CAN THE SPEED OF GRAVITY be measured directly through the observation of gravitational lensing effects?
Two scientists who monitored the deflection of quasar light as it passed very near Jupiter argue that they have derived an experimental value for the speed of gravity equal to 1.06 times the speed of light (with an uncertainty of 20%). But two other scientists claim that the lensing experiment only served as a crude measurement of the speed of light itself. Physicists have long taken for granted that the effect of gravitational force, like the effect of electromagnetic force, is not instantaneous but should travel at a finite velocity. A familiar example of this delay is the fact that when we see the sun, we see it as it was 8 minutes ago. Many believe that gravity also travels at the speed of light. The trouble is, while it is relatively easy to gauge the strength of gravity (one can measure gravity even near a black hole, where orbiting matter emits telltale x rays), it is difficult to study the propagation of gravity. Although not as heavy as a star, Jupiter still has considerable g
