JGA@MIT-MC@sri-unix (08/02/82)
From: John G. Aspinall <JGA at MIT-MC>
Date: 31 Jul 82 16:17:19-EDT (Sat)
To: space at Mit-Mc
From: decvax!harpo!npoiv!alice!rhm at Ucb-C70
Subject: Re: Gravitational Information
Article-I.D.: alice.824
Via: Usenet; 31 Jul 82 15:48-PDT
References: whuxlb.360
Are you suggesting that the gravitational force acting on a planet
acts in the direction that the sun was one light time ago? If so,
all of orbit theory has to be revised.
Exactly. Perhaps you have heard of A. Einstein?
Or perhaps you suggest that object B suddenly "decides " to move?
Exactly how can this occur, please?
Seriously, this is exactly the effect that causes the perihelic shift
of the planets, notably Mercury, that Einstein explained in 1915.
There are many ways to "explain" it, but one way to look at it is
to regard the motion of any object as being controlled by the local
space-time metric. Then the geodesic in a central force field
(the Schwarzschild metric, for you jargon fans), gives the orbit of
a mass that is negligibly small compared to the central mass.
The solution of the geodesic gives you something that looks like
Kepler's Law, but with an extra term that contains 1/c^2 . This
term disappears in the limit that c approaches infinity, which
is what you'd expect in the classical limit.smb (08/02/82)
And presumably, gravity waves -- which should in principle be detectable, especially from detonation of a supernova -- are a manifestation of the propogation time of gravity (I think). --Steve Bellovin duke!unc!smb smb.unc@udel-relay
rhm (08/02/82)
The Aspinwall theory of gravitation is incorrect. The gravitational force acting on body A at time t (measured at A) depends precisely on the distance and direction of body B at time t (also measured at A). Relativity did not do away with the measurement of time and distance. The perihelion shift of mercury and other planets is a tiny and essentially unrelated effect. Simply stated, the claim that the gravitational force on body A depends on the position of body B one light-time ago is flatly incorrect. It is not and never has been a disputed question.