What is the speed of gravity relative to light

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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


1 What is the speed of gravity relative to light

Van: patrick
Onderwerp: What is the speed of gravity relative to light
Datum: zondag 9 september 2001 19:11

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 ?


2 What is the speed of gravity relative to light

Van: Martyn Harrison
Onderwerp: Re: What is the speed of gravity relative to light
Datum: zondag 9 september 2001 22:14

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.)


3 What is the speed of gravity relative to light

Van: Tom Van Flandern
Onderwerp: Re: What is the speed of gravity relative to light
Datum: maandag 10 september 2001 19:55

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


4 What is the speed of gravity relative to light

Van: Steve Carlip
Onderwerp: Re: What is the speed of gravity relative to light
Datum: dinsdag 11 september 2001 2:34

patrick wrote:

> 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.

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


Created: 26 September 2001

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