Gravity: Speed of Light & Black Holes

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Discussion Overview

The discussion centers around the nature of gravity, particularly whether it travels at the speed of light and how this relates to black holes and the behavior of objects in gravitational fields. Participants explore concepts from General Relativity, the instantaneous nature of gravitational force, and the implications of changes in gravity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that gravity is believed to travel at the speed of light according to General Relativity, while others suggest that changes in gravity (like gravitational waves) travel at this speed.
  • A participant questions whether the force of gravity is instantaneous, proposing that changes in gravity propagate at the speed of light.
  • Another participant raises a thought experiment about an object appearing in Earth's atmosphere and whether it would feel gravity instantly or if it would take time for changes in gravity to be felt.
  • There is a discussion about the curvature of spacetime as an alternative way to understand gravity, with some suggesting that gravity is omnipresent due to this curvature.
  • Some participants challenge the idea of gravity being an effect that can be transmitted instantaneously, while others argue that gravity is caused by mass and energy.
  • One participant introduces the concept of continuity equations in General Relativity, cautioning against thought experiments involving instantaneous appearances of mass.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of gravity and its propagation. There is no consensus on whether gravity itself travels at the speed of light or if it is the changes in gravity that do. The discussion remains unresolved with various hypotheses and thought experiments being presented.

Contextual Notes

Participants reference specific astronomical events and thought experiments to illustrate their points, but these examples are not universally accepted as definitive evidence. The discussion also highlights the complexity of defining gravity and its effects in the context of General Relativity.

  • #61
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.
 
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  • #62
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
 
  • #63
The speed of gravity isn't constant though. It is different at different locations. I'll give you a link if you want.
 
  • #64
dude222 said:
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?
 
  • #65
Would surely appreciate a reliable reference explaining where speed of gravity is different.
 
  • #66
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.
 
  • #67
sorry, user name john williams
 
  • #68
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.
 
  • #69
stone1 said:
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.

stone1 said:
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.
 

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