Why doesn't gravity speed up light?

[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.)?

 

[Meizirkki]

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

 

[A.T.]

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?

- 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.

 

[phinds]

- 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.

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.

 

[Nugatory]

- 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.

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.

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

 

[A.T.]

A BODY moving in a gravitational field appears to a remote observer to move slower

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.

but the light from that body hitting the eye of a remote observer will be traveling at c,

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.

 

[phinds]

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

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

 

[mfb]

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

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

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.

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.

 

[davenn]

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

Yes as mfb said, that's wrong

gravitational lensing of light is proof

cheers
Dave

 

[mfb]

gravitational lensing of light is proof

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.

 

[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.

 

[adjacent]

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.

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

 

[A.T.]

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.

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.

 

[Yashbhatt]

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

 

[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.

 

[Yashbhatt]

Thanks Simon. You always prove helpful.

 

[A.T.]

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

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.

 

[mfb]

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.

 

[Simon Bridge]

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.

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.