# Gravity Wave & Light Wave Journeys

## Main Question or Discussion Point

Does anyone know about gravity and light waves and how they travel through space?

Are gravity waves affected by space-time curvature?

If a gravity wave and a light wave travel together will the graviton and the photon both follow the geodesics of space-time curvature or will only the photon while the graviton continues in a straight path?

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Does anyone know about gravity and light waves and how they travel through space?

Are gravity waves affected by space-time curvature?

If a gravity wave and a light wave travel together will the graviton and the photon both follow the geodesics of space-time curvature or will only the photon while the graviton continues in a straight path?
Gravity waves (or gravitons, if they exist) travel at the speed of light (the real, perfect one unachievable to even light itself) and are not affected by gravity/space-time curvature at all, they are the only thing that isn't.

Light was originally thought to travel this way, and for almost any applied purpose it does, but recent discoveries suggest that light, unlike gravity, has mass (near infinitesimally small mass) and thus doesn't travel at the actual speed of light, just very very very very very close to it.

Basically, gravity is unaffected by space-time curvature, because it is space-time curvature.

DrGreg
Gold Member
Light was originally thought to travel this way, and for almost any applied purpose it does, but recent discoveries suggest that light, unlike gravity, has mass (near infinitesimally small mass) and thus doesn't travel at the actual speed of light, just very very very very very close to it.
As far as I know, that's not true. What is true is that no experiment can prove beyond doubt that light has exactly zero mass, because there is always some experimental error, but all experiments so far (someone correct me if I'm wrong) are consistent with zero mass.

Gravity waves (or gravitons, if they exist) travel at the speed of light (the real, perfect one unachievable to even light itself) and are not affected by gravity/space-time curvature at all, they are the only thing that isn't.
Basically, gravity is unaffected by space-time curvature, because it is space-time curvature.
This is wrong. Everything is affected by space-time curvature, whether it travels at the speed of light or not. That's because everything lies within space-time, and there's nothing straighter than a geodesic, by definition.

If a gravity wave and a light wave travel together will the graviton and the photon both follow the geodesics of space-time curvature or will only the photon while the graviton continues in a straight path?
If you have the mental picture of space-time as a curved surface within a higher-dimensional space, then any "straight path" would lie outside the surface, i.e. outside of space-time, which makes no sense.

In relativity, a geodesic is a straight path. The only reason it might not look straight to an observer is because the observer is accelerating. That's true even in special relativity without gravity. That sort of curvature of a path is not what we mean by "space-time curvature".

atyy
Exercise 35.15 of MTW says gravitational waves of small amplitude propagating in a curved background has rays that are null geodesics.

Their discussion following Eq 35.28 says that the concept of a small scale ripple propagating in background of large-scale curvature breaks down if the dimensionless amplitude of the wave approaches unity.

However, light rays are bent by gravitational waves, I suppose, because the light rays are affected not by the background spacetime, but by the full spacetime consisting of background and gravitational wave.

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Everything is affected by space-time curvature, whether it travels at the speed of light or not.
Dr Greg is precisely correct, as usual. Another way to visualize gravitational interactions is to note that gravitons are self interacting, unlike photons, and that is what made the formulation of general relativity so complicated....Einsteins tensor formulation was required unlike Maxwells theory for electromagnetic pehnomena...

As for how electromagnetic and gravitational phenomena actually travel through space time...well, we can describe it mathematically, maybe starting with d = vt, but exactly how it occurs is not so clear...nobody knows precisely what space and time are....but Einstein showed us they are not fixed....

so there remain many fundamental questions to be answered by bright people just starting in physics....
Some on this forum, for example, think space is nothing but a mathematical formulation rather than a physical entity....and I assume they would claim trying to describe or analyze "what curves" in GR is fruitless....

This is wrong. Everything is affected by space-time curvature, whether it travels at the speed of light or not. That's because everything lies within space-time, and there's nothing straighter than a geodesic, by definition.
Maybe I'm not getting something here, but how can gravity affect gravity? Then wouldn't black holes cease to exist because their own gravitational fields can escape them? Is there supposed to be some defining difference between gravity and gravity waves, because I don't see it?

DrGreg
Gold Member
Maybe I'm not getting something here, but how can gravity affect gravity? Then wouldn't black holes cease to exist because their own gravitational fields can escape them? Is there supposed to be some defining difference between gravity and gravity waves, because I don't see it?
Gravity waves don't cause gravity: they are the mechanism for propagating a change in gravity, i.e. a change in space-time curvature. If nothing is changing, there are no gravity waves. Crudely speaking, you can think of a gravity wave as "ripple" in the fabric of space-time.

atyy