[net.space] Infinite Propogation Speed

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