# About the speed of light and gravity

I read (I think ) that part of relativity theory is that a strong gravitational field distorts the uniform passage of time. If this is true and a lightwave 'travelling' to Earth passes a star near its intense gravitational field (a gravity 'lens') does the gravitational field distort the 'timing' of the speed of light and for a small duration of time the light-wave slows a bit until it leaves the influence of the star's gravity? If this is so could you say the speed of light itself was slightly less while the lightwave was passing through the star's gravity?

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Possible duplicates: physics.stackexchange.com/q/98980/2451 and links therein. –  Qmechanic May 1 '14 at 13:37

We need to be a bit cautious about making over simplified statements in this area because it is a minefield for the unwary. However it is true that as observed by observers far from the star, the gravitional field does affect the elapsed time for objects within it and it also affects the distances travelled by those objects. The net result is that the speed of light does vary with distance from the star.

For more details you might want to look at the questions Photons emitted at the event horizon? and Speed of light originating from a star with gravitational pull close to black-hole strength? for a more detailed discussion of this.

The time dilation is a real effect and has been experimentally measured. In 1971 Hafele and Keating measured the affect of the Earth's gravity on atomic clocks. You ask about this in your comment to user3138766's answer, and the answer is that the time dilation slows everything. If you hovered near a black hole for a while then returned to Earth you would find you were younger than your contemporaries on Earth.

Gravitational lensing is due to the curvature of spacetime. The change in the speed of light in effect changes the refractive index of the space it's travelling through, and this bends the light in an analogous way to the bending of light by a lens.

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Gravitational fields in space distort light. The speed of light is constant, so if light is passing through this gravitational field, it does not slow down but rather time itself relative to the observer, slows down. This is how Einstein explained how the speed of light is constant, regardless of other acting forces.

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If time relative to the observer slows down, would that mean any observable phenomenon in the observer's frame of reference would also slow down? Does the lightwave bend because the gravity distorts the local space-time continuum? Is a distortion in space-time an event where any time measurements are distorted for any observer? –  user128932 May 1 '14 at 4:30

Constant $c$ is the framework of entire Relativistic Physics. You start from here. Meaning, any Time Dilation is derived from constant $c$. You can't go in reverse to alter $c$ using Time Dilation.

Think it another way: When you say someone's time is dilated, you actually compare results of own reference frame to the results of that someone's reference frame. For photons, reference frame isn't defined because there's no reference frame in which photons are at rest. So, you can't really calculate time dilation for photons.

Note: This is a strong answer. Any other answer saying different thing is plain simple wrong.

For fun, if you think about Time Dilation for photons, it's already dilated to Infinity whether it's in Gravity well or not (remember, time is dilated due to speed and they have got topmost speed possible). Now, how much further would you like to push dilation using Gravity?

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Relativistic Physics is a great true Theorem and Quantum Mechanics is a great true Theorem. Both have great success with predicting experimental outcomes yet both Theorems have not been resolved as being mutually 'compatible' or consistent I think. Is this part of what the unified field theory is trying to attain? If there is an issue of any compatibility problems between these great theorems might it be that one or both have to be altered 'slightly'? –  user128932 May 2 '14 at 5:35
@user128932 Any theory can fail in the future. No theory comes with guarantee. Why are you asking this? Your question is concerned about Relativistic Physics. Right? –  S S May 2 '14 at 7:54
What I'm saying is if relativistic physics 'can fail in the future' or it is incomplete maybe the speed of light can be different in certain extaordinary situations. Maybe after the Big Bang in the early Universe c was faster than about 300,000 km/s. There was supposed to be great inflationary expansion of the early Universe. Is Constant c an axiom of relativity physics ? –  user128932 May 3 '14 at 2:30
@user128932 Yes, constant $c$ is axiom of Relativistic Physics. And, the approach is successful so far to describe the universe (except Quantum Realm). –  S S May 3 '14 at 7:05