Quote (novocane @ Oct 18 2011 05:47pm)

Regarding why light can't escape a black hole, this is how I always understood it (taken from wikipedia):

"As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.[44] At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole."

Aight but today our physics teacher said the reason light can't escape a black hole was because its escape velocity was too low. And if you google it some shit defines black holes as having an escape velocity greater than the speed of light.

My question is why does the speed even matter in escaping a black hole? If spacetime is curved in such a way that the particle paths lead back to the black hole, a particle could travel infinitely fast and it would just travel back on itself. So what does escape velocity have to do with it?

edit -

especially since gravity doesnt directly effect light

Quote (novocane @ Oct 21 2011 12:52pm)

Thats not really true though; black holes work by bending space time so that all possible paths lead back to the center of the black hole

Quote (Matao @ Oct 21 2011 01:32pm)

those are einsteins own words ....
and the " path " , is a mathmatical path

the exact same can be said about my conveyor analogy
if the conveyer moves at light speed , the path of any object on the conveyor with a speed lower than light will always be towards the balck hole
and the path of an object moving at light speed would be " trapped " , in a linear system ( such as my conveyer ) , it would appear motionless , in a circular system ( such as a black hole ), it would appear to orbit at a fixed distance ( an event horizen )

Quote (bentherdonethat @ Oct 20 2011 09:20pm)

Saying that the escape velocity is greater than the speed of light just means that nothing can ever escape it. It's not perfect, but it's not bad for a one-line definition.

The Schwarzchild radius is the "point of no return", meaning if light (or anything else, of course) passes within that radius it can never escape. You're correct in thinking that escape velocity is no longer relevant inside that radius since space itself contorts such that escape is impossible.

Of course, if one could find a way to travel to a fourth spatial dimension that was NOT curved as much as the three spatial dimensions that we experience regularly, then you could potentially escape. However, that's hypothetical at best and extremely unlikely.

Quote (novocane @ Oct 22 2011 07:25am)

I mean you said it was his "original" thought experiment, and I'm just saying that analogy doesnt really fit well with the current theory that the black holes bend the space time back to the center of the black hole



The schwarzchild radius is (from wikipedia):"the distance from the center of an object such that, if all the mass of the object were compressed within that sphere, the escape speed from the surface would equal the speed of light."

but really the speed of light should have nothing to do with where the event horizon is in a black hole

Quote (Matao @ Oct 21 2011 02:31pm)

and the event horizon is the point at which the strength of the gravitational field equals the velocity of light

Quote (novocane @ Oct 22 2011 07:41am)

yea thats what this thread is about

escape velocity applies to a normal planet is that its the required speed need to break free of its gravitational field

but for a black hole escape velocity has nothing to do with breaking free; you could go infinitely fast and you still wouldnt escape

so why do they use it to definie the event horizon/black hole/and calculate the radius of black holes with it?

Quote (novocane @ Oct 21 2011 02:41pm)

yea thats what this thread is about

escape velocity applies to a normal planet is that its the required speed need to break free of its gravitational field

but for a black hole escape velocity has nothing to do with breaking free; you could go infinitely fast and you still wouldnt escape

so why do they use it to definie the event horizon/black hole/and calculate the radius of black holes with it?

Quote (Matao @ Oct 21 2011 02:47pm)

i feel that you are making a basic mistake , by thinking that space , and spacetime , are the same things
i think maybe the best thing at this point is for you to read einsteins book , relativity , which he wrote in 1916 , which is written for non physictist's

Quote (L4d @ Oct 21 2011 03:17pm)

Once inside of the event horizon, nothing can escape, but outside of it, there is a chance to evade it.

Quote (L4d @ Oct 21 2011 12:09am)

I will answer my own question, because I did some research that states that light is effected by gravity, so gravity will cause light to speed up towards the object that is causing gravity to pull light towards it, but the fact that space-time is contorted in such a way via black holes makes it that more difficult to escape this "trap".
I suppose if something was faster than the speed of light, that could escape the pull of a black hole, but it would have to, also, escape the contorted space-time.