C

[Guest penomni]

I was sitting with my brother yesterday, and the subject of the speed of light came about. I have always had the impression that, the new theories notwithstanding, light has a constant speed, c. But he said it can change, because gravity pulls it, and hence slows it down. So, what exactly happens when, say, a black hole pulls light to it, or the gravity of a star bends the light?

Thanks!

 

--Darren

[bascule]

Short answer: no. Light always goes c.

[Guest penomni]

And long answer? I told him that it was constant, but he kept saying it wasn't. It's a bases for Einsteins theory of relativity, so how can it change?... I'll go to Google and try and find more info on it.

 

--Darren

[uncool]

According to the general theory of relativity, light has constand speed.

According to quantum physics, velocity cannot be known exactly.

According to recent research, the speed of light has been decreasing (but only slightly).

-Uncool-

[[Tycho?]]

Gravity does not slow down light, it does decrease its energy though by making it longer wavelenght.

 

c is a constant in a certain medium. It always goes at the same speed through vacum. It will move more slowly when moving through air for example (this is not the light itself slowing down, but time added as it is abosorbed and emitted and bounced around).

[bascule]

According to recent research, the speed of light has been decreasing (but only slightly).

 

However that's a systemic effect across the entire universe (caused by inflation?) and not caused by any other influencing location-specific factors

[gaara]

but when it goes thru other medium, like glass, wat is going on. is the light slowing or not.

[Obnoxious]

Ya, light slows when moving through glass and other mediums, and we can know the speed of a moving particle, just not it's absolute position at the same time.

Also, gravity doesn't so much slow down light, but just bends it because it warps space + time.

[swansont]

According to quantum physics' date=' velocity cannot be known exactly.

[/quote']

 

 

Momentum can't be determined exactly. But a photon's momentum is not related to its velocity, so knowing a photon is moving at c is not at odds with the uncertainty principle.

[J.C.MacSwell]

']Gravity does not slow down light' date=' it does decrease its energy though by making it longer wavelenght.

.[/quote']

 

So does a black hole, or just outside the horizon not slow light in the radial direction?

[swansont]

So does a black hole, or just outside the horizon not slow light in the radial direction?

 

I think it appears to slow down because of the curvature. But the local speed is always c. (Where "local" means an area that can be assumed to be flat)

 

I think another way of looking at it is that if you were in a flat geodesic, the only way for the light to be deflected by gravity would be if it slowed down. But the curved geodesic of GR does this without affecting the speed - it's just that the path is longer that what you'd expect it to be. Once you account for the curvature, you see it's travelling at c (but note that this aspect of GR lies outside my area of expertise)

[ydoaPs]

but when it goes thru other medium, like glass, wat is going on. is the light slowing or not.

it is not slowing down. it still goes at c. it just gets absorbed and reemitted. it stops and starts several times, making it look like it slows down by taking longer to cross certain region of spacetime.

[swansont]

it is not slowing down. it still goes at c. it just gets absorbed and reemitted. it stops and starts several times, making it look like it slows down by taking longer to cross certain region of spacetime.

 

The propagation speed of the light is lower, but the photons still travel at c, i.e. the light slows down but the photons don't.

[ydoaPs]

that was what i was trying to say

[Severian]

Momentum can't be determined exactly.

 

What do you mean by that statement? Theoretically at least I can measure the momentum of a particle to whatever precision I like. The uncertainty principle is only telling me that the position of the particle will be uncertain.

 

Of course, one can't measure it infinitely accurately because one never has perfect equipment, but that is hardly a qunatum mechanical restriction.

[swansont]

What do you mean by that statement? Theoretically at least I can measure the momentum of a particle to whatever precision I like. The uncertainty principle is only telling me that the position of the particle will be uncertain.

 

Of course' date=' one can't measure it infinitely accurately because one never has perfect equipment, but that is hardly a qunatum mechanical restriction.[/quote']

 

Irt was in the context of measuring speed; I was merely pointing out that speed isn't the term in the Heisenberg uncertainty principle, it's the momentum.

[Johnny5]

Severian, and Swansont:

 

You are both encountering difficulties which arise from the use of probability theory in the uncertainty relations.

 

Not everyone who reads this thread will understand how to go from the uncertainty principle to probability, and if they cannot do that, they cannot even have an opinion on whether or not the uncertainty relations are true, much less meaningful.

 

Lest anyone challenge this, what is it about the mathematics of probablity, that led some physicists to postulate the many worlds interpretation of QM?

[Hyd]

Gravity is a misconception. It is the warp of space and time and when you "see" a photon change speed, it most likely means that the space the light is entering is the not the same as the space you are in. It is relative to the situation you are in and the situation the photon is in. If you were in that space, when the light left that space, it would appear to change speed. Plus, no matter where you go in the universe, "gravity" is always present, thus it will never be the same EXACT speed, but very very close, because of the how space and time are easily warped.

[Alligator]

Then how do we explain something like this article then: Link

[swansont]

Then how do we explain something like this article then: Link

 

By the paragraph cleverly hidden in the article:

 

The experiments don’t involve stopping the actual photons, or particles of light. Instead, information about the light wave is gradually transferred to specially prepared atoms trapped within a glass chamber, and then turned back into a replica of the original light wave.

 

The light is absorbed in atomic states. But because of the way the atoms are prepared, the coherence information is retained.

[brokenbin]

the speed of light is constant, but if there is enough gravity, such as near a blackhole, light can be bent, but bending it does not mean that the speed is changed, it's just like going in a tunnel. light can be bent in an optical fiber, but it does not get slowed down. the only time it does get slow down is when it is passing through an object with density higher than vaccum, such as water, or air, but even that is by a fraction of a fraction of it's speed, which does not really matter.

[Alligator]

Then if you think about it, light has a primary velocity (speed plus a first direction). What gravity is doing near a black hole is merely causing light to speed up in a second direction. This second velocity would be a mere fraction of the original speed. But one could argue that the original velocity of light never really changes due to gravity.

[Mart]

Can light or photons have a velocity?

[swansont]

light can be bent in an optical fiber, but it does not get slowed down.

 

Optical fibers are made of materials with an index >1, so light is actually slowed down in a fiber.

[Mart]

the only time it does get slow down is when it is passing through an object with density higher than vaccum, such as water, or air, but even that is by a fraction of a fraction of it's speed, which does not really matter.

Check the speed of light in water.