| 1 patrick |
What is the speed of gravity relative to light | zondag 9 september 2001 19:11 |
| 2 Martyn Harrison | Re: What is the speed of gravity relative to light | zondag 9 september 2001 22:14 |
| 3 Tom Van Flandern |
Re: What is the speed of gravity relative to light | maandag 10 september 2001 19:55 |
| 4 Steve Carlip | Re: What is the speed of gravity relative to light | dinsdag 11 september 2001 2:34 |
There is a paradox in that black holes radiate gravity to cause stars to rotate around them but the perceived wisdom is that gravity travels at the speed of light and as such gravitational waves can be created as matter dissapears into black holes. Millions are being spent on gravitational wave detectors but I cannot find any reference to any experiment which sets out to measure the speed of gravity relative to light. any Ideas any one ?
They all laughed, when patrick@deeperspace.com (patrick) said:
| > | There is a paradox in that black holes radiate gravity to cause stars to rotate around them but the perceived wisdom is that gravity travels at the speed of light and as such gravitational waves can be created as matter dissapears into black holes. Millions are being spent on gravitational wave detectors but I cannot find any reference to any experiment which sets out to measure the speed of gravity relative to light. any Ideas any one ? |
Yeah, go ask in sci.physics.relativity rather than sci.astro. It isn't about speeds and things, but warped spacetime.
Addressing this in a newtonian fashion by talking about escape velocity won't get you anywhere (or so I would imagine.)
patrick@deeperspace.com (patrick) writes:
| > | There is a paradox in that black holes radiate gravity to cause stars to rotate around them but the perceived wisdom is that gravity travels at the speed of light and as such gravitational waves can be created as matter dissapears into black holes. Millions are being spent on gravitational wave detectors but I cannot find any reference to any experiment which sets out to measure the speed of gravity relative to light. any Ideas any one ? |
The six experiments that attempt to answer this question are described in "The speed of gravity - What the experiments say" in Phys. Lett. A, v. 250, pp. 1-11 (1998), also available at http://metaresearch.org , "Cosmology" tab, "Gravity" sub-tab. That paper also contains discussion of the black-hole case.
The "speed of gravity" is also the subject of a current thread in sci.physics. You might find that discussion of interest. -|Tom|-
Tom Van Flandern - Washington, DC - see our web site on replacement astronomy research at http://metaresearch.org
patrick
A direct test of the speed of gravity is quite difficult. We
can accurately measure the speed of light because we have
sources of light that we can turn on and off. We don't
have this luxury with gravity, and the interpretation of
observations of gravitating sources depends on theory as
well as experiment.
The standard example of this is electromagnetism. If you
observe the electric field of a charge moving at constant
velocity, you will find that it points to the ``instantaneous''
position of the charge. If all you knew about electric fields
was Coulomb's law, you would interpret this as meaning
that the electric field propagates instantaneously. But
Coulomb's law is wrong, of course, or at least incomplete,
when applied to a moving charge; you really need the full
set of Maxwell's equations. If you do the calculation, you
find that the direction of the field depends on the charge's
velocity as well as its position, and that the field points to
an ``extrapolated'' position of the charge even though it
propagates at the speed of light.
For electromagnetism, this is testable. All you have to
do is to abruptly stop the charge, and see whether the field
instantly stops changing. It doesn't. Instead, for a while
it continues to point to the ``extrapolated'' position, where
the charge *would have been* if it hadn't stopped, even
though the charge is never actually at that position. Then,
after a delay (equal to the light-travel time from the charge
to the observer), the field abruptly ``corrects'' itself and
points to the place where the charge has actually stopped.
You observe this phenomenon every time you turn on a
radio---the radio wave that moves the electrons in your
antenna is just this ``correction'' of the electric field of
the charges in the transmitter.
The reason I went through this example is to show why the
corresponding observation is so hard for gravity. It's not
just that it's hard to swing around a big enough mass to
see how its gravity changes; it's also that whatever you
swing the mass with produces its own gravitational field,
which tends to cancel the effect. (The center of mass of
the ``mass/thing-to-swing-the-mass-with'' system can't
move, so to first order the gravitational field remains
constant.)
There is one recent proposal for a direct test of the speed
of gravity---see S. Kopeikin, Astrophys.J.556:L1-L6,2001 ,
http://xxx.lanl.gov/abs/gr-qc/0105060
---by looking for the effect of changes in
Jupiter's position on the time delay of a passing light ray.
I'm not sure if it's practical, but it might be.
Steve Carlip
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Millions are being spent on gravitational wave detectors
but I cannot find any reference to any experiment which
sets out to measure the speed of gravity relative to light.
Created: 26 September 2001