# Does the event horizon change as large objects approach it?

1. Jan 26, 2016

### newjerseyrunner

If there is a large mass near a black hole, is the event horizon teardrop shaped? Today I was thinking if it was possible to violate the idea that anything that passes the event horizon is gone forever. I forgot to label my picture, Figure 1 is the top and Figure 2 is the bottom.

I imagine a black hole being approached by a massive hypervelocity star, it's nearing it's breaking point because of the tidal forces, but still in one piece. My thought experiment involved taking a trip on a neutrino that is ejected by the star traveling at nearly the speed of light and enters the black hole at an extremely shallow angle.

The neutrino goes into a spiral path inwards, making one full "orbit" around on the inside. Normally in a black hole, it'll never be closer to the event horizon in the future as it is in the present. [Figure 1] But if there is a large object near it, the gravity of the other object should pull on objects close to the event horizon, wouldn't that cancel out a small part of the gravity of the singularity and allow the neutrino to pop back out? Like how when an asteroid slingshots around the Earth and the gravity from the Moon pulls it away? [Figure 2]

If the event horizon gets pushed back towards the singularity on the side of it closest to the star because of the star's gravity, then on the opposite side, where the gravity of both the star and the black hole are acting, the event horizon should be stretched out. So imagine a relativistic proton approaching the black hole, but if the larger star were not there, is not on a collision course with the event horizon, it'll be ejected, but it passes extremely close to it. [Figure 1] With that more massive star there, it would pass the event horizon, could it make half an "orbit" and come out the other side where the event horizon has been pushed back? [Figure 2]

I know this is classical physics, but since everything is in the same reference frame, I'm thinking that that's ok? If any of this is true, would that also imply that if the black hole has a big object orbiting every close to it, would there be Lagrangian points inside the black hole?

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2. Jan 26, 2016

### DaveC426913

That is an intriguing question...

A diagram:

There has been some research into merging black holes. This will surely shed light on your question, since the last moments before a merge will essentially replicate the condition you are positing.

Last edited: Jan 26, 2016
3. Jan 26, 2016

### DaveC426913

Here is an article containing a simulation of two black holes colliding.

http://www.theevolvingplanet.com/happens-two-black-holes-collide/

A screen shot (taken at 2:14) certainly seems to show your teardrop shaped EHs:

We can extrapolate then, to posit that an object of sufficient mass near a black hole will later the shape of the BH's EH.

I don't see how you could use that to essentially escape. It seems to me, it means there is an even larger volume in which you can find yourself trapped within the EH.

4. Jan 26, 2016

### phyzguy

While it is true that the event horizon becomes distorted, it is not true that an object can cross the event horizon and then come back out. Once it has crossed the EH, even a distorted one, it will not reappear.

5. Jan 26, 2016

### DaveC426913

Right.

I think he's asking if it causes the EH to shrink at some point, meaning you could actually get closer than would normally be allowed by a spherical EH.

6. Jan 27, 2016

### Chronos

An EH is, afterall, just a coordinate surface without any physicality Spacetime is however curved near the EH. Whether this is manifested in any non trivial way on the geometry of nearby space appears a tricky question. I doubt any such effect would permit a particle that had 'cross over' to exit the EH. AFAIK no weird geometry is evident in galaxies that host SMBH's.

7. Jan 27, 2016

### newjerseyrunner

I've seen this guy before, I don't think that's a mathematically sound simulation. Could you link to one of those research papers you mentioned? Here is why I think the simulation is just CGI and not based on actual physics: the teardrop is a mirror image of what I expected it to be. In the screenshot, the two lobes are facing each other, that doesn't make any sense, shouldn't the lobes be facing away from each other? It makes sense that gravity would be strongest when the two black holes are aligned with each other, not when you're in between them. Maybe there is some obscure dragon in GR that I don't understand, but the screenshot doesn't seem to fit my understanding of it.

Exactly, like how in between the moon and the Earth, the escape velocity is slightly less than it is on the opposite side of the planet because the Moon has it's own pull. The moon allows for objects that would otherwise be in death spirals in our gravity well to pop back out. Essentially, creating an L1 point in between them. In a two body system, doesn't there have to be an L1 point and between the L1 point and the center of mass of the larger object, the escape velocity is slightly less than it is everywhere else at the same radius? Like in the diagram that DaveC426913 posted.

I wouldn't think so, the gravitational effects of a large star near a SMBH is negligible, in my though experiment, I was thinking of a stellar black hole, where the gravity of a nearby massive star would be more noticeable. I should have made that clear.

I have a follow up scenario: a photon. Make the assumption that black holes EH do in fact form teardrop shapes near other huge objects (the direction that it's pointing doesn't matter.) Say the radius of the greatest extension is 5km and the shortest is 3km. Is it not possible that there is an angle of entry for the photon in which the geodesic doesn't curve inwards the 2km required to keep it inside the EH?

8. Jan 27, 2016

### DaveC426913

There's a bit of a disconnect happening here.

The event horizon is defined as the virtual boundary where photons cannot escape. i.e. if a photon can escape, then it is, by definition, outside the EH.

9. Jan 27, 2016

### newjerseyrunner

I thought it was defined as the virtual boundary where the escape velocity is equal to the speed of light?

Meaning that if it's not perfectly round, it'd be possible that the escape velocity at radius r on one side could be greater than c, and the same radius on the other side could have an escape velocity less than c.

If it's defined as the boundary where it's completely impossible, then that's the part that I didn't understand. That would also resolve my disconnect on which side the lobes of the teardrop are. Is this correct?

10. Jan 27, 2016

### sevenperforce

A spinning black hole drags a region of space around its equator called an ergosphere. This is a region of space where depending on what direction you're going, you will either pass through just fine, or you will be swallowed up. If you approach a black hole near the pole, you can come closer to the center than you would be able to if you were approaching near the equator.

Merging black holes, or a hypervelocity star passing very close to a black hole, would produce the same sort of effect.