a single, stationary black hole, the two horizons coincide; but in a spacetime in which two black holes merge, they don't. In the case of a black hole merger (or more generally in any case where a black hole gains mass by something falling into it), the absolute horizon will be *outside* the apparent horizon. This is important because the degree of "tipping" of the light cones corresponds to the *apparent* horizon, not the absolute horizon, which means that a point where the light cones are tipped enough so that the outgoing side is vertical (i.e., outgoing light stays at the same radius--the apparent horizon) is already *inside* the absolute horizon, i.e., already inside the black hole.

So, for example, suppose you are exactly halfway between two black holes of equal mass that are falling towards each other. Initially, the two holes are far apart, and you are outside both of them: you are able to send light signals to infinity. In your vicinity, the light cones are not tipped at all; you can indeed view this as the gravity of the two holes cancelling. As the two holes fall towards each other, the light cones in your vicinity still are not tipped; but the light signals you send out towards infinity will become more and more redshifted, as seen by an observer very, very far away from you and from both holes. At some point, you will become unable to send light signals to infinity at all; in other words, you will be inside the event horizon (which you would view as the horizon of the combined black hole formed by the two original holes joining). But at this point, there will be *no* apparent horizon anywhere in your vicinity; you will still have seen no tipping of the light cones near you.

Once you are inside the event horizon, you will eventually reach an apparent horizon; you will eventually see outgoing light in your vicinity no longer move outward. But at the point, you are already inside the event horizon of the combined hole, and there will be no simple way to relate the location of the apparent horizon you reach to any apparent horizons that someone falling into one of the original holes would have encountered.

Last edited: Sep 9, 2014
14. Sep 10, 2014

### Algr

Edit:
I think I get it now. A straight light cone does not necessarily mean that you are outside the (absolute) event horizon. There would be a zone that is "outside" the absolute horizon - but surrounded by it in every direction, and thus inside the apparent horizon. Is that right?

Hmmm... If the two black holes were orbiting each other - this zone could last for millennia. You couldn't escape, but neither would you be destroyed. You'd need to keep exerting energy to stay in the middle, but not too much if the zone was big enough. Then you could escape once the black holes evaporate.

Of course this depends on the two black holes NOT being within each other's absolute horizons - if they were, they'd need to orbit faster than light.

Last edited: Sep 10, 2014
15. Sep 10, 2014

### Staff: Mentor

[Edit: this portion of your post was edited, but I think these comments are still relevant and useful to clarify the relationship between the two types of horizons.]
The apparent horizon is not an event horizon. The apparent horizon is the point at which the light cones are tipped over enough so that the outgoing side is "vertical" (i.e., outgoing light stays at the same radius). So if the light cones are straight (i.e., not tipped), then you *are* outside the apparent horizon.

Also, if the light cones are not tipped at all (i.e., they are like they are in flat spacetime), then you are most likely outside the event horizon at well. If two black holes are merging, the event horizon will be outside any apparent horizons, but not so far outside that the light cones won't be tipped at all. They still will be, just not to the point of the outgoing side being vertical.

No, that's not possible. If there is a region inside the absolute (i.e., event) horizon in every direction, then there is no way to send light signals to infinity, so you must be inside the event horizon.

There's no such thing. The event horizons *define* the boundary of a black hole; you can't have one hole inside another. You just have two holes merging into one, and a single event horizon that has two "branches" in the past (like a pair of trousers, if we view time as vertical).

Last edited: Sep 10, 2014
16. Sep 10, 2014

### Algr

Oops, I edited my post just as you posted yours.

17. Sep 10, 2014

### Staff: Mentor

Responding to the edited portion of your post:

No. The absolute horizon is outside the apparent horizon in the case we're discussing. Also see the comments in my previous post.

18. Sep 10, 2014

### Algr

Okay. I've been using "Black Hole" as synonymous with "Singularity". I seem to recall that if you enter the event horizon of a large quiet black hole, you won't notice anything unusual in your vicinity. Distant stars would look strange, but your ship would seem normal until you (inevitably) got close enough to the singularity for spaghettification to occur.

19. Sep 10, 2014

### Staff: Mentor

Some people do this, but it's not really correct. The correct definition of "black hole" is the region that can't send light signals to infinity, i.e., the region inside the absolute horizon.

This is true, but it doesn't change anything I said. The "tipping" of the light cones is not really a local phenomenon, nor is the absolute horizon. For example, if your ship is falling into a black hole, there is no way for you to tell just by local measurements that you have crossed the absolute horizon--to know for sure, you would have to know the entire future of the spacetime, so you could know exactly which light signals will eventually reach infinity. You don't need to know quite that much to know whether you've crossed the apparent horizon; but you would need to measure the trajectories of light signals that you emitted in different directions, for long enough to see whether they were converging or diverging, and that's not really a local measurement either.

20. Sep 10, 2014

### Algr

So in those terms, I am describing a stable area within the black hole where regular mater and information could exist.

In this one dimensional diagram, the red time cones mark the event horizon as I understand it. The green time cone is not tipped, but any light from it must enter one of the two event horizons. In one dimension I can see that the two singularities must merge, but in two dimensions they could instead orbit each other. Then the area near the green time cone would be stable, yet unable to reach the outside world.

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