[boris]

Eddington (innocent of General Relativity) and certain obtuse individuals today argue that gravity must propagate at infinite velocity.

The argument has superficial plausibility but has been convincingly refuted by (e.g.) Prof. Carlip at UC Davis.

The basic argument relies on something like the inverse of the Poynting-Robeson (spelling?) effect. Light moving radially from the sun encounters Earth moving 'forward' and thus falls at a slight angle relative to the radius vector from Earth to Sun. The effect of this is to exert a slight retrograde pressure due to Earth's motion relative to the radial lines from the Sun.

Gravity, it is argued, would have the same vector triangle except that it would be of opposite sign (since gravity 'sucks' and light 'pushes'). This, says the argument, should cause an acceleration of Earth relative to the Sun, which in a remarkably short period of time (a few thousand years) would result in a doubling of Earth's orbital distance, and in millions of years would fling us out of the solar system entirely. Therefore gravity must propagate at infinite speed, QED.

The problem is that it can be shown that under General Relativity, gravity waves are radiated which exactly equal the increased energy due to the 'couple'. The extra momentum, in other words, is leaked away via gravity waves and the solar system remains stable.

--Boris

[RightWhale]

gravity waves are radiated

Assuming this is going on, should we expect to be able to detect these gravity waves somehow? They would be coming from all directions and we ought to notice them if they are 'bright' enough and perhaps even form images. Or are they so weak and smooth that we can't devise instruments sensitive enough to register them?

[RightWhale]

gravity must propagate at infinite velocity

Maybe gravity waves propagate at a finite speed. However, they are unlike light in that: light is generated by various reactions and must be generated or it goes out, ceases, whereas gravity is always just there, static, not necessarily propagating at all. Maybe gravity waves are possible, but just an ancillary phenomenon, something that gravity can do but doesn't need to do.

[Southack]

"Eddington (innocent of General Relativity) and certain obtuse individuals today argue that gravity must propagate at infinite velocity. The argument has superficial plausibility but has been convincingly refuted by (e.g.) Prof. Carlip at UC Davis."

Ah, you're calling my name. Would you settle for near-infinite rather than "infinite speed"?

How does the speed of light squared strike you, so to speak?

Gravity is the Energy of a Mass (G=E/M).

Energy is equal to the Mass times the speed of light squared (E=MC^2).

Solving the two equations for Gravity yields G=C^2.

Well OK, that would all be fine except the units look pretty funny, so how do we *observe* whether or not the math adds up?

Two observable phenomenon come to mind:

1. recent experiments in which Light was reduced in speed to under 50 mile per hour show that Light is NOT bent by Gravity at slow speeds. This would seem to indicate that *speed* matters. And since Gravity hasn't been shown to be influenced by Light, but Light can be shown to be influenced by Gravity, a reasonable person could speculate that Gravity is either a much stronger force or a much faster force than is Light.

2. The Earth and other planets in our Solar System. We already know that our entire Solar System is hurtling through space in one constant overall direction at some great speed. Fine. We also know that the planets orbit the Sun in a horizontal plane. Now, if there is a *lag* between the time that a Gravity wave leaves the Sun and reaches each planet, then it should be a more pronounced lag as one moves from Mercury to Pluto, simply because there is more distance to cover.

Now keeping in mind that all of those planets are hurtling through space in the same overall direction (let's say, North) along with the Sun, we should see planets orbit where the Sun *was*, rather than where the Sun *is*.

If it takes 18 seconds for Light to reach Earth from the Sun, then the Earth would be orbiting where the Sun was 18 seconds ago if Gravity travels at the speed of Light, with an even more pronounced lag in horizontal plane orbit for Mars, Saturn, etc.

But we don't see that. Instead of successive planets orbiting more Southward as one looks away from the Sun, we see the planets pretty well line up on the same horizontal plane, as if Gravity was MUCH faster than Light.

And this observation agrees with what we see in the first experiment above when we slow Light down in the lab, which also seems to agree with the math above.

so in my opinion Gravity propagates at somewhere around 3.4703029E10 miles per second (or is that per second squared??).