# Why doesn't gravity speed up light?

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1. Jan 31, 2014

### Yashbhatt

We know that gravity speeds up a body, like a meteor which enters the earth gets constantly speeded up by earth's gravity.
And from relativity we know that light bends near a massive body because newton's law of gravitation is just an approximation and actually gravity depends on energy and momentum.
So my question is: If a ray of light is aimed exactly at the centre of a body, then will it get accelerated like a meteor? And if does get accelerated, then won't it surpass the universal speed limit of 3,00,000 km/s (approx.)?

2. Jan 31, 2014

### Meizirkki

Gravity does not effect light because light does not have mass.

3. Jan 31, 2014

### A.T.

- To the local free falling observer it will always move at c.

- To a distant observer it will appear to be slowed down. Light needs more time to pass a region containing mass/energy, than a region of vacuum with the same outer dimensions.

4. Jan 31, 2014

### phinds

Well, I'm not sure that's quite right. A BODY moving in a gravitational field appears to a remote observer to move slower but the light from that body hitting the eye of a remote observer will be traveling at c, it's just that it will be red-shifted.

5. Jan 31, 2014

### Staff: Mentor

I expect that A.T. is talking about the Shapiro delay.

Last edited: Jan 31, 2014
6. Jan 31, 2014

### A.T.

No, a massive body fired radially at a mass with v<<c will be accelerated, so it will move faster than without the mass. So it needs less time to pass a region containing mass/energy, than a region of vacuum with the same outer dimensions.

Locally every free falling frame has light traveling at c. That doesn't change the fact that light needs more time to pass a region containing mass/energy, than a region of vacuum with the same outer dimensions.

7. Jan 31, 2014

### phinds

Interesting. I didn't even know about that. Thanks.

8. Jan 31, 2014

### Staff: Mentor

That is wrong, and numerous experiments showed a clear effect.
Light has energy, that is sufficient to interact via gravity.

And for v~c, there should be a speed where the non-relativistic effect (increased speed) and the relativistic effect (Shapiro delay) have the same size, so the total effect is zero.

9. Jan 31, 2014

### davenn

Yes as mfb said, that's wrong

gravitational lensing of light is proof

cheers
Dave

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10. Jan 31, 2014

### Staff: Mentor

And the deflection of starlight by the sun, the Shapiro delay and gravitational redshift of light emitted from massive objects like stars and black hole accretion disks.
Modern atomic clocks are so precise, gravitational redshift from earth can be measured with GPS satellites and even within a few meters of height difference in labs.

11. Feb 3, 2014

### Yashbhatt

What is Shapiro delay?
And someone said that it would appear to move slower for a distant observer but that's not true. The value of c is same for all reference frames.

12. Feb 3, 2014

http://en.wikipedia.org/wiki/Shapiro_delay

13. Feb 4, 2014

### A.T.

Light speed propagates at c in inertial frames of reference, which exist only locally in curved space time. Distant observers can observe light to propagate at other speeds than c.

14. Feb 8, 2014

### Yashbhatt

But is Shapiro delay the thing which prevents light from speeding up further?

15. Feb 18, 2014

### Simon Bridge

There have been no replies for a while.
iirc: You are in High School.

The High-School level explanation is that it is a property of the Universe that all observers will measure the same speed for light in a vacuum. This is why gravity does not speed light up. As a reason it amounts to "just because" but that's the nature of science: some things just are. Finding those things that "just are" is one of the deeper aims of physics.

OTOH:
- because the speed of light is always the same, gravity can make the light change color ;)
- because the speed is always the same, gravity is understood in terms of curving space (+time).

Light is affected by gravity because gravity curves space (+time) - in zero gravity, light travels in straight lines but in gravity light travels in curves.

The Shapiro delay is, loosely, showing you that it takes longer to go in a curve than in a straight line.

The advanced folk are probably having fits over this description - I've left a LOT out so be careful not to draw general conclusions from this. It is quite difficult to do this physics justice at HS level.

16. Feb 19, 2014

### Yashbhatt

Thanks Simon. You always prove helpful.

17. Feb 21, 2014

### A.T.

The OP asked about light traveling radially, in a straight line. There is no light bending here, but there still is a delay compared to empty space.

18. Feb 21, 2014

### Staff: Mentor

There is light bending in 4-dimensional spacetime.
It is pointless to try to understand GR in 3-dimensional space. It just does not work.

19. Feb 21, 2014

### Simon Bridge

Of course, so it is probably worth reiterating that the bending happens in "space (+time)" and that I've "left a LOT out" of the high school level description.
That's all in post #15 but may not be clear enough.

I agree with mfb.
This is where people usually get caught out from staring too hard at rubber sheets.