ech (08/02/82)
floyd!rhm's statement that: 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. is absolutely correct. However, he proceeds to draw (or imply) the erroneous conclusion that the position of B, measured at A at time t, IS the position of B. Sorry, not so. The emphasis is on the phrase "as measured at A," and the information AVAILABLE at A is always retarded by (at least) the speed of light limit. By analogy, imagine that you are chasing a train, in the dark, by the sound of its passing; you are "attracted" by the sound, but will always move toward the point the train was at WHEN THE SOUND WAS EMITTED. Obviously, if sound had an infinite propogation speed, you'd be headed directly for the train, but a finite propogation speed implies that you will head for where it's BEEN. The same applies to the gravitational problem in question: A is attracted to B based on "best information available". That is the actual location of B ONLY if gravitation propogates at infinite speed. By the way, infinite propogation speed of gravity implies the ability to build an infinite-speed "radio:" just vibrate a mass and measure the position of the mass at some arbitrary distance. By modulating the vibration you can transmit a signal at infinite speed. The existence of such a "radio" would blow all omodern cosmology right out the door, by the way, destroying relativistic causality... =Ned Horvath=
Physics:hal (08/03/82)
With respect to the finite propogation time of gravitational information, I have always thought it would be an interesting experiment to do the following: Position yourself at the center of the sun, (without burning up, preferably) Destroy the sun in some non-cataclysmic, instantaneous way. Watch the Earth continue to orbit for ~16 minutes. This would all be done for the sake of Science, of course!! Hal Chambers decvax!pur-ee!Physics:hal