The Speed Of Gravity

By Johny Jagannath

Light from the sun takes 8 minutes to reach the Earth. If gravity were an effect that travels at the speed of light, then during its transit time [of 8 minutes], the earth and the sun would have moved away from their current positions causing a visibly unstable earth orbit. But observations tell us that the planetary orbits have always been stable. Therefore, Newton postulated that gravity travels instantaneously which is why to him the planetary orbits must be stable. Seems like a simple assumption and his mathematics is also very simple.

On the other hand, Einstein's theory of gravity assumes that gravity travels at the speed of light! This very assumption goes against the thousands of observations that were made since centuries.

It's not just Einstein, many physicists before Einstein tried to set a speed limit to gravity and they worked around the problem of the instability-of-orbits by adding new mathematical entities [to fix a problem that they had created]. The justification for making such a wrong assumption is stated below [to combine Newton's laws with the laws of electrodynamics]. 

At the end of the 19th century, many tried to combine Newton's force law with the established laws of electrodynamics, like those of Wilhelm Eduard Weber, Carl Friedrich Gauss, Bernhard Riemann and James Clerk Maxwell. Those theories are not invalidated by Laplace's critique, because although they are based on finite propagation speeds, they contain additional terms which maintain the stability of the planetary system. Those models were used to explain the perihelion advance of Mercury, but they could not provide exact values.
Einstein takes the same route as his predecessors by adding new mathematical entities to fix the instability introduced in the planetary orbits.
For consistency, just as in the case of electromagnetism, a cancellation of the effect of retardation must occur, but it must now be even more complete—that is, it must hold to a higher power of v/c.  This is exactly what one finds when one solves the equations of motion in general relativity.
One really wonders, whether one has to make provably wrong assumptions [and then come up with complicated mathematics to mask the wrong assumptions] in order to combine Newton's laws with electrodynamics.


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