# Why is gravity assumed to be a wave

## Main Question or Discussion Point

apart from the fact gravity propogates at the speed of light, what leads people to assume it is a wave? if it were wouldnt even gravitys line of propogation be bent by proximity to mass, after all refraction and defraction occcur to waves, this would mean the moon would bend the suns lines of gravity propogation.

## Answers and Replies

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Gravity isn't a wave, and neither is electromagnetism. In general relativity gravity is a psuedoforce and electromagnetism is a force. And just like electromagnetic waves (ripples in the electromagnetic field) can be produced (light), there are solutions to general relativity where gravitational waves (ripples in spacetime) are found. Gravitational waves are a phenomenon that is believed to occur, but they've never been observed so we're not even entirely sure they exist. Taking the analogy further, everything we've ever analyzed through quantum mechanics so far that has at one time presented itself as a wave to us has been capable of being described as a particle also. It is because of this that the graviton has been proposed. Since we can find instances in Einstein's equations where gravitational waves occur, it is believed there should be some way to also describe these waves as particles.. gravitons.

pervect
Staff Emeritus
Science Advisor
As a rough anology to compare E&M to gravity

mass (more accurately, energy) in GR is like charge in E&M. It's the "source" of gravity.

The gravitational force between two masses is like the electrostatic force between two charges (the coulomb force).

Moving masses cause frame-draging effects known as "gravitomagnetism", just as moving charges cause magnetism. Gravitomagnetism is, however, very weak. Gravitomagnetism has been detected already in a few experiments, however the best and most accurate test, Gravity Probe B, is currently underway.

Gravity waves are analogous to E&M waves, but instead of electric and magnetic fields gravity waves have gravitic and gravitomagnetic comonents. Note that E&M waves are related to, but different from, the coulomb force between two charges. The same thing is true about gravity and gravity waves.

what im getting at is..........so obviously a gravity wave must propogate at a speed,c, as waves cant occur otherwise, does gravity itself propogate at a speed, by this i mean, if 2 masses appeared from nowhere at the same time, instantly in space, would they both experience gravity of the other instantly or would we have to wait for a split second(c's worth of time over the distance between them)

jim_990 said:
what im getting at is..........so obviously a gravity wave must propogate at a speed,c, as waves cant occur otherwise, does gravity itself propogate at a speed, by this i mean, if 2 masses appeared from nowhere at the same time, instantly in space, would they both experience gravity of the other instantly or would we have to wait for a split second(c's worth of time over the distance between them)
Gravity doesn't act instantaneously, it travels at c, so if the Sun randomly disappeared, we'd stay in orbit for 8 minutes until both the light waves and gravitational effects left. The fact that Newton's theory of gravity is non-local (gravity travels instantaneously) was one of the major indications that it was wrong. The special theory of relativity doesn't mesh well with non-locality.

pervect
Staff Emeritus
Science Advisor
jim_990 said:
what im getting at is..........so obviously a gravity wave must propogate at a speed,c, as waves cant occur otherwise, does gravity itself propogate at a speed, by this i mean, if 2 masses appeared from nowhere at the same time, instantly in space, would they both experience gravity of the other instantly or would we have to wait for a split second(c's worth of time over the distance between them)
Two masses can't appear from nowhere. An attempt to apply the rules of GR to that case fails, just as an attempt to find the electric field of a charge "appearing from nowhere" fails.

However, one can show that if you perturb a gravitational field, that changes in it will propagate at a velocity that's equal to 'c' or less.

You might want to take a look at the sci.physics.faq on this topic:

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html