Index Key:  PHY064
Author:     Planck
Subject:    Can information out speed light?
Text:       According the theory of relativity, nothing can surpass the speed 
of light, even the flow of information.  But what of events that necessarily 
take place instantaneously?  Ex:  An object moves, and the gravitational force 
instantly changes proportionately a light-year away.  I believe that Einstein 
resolved this difficulty in terms of gravity's warping space - time, but I 
still do not understand his explanation.  And on a more basic level, if I were 
to shake a move a one light- year long pole, would not the tip move 
instantaneously, beating light by roughly 12 months?  Or would some sort of 
spring action occur just to thwart my attempt of disproving relativity?

Response #:  1 of 1
Author:      A. Smith
Text:        Actually, it turns out that all forces, including gravity, the 
electromagnetic and other forces, are what is called "retarded".  That means 
that although they look like they act at a distance, they do not act 
instantaneously at a distance.  In your example, the gravitational field one 
light-year away would not start changing until exactly one year later.  Time-
dependent forces are tricky.  Concerning a pole that was extremely long - how 
do you suppose forces move from one end to the other?  They actually cannot 
move any (or at least not much) faster than the speed of sound in the pole.  
Very rigid poles have very high speeds of sound but far less than the speed of 
light.