[Gary Boldwater]

I don't think gravity does propagate at superluminal velocities. All action at a distance (EM, gravity, light) propagates through the same medium, the ether. If gravity was a summation of waves travelling in both directions (as in standing waves) then it (as a disturbance) could propagate at superluminal velocities. But if you take into account the time it takes to set up a standing wave pattern, it's not superluminal.

[Physicist]

BTW, did you formerly have the screen name "Barry Goldwater"?

[aruanan]

I don't think gravity does propagate at superluminal velocities.

Sure it does. This is why computations of orbits have to treat gravity as a more or less instantaneous force. If gravity propagated at the speed of light, nothing would stay in orbit around the sun for very long. So where you see the sun in the sky is 8.3 minutes behind where it actually is. However, the acceleration of the earth with respect to the visible sun is toward a point in the sky 8.3 minutes into the future, that is, toward where the sun actually is in the sky but is not seen to be because that light has only just left. Measurements put the speed of gravity at 2x10¹⁰ C, a bit faster than LaPlace's lower limit of 108 C.

You can see a more complete description of this here: 2. Gravity and light do not act in parallel directions.

There is no cause to doubt that photons arriving now from the Sun left 8.3 minutes ago, and arrive at Earth from the direction against the sky that the Sun occupied that long ago. But the analogous situation for gravity is less obvious, and we must always be careful not to mix in the consequences of light propagation delays. Another way (besides aberration) to represent what gravity is doing is to measure the acceleration vector for the Earth's motion, and ask if it is parallel to the direction of the arriving photons. If it is, that would argue that gravity propagated to Earth with the same speed as light; and conversely.

Such measurements of Earth's acceleration through space are now easy to make using precise timing data from stable pulsars in various directions on the sky. Any movement of the Earth in any direction is immediately reflected in a decreased delay in the time of arrival of pulses toward that direction, and an increased delay toward the opposite direction. In principle, Earth's orbit could be determined from pulsar timings alone. In practice, the orbit determined from planetary radar ranging data is checked with pulsar timing data and found consistent with it to very high precision.

How then does the direction of Earth's acceleration compare with the direction of the visible Sun? By direct calculation from geometric ephemerides fitted to such observations, such as those published by the U.S. Naval Observatory or the Development Ephemerides of the Jet Propulsion Laboratory, the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes. In other words, the acceleration now is toward the true, instantaneous direction of the Sun now, and is not parallel to the direction of the arriving solar photons now. This is additional evidence that forces from electromagnetic radiation pressure and from gravity do not have the same propagation speed. [emphasis added]