gravity's speed

[Paolo Cavallo]

Dear Matt,

the point you stress is important. I'm a physics teacher but have no special
training in general relativity. My first remark was due to the simple fact that
the graviton has zero rest mass, so it must have velocity c (for real, not
virtual, gravitons). Your remark was very useful and interesting to me, and I
traced it to the source in MTW (alas, not all the way to Einstein). I looked
also in Weinberg's Gravitation and Cosmology and in Landau & Lifshits's Theory
of Fields. Weinberg discards altogether the problem of strong gravitational
radiation, working out the velocity=c result for weak radiation. Landau &
Lifshits state this too, and work out the result for strong radiation, finding
a strange kind of singularity. I'm not able to follow the entire argument.

I would summarize the former as:
1) ordinary gravitational interactions (virtual gravitons?) travel at light
speed - if the Sun change position at an almost uniform speed, the Earth learns
it 8 minutes later;
2) weak gravitational waves travel at light speed;
3) strong gravitational waves, like the ones produced in gravitational
collapse, can be treated as packet waves, so you need to speak separately of
wave speed and group speed - and you can find a group speed larger than that of
light, if I remember it well.
Uff! I'm almost exhausted. If you can tell some mistake or worse, please shout.
Thank you again.

P.S.: All the way from USA to Italy, it came to me today. M&D, I mean!

============================================================
" If patterns of ones and zeros were 'like' patterns of hu-
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        could be represented in a computer record by a long
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Paolo Cavallo              of creature would be represented
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40127 Bologna ITALY                                deaths? "
ton0621 at iperbole.bologna.it         THOMAS R. PYNCHON (1937)

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                               WALLACE STEVENS (1879 - 1955)
============================================================
----------
> From: Matthew B Hoyt <fibers at juno.com>
> To: ton0621 at iperbole.bologna.it; pynchon-l at waste.org
> Subject: gravity's speed
> Date: lunedì 26 maggio 1997 13.19
> 
> 
> On Fri, 23 May 1997 09:14:38 +0200 "Paolo Cavallo"
> <ton0621 at iperbole.bologna.it> writes:
> 
> >Every fundamental interaction in Nature propagates at the speed of 
> >light - not
> >only gravity. It's required by special relativity.
> 
> Well, only sorta kinda.  During BIG gravitation events this changes. 
> [snip] 
>
> As one with a BA in physics I translate to mean that big gravitational
> changes propagate slower
> and faster that the speed of light, if for no other reason than that the
> gravitational change is manipulating
> the spacetime the wave would propagate through.  Though in any case the
> overall speed of
> progation is near the speed of light (faster or slower), rather than
> instanteously (whatever that
> could mean in light of special reletivity).
> 
> Matt