[Forwarded to TML (came to list admin only) -- James]
> Time for a stupid question:
>
> If gravity goes at the speed of light, and light goes at the speed of light,
> how can gravity catch up with light? If it can't, why can't light escape
> from a black hole?
Mass has the effect as though it has curved spacetime. Anything (even photons)
are effected by the curvature. If the curvature is great enough photons
can not climb out of the well cause the total energy of the photon is
less then the potential needed to escape. If the mass were suddenly
removed (Zot!) space would become flat but it would do so a the speed of
light - meaning if you were 1 light day away, there would be NO WAY for
you to detect the sudden loss of mass for 1 day while it propagates out
to you. Hence gravity travels at the speed of light but can still "catch"
other photons.
>
> Make that two stupid questions:
>
> What do gravity waves do when they hit a black globe? If they get absorbed
> by the black globe, then you can have a nice perpetual motion machine - have
> a small vehicle carrying a black globe which switches on said globe and floats
> up, then switches off the globe and falls down, then switches the globe back
> on before it hits the ground and floats up, etc.
If the globe absorbs gravity (which is actually a fairly weak force) then
you would gain energy - but you wouldn't float up since the only thing
that would cause you to rise would be air pressure caused by air molecules
impacting on the globe which would be absorbed and not transfered to the
ships momentum. Switch it off and you drop like a rock. Back on again
and you...drop like a rock - no change in momentum - you are headed for
a big crash!
>
> : According to the seminar on gravitiation I sat through,
> : gravity propogates at the speed of light.
>
> So, can you tell me why?
You can derive the velocity from the tensor equations. Anything else is
just handwavy. Basically, nothing with mass can travel at or above c and
massless particles (photons, neutrinos, etc) travel at c.
> Are you convinced?
Yes. The effects have been measured. If the current theory's predictions
are wrong, they are wrong by smaller amounts than we can currently
detect, or at least in the areas we have looked.
>
> Einstein's GR called gravitation the curvature of space-time. How does
> a universal geometry propogate?
The is a way of viewing the mathematic. It is a model not necessarily
real life. All math is just a model of reality. It is only important that
it produce accurate predictions not be an accurate representation of what
is going on. In any case, take a sheet and "snap" it. The ripple travels
at a finite speed through the sheet. A propagation of curvature.
>
> Are you sure? I thought that gravity propagated instantaneously. As
No, see above. Gravity is DEFINITELY a finite speed effect.
> far as I know, they're still working on finding a gravity wave/particle.
Yes, they are still trying to detect the effect in a controlled way. They've
had some successes but nothing conclusive (enough) yet. I should point out
that the experiment is not the prove anything (everyone already accepts
their existance) but more to demonstrate. No experiment ever proves
anything, they can only disprove things.
- --
This is a test signature. If this had been an actual signature, something
cute would be written here.
Tony L. Hayes - Lincoln Laboratory - [email removed]