Does gravity travel at the speed of light?

c doesn't have a limit on it's sig figs. it's defined as a whole number of meters (or meters are defined as a rational fraction of c, whichever you prefer). it has sig figs like the 2 in perimeter=2r*pi does.

one quick question about black holes, since it came up and we seem to have a physics guru on the line. imagine a black hole moving at 0 velocity relative to us. my understand is that as an object approaches the event horizon, it would theoretically look like it was slowing down because of the warping of space/time, and it would hence appear to approach the event horizon asymptotically. but this would only be the appearance in from our frame of reference, because by its own frame it would pass right through the event horizon with increasing speed. So, if i'm right in all this, we would continue to see the object forever in the event horizon, despite the fact that by it's own frame of reference it's already been swallowed up. by our frame of reference, then, a black hole should never be able to swallow anything; it should never grow. is that right? the only alternative i see to that is that we would observe that the object dimmed as it got closer to the event horizon, but i don't remember if that's the theory or not.
 
i'm not sure what you mean by asymptotically

we watch a red ball being sucked towards a black hole, as it gets closer and closer the variable gravity between the front of the ball and the back of the ball would stretch it into an ellipsoid and eventually it would pass the event horizon, where no light would return to us, and it would appear black

any light transmitted to us before the EH we see, as soon as it passes it appears black

hence "black" hole

I don't follow you on the asymptotic thing..and I don't see why it would appear to slow down. It would be speeding up and we could measure this. It would also be stretching
 
(edit: wrong quote :) )
That is not correct. Once the object passes the event horizon (from its point of view), then its photons stop being emitted, and we stop seeing the object.

I dont see why the warping of space-time would affect the observer, since te observer's frames are not affected by the black hole. I think of it like a big planet that some rock crashes into. Wherever you are, you just see a rock that crashes onto a planet.
 
I thought it slows down because the distortion in space-time caused by the ridiculous mass would make the light getting to us from the object take longer and longer to get to us (since the space between the object and us would be getting furhter and further stretched), until it would take infinite time to reach us from the eh itself.

but i guess that would just mean it gets dimmer and dimmer. still, it should be apparent long after it's slipped into the eh.
 
one thing about why light doesn't escape a black hole: it's not that the particles are being pulled towards the black hole (like we are pulled towards the earth) and hence stopped from going out. you can think of it better as the light is still travelling away from the black hole (at c), but the mass behind it is stretching space out faster than the light can traverse that space. so the light particles are getting further from the center of the hole, but they're also getting further from us. at the eh, the photons are moving at c toward us but the distance they have to cover to get to us increases at the rate of c. at slightly outside the eh, the photons are moving at c and the space is being stretched out just a bit slower than c, so it would take a really long time to get to us.

at least that's my understand of black hole theory....maybe i'm wrong.
 
well it really depends specifically on where you're watching from

yeah it would be apparent long after it "really" past the EH, that is just normal c-is-constant behaviour

Remember it's accelerating towards the black hole. If it slowed down as it got closer, well you're right it would never get there. But it's gravity we're talking about - gravity causes acceleration which means it's going faster. Gravity doesn't cause deceleration.

The light would be more and more redshifted as it went closer to the black hole because the object would be moving faster and faster
 
(^that was for your 1st message)

What you described (in 2nd post) is impossible by definition. You say that the light would be trying to move away from the black hole, but that space-time would stretch faster than the particle. Since the particle moves at c, space-time would ahev to stretch faster than c.
 
Forensic said:
well it really depends specifically on where you're watching from

yeah it would be apparent long after it "really" past the EH, that is just normal c-is-constant behaviour

Remember it's accelerating towards the black hole. If it slowed down as it got closer, well you're right it would never get there. But it's gravity we're talking about - gravity causes acceleration which means it's going faster. Gravity doesn't cause deceleration.

The light would be more and more redshifted as it went closer to the black hole because the object would be moving faster and faster
Click to expand...

it would only go faster according to its own frame of reference though, which is within a very warped space (more importantly, is in an area that is continually being more and more warped).

maybe slowing down isn't the best way to say it. what i meant more specifically is that by our frame of reference it would never seem to reach the event horizon, because the eh is the sphere around the hole where space is being stretched at a rate of exactly c (to reach it, you'd have to go c, and to get closer to the center of the hole from the eh you'd have to go greater than c).
 
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TheMaster_C said:
(^that was for your 1st message)

What you described (in 2nd post) is impossible by definition. You say that the light would be trying to move away from the black hole, but that space-time would stretch faster than the particle. Since the particle moves at c, space-time would ahev to stretch faster than c.
Click to expand...

isn't that the theory? that the hole is forever collapsing, so it is forever "falling" into space-time, stretching it out. stretching space shouldn't have a speed limit any more than density has a limit.
 
i think you're overcomplicating things

the warping of spacetime effects us through accelerations

i don't see why a particle couldn't collapse to the "center" of a black hole

the EH just determines the point where light is overpowered by gravity
 
so...the particle just stops? it couldn't stop. or maybe moves sideways? there would be no force on it to move sideways.

i'm really asking. the reason i brought this up was cuz i'm confused by the theory
 
so you're asking what the effect of a continually increasing stretch of spacetime is?

the effect is a continually increasing gravitational force ;)

this reminds me of the question..

You black off in a rocket with a clock onboard, and there's a clock on the ground. The idea is that you have to be back when the clock on the ground says one hour has passed. Now you want it so that when you come back, your own clock is as far ahead as possible. According to Einstein, the higher you go in a gravitational field the faster your clock goes. But if you go too high, since you've only got an hour, you have to go so fast to get there that the speed causes your own time to slow down. So you can't go TOO high.

The question is, exactly what program of speed and height should you make so that you get the maximum time on your own clock?

If you're good with your physics you should be able to answer this, without calculations :D
 
phoenix. See it in a simple way.

Think of a black hole as a very massive rock.
Think of a photon as a very elastic rubber ball.

The rubber ball goes toward the rock due to the rock's gravity. At some point, it hits the rock and rebounds. However, after the rebound, it is still affected by the rock's gravity, and will, at some point, stop and come back towards the rock and bounce again (the same way as if you dropped a rubber ball on earth, it would rebound, go back up, but lower than its initial height, and go back down).

The event horizon is the distance after which it is impossible for the rubber ball to come back (if ur on earth and u throw the ball very high, it wont come back since the higher you are, the less gravity there is. see anything we launch into space.).
 
TMC: that model doesn't work for black holes because photons don't act that way. they don't go back towards a massive rock the way a ball or an improperly launched satellite returns to earth.

Forensic: i have no idea how to solve that, but let's see....i'm assuming you mean there's a constantly increasing grav field overhead. so for every point x above point y, time is moving more quickly at x (according to an observer at y), but to get to x, you need to move, which slows down your time (relative to y)? are there equations that relate how relative time is related to relative speed or grav-field. I just don't know? and my cousin just rang my door bell so i'm going to eat some sushi. post the answer cuz i'm really curious.
 
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