Does Gravity Have the Power to Slow Down Light?

[Kerry Tawadros]

Hey guys,

I'm itching to find out what you know or theories about gravity and light...

My first question that I pose to you is this..

If the cosmic speed limit is set by light.. then why can gravity affect it?

Consider this.

Light is traveling away from a black hole.. gravity is so immense in the black hole that "not even light can escape".. so for gravity to affect light it must catch it (and therefore move quickly enough to affect it) and slow light down .

It's my understanding that 2 things have to be occurring here.

1: Gravity is moving at the speed of light to affect it?
2: The cosmic speed limit in that area of space is being altered?

At what speed does gravity travel?Thoughts?

 

[tom.stoer]

Sorry to say that, but have you ever considered to look at the mathematical formalism (from which interpretations could be derived) instead of discussing interpretations and intuition w/o having any formal basis?

What you are asking is certainly not "beyond the standard model" but "standard textbook general relativity".

 

[Kerry Tawadros]

My apologies.. should i remove my question?
I am merely here to learn .. and I'm not exactly well educated as to formulas and figures. I just have questions to ask and was hoping to learn something interesting.

 

[tom.stoer]

Have you ever tried to understand general relativity?

 

[Kerry Tawadros]

Yes I have.If general relativity can explain precisely this scenario then I should probably read it more carefully. ( I am probably a bit too green for these forums i understand )

 

[jbriggs444]

so for gravity to affect light it must catch it (and therefore move quickly enough to affect it) and slow light down .

It's my understanding that 2 things have to be occurring here.

1: Gravity is moving at the speed of light to affect it?
2: The cosmic speed limit in that area of space is being altered?

Rather than trying to reason about a black hole, try reasoning about a pulse of light crossing an elevator that is accelerating upward in a shaft. From the standpoint of a rider on the elevator, does that pulse of light follow a straight path or a curved path?

 

[Kerry Tawadros]

Straight

 

[jbriggs444]

Straight

Wrong.

 

[Kerry Tawadros]

Ah i see...

 

[tom.stoer]

Currently I am busy, but later I can give you some hints regarding "speed of light" and "speed of gravity".

 

[jbriggs444]

Ah i see...

Now instead of an elevator accelerating upward at 1g in a shaft in empty space, leave the elevator stationary on the surface of the Earth.

The equivalence principle says that a rider in the elevator cannot distinguish between the two scenarios. If he sees a pulse of light curving in the one situation, he must see it curving in the other as well. This is one of the starting points for General relativity.

 

[phinds]

If the cosmic speed limit is set by light

It is not. I gather from your questions that your knowledge is from pop-science presentations which get pretty much everything wrong to one degree or another. There is a "universal speed limit" that is followed by all massless particles. As far as is known, light is massless and follows the speed limit but if we found out tomorrow that light has a tiny tiny mass, the universe speed limit would not change, we would just conclude that the tiny tiny mass was SO tiny that the slight slowing of photons slightly below the speed limit is not measurable by our instruments. Oh, and I should add, it really isn't correct to say that light travels as photons (as I just did). It travels as a wave and only becomes photons when it interacts with mass.

Light is traveling away from a black hole.. gravity is so immense in the black hole that "not even light can escape".. so for gravity to affect light it must catch it (and therefore move quickly enough to affect it) and slow light down .

You misunderstand gravity. Gravity is a FIELD and does not need to move at all to affect things. Only CHANGES in gravity move and they move at the universal speed limit. Black holes have an Event Horizon which is a mathematical (not physical) sphere inside of which the gravity field gradient is so strong that even light cannot escape. Outside the EH, light can move away from the BH but it gets red shifted.

 

[PeterDonis]

Gravity is a FIELD

Actually, according to GR, gravity is spacetime curvature. That's probably the best viewpoint to adopt in this thread.

Black holes have an Event Horizon which is a mathematical (not physical) sphere inside of which the gravity field gradient is so strong that even light cannot escape.

One reason why the spacetime curvature viewpoint is a better one to adopt in this thread is that the concept of "gravity field gradient" breaks down at the event horizon of a black hole. But the concept of spacetime curvature works just fine. The reason light can't escape from a black hole is that spacetime is curved in such a way that the light cones "tip" inward to the point where the outer edge of the light cones--i.e., the path through spacetime followed by radially outgoing light--is "vertical"; it stays at the same radial coordinate forever.

 

[Mister T]

Light is traveling away from a black hole.. gravity is so immense in the black hole that "not even light can escape".. so for gravity to affect light it must catch it (and therefore move quickly enough to affect it) and slow light down .

I'm no expert, but I don't think that gravity is moving in this example. There are no gravitational waves being emitted by a source, you just have a static gravitational field, like what you have here on Earth only much stronger.

 

[PeterDonis]

I don't think that gravity is moving in this example.

This is correct, but not for quite the reason you give. See below.

There are no gravitational waves being emitted by a source

Correct.

you just have a static gravitational field, like what you have here on Earth only much stronger

The problem with this viewpoint is that it only works outside the black hole's horizon. At and inside the horizon, the concept of "gravity field" doesn't make sense. The only viewpoint you can really use that works in this regime is the spacetime curvature viewpoint. In that viewpoint, gravity never "moves" because spacetime curvature never "moves". It just is--it's a 4-dimensional geometry.

 

[phinds]

Actually, according to GR, gravity is spacetime curvature. That's probably the best viewpoint to adopt in this thread.

You are right, as always

 

[Kerry Tawadros]

Awesome :) thanks.