# Does gravity affect the speed of light on Jupiter ?

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1. Feb 14, 2014

### jacob.

i have asked my physics and maths teacher a question which they could not answer, the question follows:-

Does the large gravitational pull of Jupiter reduce the speed of light (on jupiter) ?

Because what i understand so far that a photon has near no mass however it does a micro mass so it should be effected by gravity

2. Feb 14, 2014

### VantagePoint72

No, light always travels at the same speed through a vacuum. It can be deflected by gravity, but not slowed down. Photons do not have "micro mass", they have zero mass.

3. Feb 14, 2014

### Staff: Mentor

No, but that doesn't mean gravity doesn't affect light. It does. See below.

A photon has zero invariant mass (or "rest mass", which is not a very good term since a photon can never be at rest), but it does have energy, and anything that has energy is affected by gravity.

However, "reducing the speed of light" is not a good description of what gravity does to light. A better description is that, as light climbs out of a gravity well (for example, as it goes upward from Jupiter's surface), it redshifts. That changes the light's frequency (redshift means the frequency decreases), but not its speed.

Gravity can also deflect light that is traveling "sideways" past a gravitating body. For example, it has been experimentally verified that the Sun bends starlight that passes close to its surface.

4. Feb 15, 2014

### slakker

This is more adding to the question... is the light actually "deflected" or is it the mass of Jupiter changing space time in such a way that the light appears to be deflected or bend as it travels by Jupiter?

5. Feb 15, 2014

### Staff: Mentor

What's the difference?

6. Feb 15, 2014

### A.T.

It is actually deflected by changing space time in such a way that it is actually deflected.

Here a similar thread:

7. Feb 15, 2014

### slakker

Was trying to reconcile the idea that light always travels in a straight line with it being deflected.

8. Feb 15, 2014

### WannabeNewton

When in free fall, a light ray always travels on a null geodesic of space-time meaning locally (at a single event) its world-line looks straight and the tangent to the geodesic has identically vanishing "length". This is not at all the same thing as saying the spatial path of the light ray is straight or bent. First of all "space" has no invariant meaning but rather only exists relative to a specified space-like foliation of space-time. Secondly, given such a foliation, the "spatial" path of light can certainly be bent whilst still having a locally straight world-line in space-time-the latter of which is an invariant property of the freely falling light ray.

This is not unique to light rays by any means. All the classical Keplerian trajectories, including bound orbits and scattering trajectories, are curved trajectories in "space" but they still correspond to time-like geodesics in space-time which are also locally straight.

9. Feb 15, 2014

### slakker

So is it correct to say then that from the localized view if the light, it is travelling straight but to a distant observer, the light appears to have deflected?

10. Feb 15, 2014

### WannabeNewton

No. I'm not sure how you got that from what I said. First things first you have to distinguish between trajectories in space-time and trajectories in "space". You're confusing the two.

11. Feb 15, 2014

### slakker

I'm confusing something alright...

12. Feb 15, 2014

### A.T.

Light only travels in a straight line though space, when viewed from an inertial frame, which exist only locally in curved space time.

All free falling objects (including photons) advance along geodesics (locally straight lines) in space-time.

13. Feb 15, 2014

### slakker

That's what I thought I said… with "So is it correct to say then that from the localized view of the light, it is travelling straight but to a distant observer, the light appears to have deflected?"

Maybe my terminology is not correct...

14. Feb 23, 2014

In GR, this is correct, as light moves slower close to a large mass (Jupiter) compared to farther away - according to a single clock at any one of the two locations under consideration. This is Shapiro delay/effect.

In SR, you would have to measure the speeds locally, so you would have to use two different clocks, one at each location. According to those two clocks, the local speed of light would be the same constant $c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}$.

15. Feb 23, 2014

### Staff: Mentor

The Shapiro delay is real (it's been measured) but GR does not explain it by saying that light moves slower closer to a larger mass. Instead, we say that the mass warps the nearby spacetime in such a way that the light has farther to travel to get from point A to point B - and of course it takes longer to cover a longer distance.

16. Feb 23, 2014

### pervect

Staff Emeritus
If you measure the speed of light with local clocks and rulers on Jupiter, it doesn't change. Sometimes, though, people talk about the coordinate speed of light. That can vary, but the details depend on the coordinate system you use. So there is some ambiguity in the question, it depends on how you measure the speed.

Photons don't need "mass" to be affected by gravity. Passively, as test particles, they just follow geodesics in space-time. Actively, the fact that they have energy and momentum is sufficient for them to cause gravity according to general relativity - the idea that "mass" is what causes gravity is appropriate for Newtonian theories of gravity, but not for General Relativity, where the source of gravity is the stress-energy tensor.