Physics Questions People Ask Fermilab

Gravitational Force

You wrote:
Is the gravitational force instantaneous? For example, if I move a mass here,how long will it take for it to affect another mass far away? Does this take a finite amount of time? If it is in fact instantaneous, doesn't this mean that a gravitational signal should be able to exceed the speed of light? This puzzles me. If you do not know can you please forward this to someone who does know?

- Mike

Dear Mike:
Is the gravitational force instantaneous?

Good question, and you guessed the answer yourself. Forces, whether electromagnetic or gravitational, are NOT transmitted instantaneously. The information "that a force is coming your way" is submitted at the speed of light (or, in some cases, slower). Otherwise the very important concept of causality would be violated. And I would hate to hear the sound of broken glass before I dropped the bottle.

Physicists think that there are force carriers for all forces. For the electromagnetic force it is the photon, for the strong force the gluon, for the weak force the weak bosons W and Z. In the case of the gravitational force scientists believe that there must be a similar force-carrying particle called the graviton. No one has seen it yet, and no one will see it any time soon, if it exists. (The interaction of a single graviton would be way too weak.) Physicists hope, however, to detect gravitational waves. (Think of a large number of photons making light waves.) There are several experiments under way around the world. For example, LIGO is the name of one of those experiments located in the United States. Check

Best wishes,

Kurt Riesselmann Fermilab

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