# Colliding Black Holes & Einstein's Equations

1. Apr 24, 2013

### Yosty22

I was watching Neil deGrasse Tyson video, in which he describes a scenario of colliding black holes. He mentions that when two black holes collide, there is a huge distortion of the space time between those two black holes as each of their even horizons intersect (i.e. each black hole has passed the others' event horizon). He says that, according to Einstein's Equations, the severely distorted space time that "there is a path you can take around two moving black holes that haven't quite collided where you can end up in the past of when you started that journey."

From what I understand, light can't escape a black hole because the space time loops back in on itself so to say so that there is no path out, making the escape velocity larger than that of light. However, when two of them collide, the space time gets even more distorted. I was wondering: how exactly is it possible that space time can be warped so severely that it can allow for a method of time travel?

2. Apr 24, 2013

### Chronos

Dr Tyson's comment came around minute 48 of the video where he was discussing a paper on black hole mergers where he inferred the author had concluded reverse time travel was a possible outcome. It was an impromptu remark about a paper he admittedly did not understand and he [or the author] may easily have drawn an incorrect conclusion. Time travel to the past presents a cornucopia of paradoxes with unknown consequences. Perhaps trapping such events behind an event horizon avoids the potential issue of paradoxes, but, also renders the hypothesis untestable.

3. Apr 25, 2013

### ImaLooser

Well, I don't know. I do know that in such extreme circumstances the whole idea of time and pastpresentfuture loses all meaning. An opaque mystery, as far as I'm concerned. There is a professor at University of Col at Boulder who studies this stuff, so you might look that up.

4. Jun 11, 2013

### ruspj

just reading through & got me thinking.
wouldn't you need to take time dilation close to the black holes into account ?
the closer an object is to the event horizon the slower it would appear to an observer.

surely they could never ultimately collide as seen by an observer ?
( ok not actually SEEN but compared to the time away from the event horizons )

5. Jun 11, 2013

### Yosty22

I'm not sure, but as "seen" by the observer, I don't think they would collide. It kind of confuses me reading through this again, because if you are observing it from afar, you never "see" them collide, so how would you have any sort of guess what could be happening on the inside of the possible collision?

6. Jun 11, 2013

### ruspj

oh :( didn't realize items entering a black hole needed to be observed for time dilation to be effecting them.

surely their edges could both be seen if matter was falling into them, due to gravitational lensing, or possibly other effects. aren't they theoretically supposed to emit low level heat.