We know that: 1. Gravity propagates at the speed of light (as I know General Relativity concludes it) 2. At the Event Horizon the value of Escape Velocity is equal to speed of light, that’s why the light cannot escape from the Black Hole. If both statements are true then the question arises: how the gravity escapes from the black hole at all? As we see from these two statements the gravity also (like light) should not be capable for escaping from the black hole, right? If so, the black hole cannot act on the matter near it, cannot attract it…..so, where is solution to this question? Please Register or Log in to view the hidden image!
Gravity waves propagate at the speed of light. Gravity waves, are not however responsible for the gravitational force. The force is due to the gravitational field. What gravity waves carry is information about changes in the field. So, consider what happens when something collapses into a black hole. The instant the event horizon forms, no more information can leave, that includes information carried by gravity waves. Which means that the last information the gravity field outside the event horizon gets is that which left just before the event horizon formed. Thus the Gravity field is "frozen" in the state that it was when the BH formed. (This is why one term for a black hole id "Frozen Star")
tashja Thanks, I am not Physicist, but somehow I understood it Please Register or Log in to view the hidden image! Janus58 I see Please Register or Log in to view the hidden image! but at what speed does the gravitational field/force propagate? Imagine that the BH is attracting some celestial body falling into BH and then due to some reasons the mass of the BH increases 2 times. Question: how quickly the falling body will “feel” this change? Please Register or Log in to view the hidden image!
You should ask this question again, but next time ask how gravitons escape from a black hole! But to offer my own twopennyworth on the OP, like some of the guys have said, a gravitational field is different to a gravitational wave. It's just there, it isn't "going" anywhere, speed doesn't apply. Think about the bowling ball on a rubber sheet analogy. The depression caused by the bowling ball is a gravitational field. If you shook the bowling ball, the resultant ripples in the rubber sheet are the gravitational waves. They're subject to speed, but the gravitational field isn't. A black hole is like a very small very dense bowling ball that makes a depression in the rubber sheet that's so deep you can't see the bottom of it.
I thought you didn't believe in curved space. Anyway, the bowling ball on a rubber sheet analogy for GR is not a very helpful one because you need a gravitational force to pull the bowling ball down, and there is no analogue of that in real gravity.
Who, me? I say the electromagnetic field is curved space. Yes, the bowling ball on a rubber sheet isn't ideal. You'll doubtless have heard me say in Gravity Explained that "it's wrong because it relies on gravity to pull the cannonball down in the first place". A better analogy would involve a stress-energy-density or pressure-gradient in inhomogeneous space, but people aren't accustomed to it, and tend to reject something that isn't in their textbooks. I didn't feel it was appropriate here.
In what sense is the electromagnetic field curved space? Believe it or not, people that do physics know about physics. I don't know whether "gravity explained" is a book, blog or thread but I can tell you I haven't read it.
Actually I might have an intuitive answer for the OP, depending on the type of black hole: For a black hole formed by collapsing matter, you don't feel the black hole itself. You feel the gravitational field caused by matter falling into the black hole. Just as an outside observer never sees the matter cross the event horizon in finite time, they never stop feeling the gravitational field it caused. For an eternal black hole, the Schwarzschild black hole solution requires the existence of a corresponding white hole. In that case, it's the white hole's gravitational field you feel.
That's a good answer. It makes me wonder what the gravitational analogy is to the redshifting of light. It seems none could exist, otherwise the gravitational forces of a BH would change with time regardless of mass accumulation and/or Hawking radiation. Ahh hell przyk now you're whet my appetite for asking some questions about Kruskal coordinates...
