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Speed of gravity

The speed of gravity is the speed at which changes in the location of an object propagate their gravitational effects to all other objects in the Universe.

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Newtonian mechanics

Newton's mechanical systems included the concept of a force that operated between two objects, gravity. The quantity of force was dependant on the masses of the two objects, with more massive objects exerting more force. This led to a problem: it seemed that each object had to "know" about the other in order to exert the proper amount of force on it. This troubled Newton, who commented that he made no claims to how it could work.

Given two bodies attracting each other, the question then arises as to the speed of propagation of the force itself. Newton demonstrated that unless the force was instantaneous, relative motion would lead to the non-conservation of angular momentum. This he could observe as not being true, in fact the conservation of momentum was one of the observations that led to his theory of gravitation in the first place. He therefore assumed that the speed of gravity was infinite.

Field theories

Michael Faraday's work on electromagnetism in the mid-1800s provided a new framework for understanding forces. In these "field theories[?]" the objects in question do not act on each other, but on space itself. Other objects react to that field, not to the distant object itself, and in this was there is no requirement for one object to have any "knowledge" of the other. With this simple change, many of the philosophical problems of Newton's seminal work simply disappeared while the answers stayed the same, and in many cases the answers were easier to calculate.

Along with this change came the possibility that the field did not move instantaneously. If the field took time to "set up" it would fall prey to the same problems that Newton had originally noticed, although to a lesser degree because the relative motion was no longer a problem, only the motion of the "other" mass which created the field.

General Relativity

In general relativity (GR), the field is elevated to the only real concern. The gravitational field is equated with the curvature of space-time, and propagations (including gravity waves) can be shown, according to this theory, to travel at a single speed, cg.

Measurements of various sorts, notably orbiting neutron stars, have shown that cg must be very close to c, the speed of light.

This finite speed leads to the exact sorts of problems that Newton was originally concerned with. Simple calculations show that in order to avoid seeing these effects over the period of historical astronomical measurements, the speed of gravity would have to be millions of times faster than the speed of light.

In fact the theory itself explains this. The nature of gravity makes these aberrations only appear when v³/c³ is measureable, and due to the magnetude of c, this turns out to be basically invisible in all astronomical measurements made so far.

Experimental measurement

In September 2002, Ed Fomalont and Sergei Kopeikin made an indirect experimental measurement of the speed of gravity, by observing the transit of Jupiter across the line-of-sight of a bright radio source. The speed of gravity, presented in January 2003, was found to be somewhere in the range between 0.7 and 1.2 times the speed of light, which is consistent with the theoretical prediction of general relativity that the speed of gravity is exactly the same as the speed of light.

Other physicists have criticised the conclusions drawn from this experiment on the grounds that, as it was structured, the experiment was incapable of finding any results other than agreement with the speed of light.

See also:

Tom Van Flandern

External links:

New Scientist story on experimental measurement of the speed of gravity (http://www.newscientist.com/news/news.jsp?id=ns99993232)
A critism of the above (http://space.com/scienceastronomy/gravity_speed_030116), another (http://www.newscientist.com/news/news.jsp?id=ns99993282)
Aberration and the Speed of Gravity (http://xxx.lanl.gov/abs/gr-qc/9909087)

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