# Why does Light get caught by Gravity, when both are travelling at the Speed of Light?

Gravitational waves travel at the speed of light waves.

1. So then, why do light waves get caught by gravitational waves (eg, black holes)?

2. And if it is about the strength of the photon field, then why are light waves from a stronger light source (eg, a quasar) still caught by the gravity of black holes, instead of passing by ?

• "I was traveling exactly the same speed as the other guy. There's no way we could have collided!" – WillO Jan 21 at 19:25
• The term gravity wave refers to a different thing. The term you want is gravitational wave. However, gravitational waves propagate changes in the gravitational field, they aren't relevant to the effects of a static gravitational field. – PM 2Ring Jan 21 at 19:35
• Gravity causes curvature of space time, light travels in a straight line through the curved space time. – Adrian Howard Jan 21 at 19:45
• @AdrianHoward I think what's tripping up OP is that changes in that curvature also propagate at the speed of light. That's not the same as saying the curvature itself cannot affect something moving at light speed though, which is probably where the misunderstanding sets in. – JMac Jan 21 at 19:52

Lets get things straight.

Gravity waves travel at the speed of Light waves.

Gravity waves are different than gravitational waves. It is gravitational waves you must mean.

They are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light.

So then, why does Light waves get caught by Gravity waves (eg. Black holes) ?

Black holes are not gravity waves ( see link above for definition of gravity waves)

They do not emit gravitational waves unless accelerating, as happens with the merging of black holes.

And if it is about the strength of Photon field,

Photons are elementary particles in the standard model of particle physics. The photon field is part of the quantum field theory that describes elementary particle interaction.

then why are Light waves from a stronger light source (eg. Quasar) still caught by gravity of Black holes on the way ?

If by "caught by gravity" you mean the gravitational lensing observed on light from stars passing close to heavy masses, it is explained by General Relativity.

In general relativity, light follows the curvature of spacetime, hence when light passes around a massive object, it is bent. This means that the light from an object on the other side will be bent towards an observer's eye, just like an ordinary lens. In General Relativity the speed of light depends on the gravitational potential (aka the metric) and this bending can be viewed as a consequence of the light traveling along a gradient in light speed.

Light is not "caught by gravitational waves", so the fact that these propagate at $$c$$ is not relevant. Black holes are not gravitational waves. It is also unclear what is meant by the "strength of the photon field". The frequency of the light is not relevant to the capture.