Does gravity travel at the speed of light and how do black holes trap photons?

[gtimespace]

In his book The Fabric of the Cosmos, Brian Greene writes, "Einstein was able to work this out, " and he found that the speed at which gravity can transmit it's influence is exactly the speed of light. "...the warps and ripples-gravity, that is-do not travel from place to place instantaneously, as they do in Newtonian calculations of gravity." Greene then gives an interesting example of what would happen if aliens removed the moon from its orbit. It would take one and a half seconds for the tides to be affected. Exactly the time it would take for us to see that the moon had vanished.

 

[HarryWertM]

This thread has been interesting reading. Here is my best understanding:

The "speed of gravitational information" is c. The "speed of electrical information" is also c. Both can easily escape a black hole. But nothing inside the black hole can generate any "waves". Only "normal" fields escape, kinda like the poster who mentioned infinitely long red shifting.

So: Charged particles fall into a black hole. The charge on the black hole changes. This this electric field change propagates out at the speed c. But if charges are somehow moving around inside the black hle - [[An entirely different question: Can they?]] - then no apparent effects from the moving charges would go outside the hole. Likewise, if massees fall into the hole, the increase in gravity goes out at c, but if masses are spiraling around inside the hole, no "gravity waves" would exit the hole.

Right?

-Harry Wertmuller

 

[dude222]

The speed of gravity isn't constant though. It is different at different locations. I'll give you a link if you want.

 

[Chaos' lil bro Order]

The speed of gravity isn't constant though. It is different at different locations. I'll give you a link if you want.

Umm. You should give that link, because its currently accepted as a constant. Are you referring to the theory of MOND?

 

[HarryWertM]

Would surely appreciate a reliable reference explaining where speed of gravity is different.

 

[dude222]

I can give you a link for a uniform gravitational field (or uniform acceleration). I'm thinking of posting a more complete account. I post on hi5, username johnwilliams22.

 

[dude222]

sorry, user name john williams

 

[stone1]

I have been curious to find an answer to that question too.

Suppose that at 12pm one marks the position of the Sun (as it was ~8 min in the past) and also at 12pm one marks the position of the gravity pull (I don't know whether there are instruments that can do that yet). Would the two positions be the same?

A similar question is whether the Earth rotates around the actual position of the Sun or around the position of the Sun as it was ~8 min in the past.

 

[DaveC426913]

I have been curious to find an answer to that question too.

Suppose that at 12pm one marks the position of the Sun (as it was ~8 min in the past) and also at 12pm one marks the position of the gravity pull (I don't know whether there are instruments that can do that yet). Would the two positions be the same?

Yes. Light and gravity both lag by 8 minutes.

A similar question is whether the Earth rotates around the actual position of the Sun or around the position of the Sun as it was ~8 min in the past.

"Actual" is a tricky term here. Since nothing can travel faster than c, it's kind of moot to talk about "actual" position.