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Q & A: waves from black holes

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Most recent answer: 01/25/2017
Q:
If nothing can escape a black hole, how did gravitational waves escape from them? Recently the LIGO experiment proved their existence from the merging of two black holes. But these waves must not have traveled from the new b.hole as nothing can escape from them right?
- Deepak (age 23)
India
A:

The gravitational fields already extend outside the black hole horizons. So the waves don't have to cross the horizons.

Mike W.


(published on 01/10/2017)

Follow-Up #1: more on black hole gravity waves

Q:
Okay. But I am finding it a bit tough to digest it. Let's say there is a star losing its vigor. Let r be the radius from the center of the star to the future event horizon (after the star becomes b.hole). The star has not turned into a black hole. Now, the gravity at a distance r from the star's center is, say, g1. After the star becomes a b.hole, the gravity at r (now this r is event horizon) be g2. Certainly, g2 > g1, as no one can escape from the event horizon but no such thing exists in the case of a star. So somehow the information (gravitational waves) must have traveled from the center of our new b.hole to distance r. Certainly, it can't be an instant change right ?. So for me, it looks like, it is violating Einstein's maximum speed limit.Okay, let me ask it in another way:The LIGO thing detected the merging of two black holes, which happened 1.3 billion years ago. So the two b.hole before merging were present and the gravity due to them on earth be g1. After the merging, let it be g2 (I guess g2 is not equal g1, then only one can talk about a WAVE(not field) and discuss its detection). This being the case, it looks like the gravitational wave has traveled from the new b.hole violating special relativity. Please clear my doubts.
- Deepak Muthukrishnan (age 23)
603202
A:

"So somehow the information (gravitational waves) must have traveled from the center of our new b.hole to distance r." Why from the center? The field was spread out, originating in all the areas where there is mass, so the changes come from all those parts. 

With the two black holes merging, there's still a field outside their horizons, and later outside the horizon of the merged black hole, and changes in that field propagate as waves.

Mike W.


(published on 01/25/2017)

Follow-up on this answer.