# Variability of the speed of light

## Main Question or Discussion Point

We all know the constant which stands for the speed of light in a vacuum.
However the speed of light is not constant, we can slow its speed to near zero right here on Earth (Scientific American). My question is, if the speed of light can be slowed, can it not also be increased? Supposed we could formulate an exotic material that could increase the speed of light when it passes through or near the material.
It is by the speed of light that we know the age of our Universe, just find the boundry and calculate the distance in light years it had to travel. But if light speed were variable, the Universe could be an infinite number of shapes.

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Light speed isn't variable in so far as the speed within a vacuum goes.

Light travelling through a vacuum is it's maximum possible speed.

What could we possibly do to speed it up?

Note that when light slows down, it is not done under standard vacuum conditions. The light has to be travelling through a some soft of medium (glass for example).

I heard an explanation once that went, "Any speed that you could travel, light will always go faster." As light travels through a substance that approaches absolute zero, its speed goes down. This suggests light is influenced by radiation (heat). My question revolves around what might happen to light on passing by black holes. Bending light is easy to do but suggests a part of the frequency is faster then the rest. Light that is far, far outside the visible spectrum might get a boast from passing just the right distance from a high energy source.

You're describing a slingshot orbit involving light?

To say it again, the speed of light through a vacuum is constant. You cannot change it.

Light through another medium =/= light through a vacuum. There is nothing 'better' than a vacuum for it to travel through.

You're describing a slingshot orbit involving light?

To say it again, the speed of light through a vacuum is constant. You cannot change it.

Light through another medium =/= light through a vacuum. There is nothing 'better' than a vacuum for it to travel through.
How do you know?

Let's stick to established mainstream science shall we and not your own overly-speculative ideas (recommend you review PF rules on that matter).

russ_watters
Mentor
How do you know?
Every experiment ever performed on the subject agrees with the theory. That's as good as knowing ever gets.

jtbell
Mentor
See section 3 of

Experimental Basis of Special Relativity

Of course, tomorrow somebody might publish the results of an experiment indicating a variable value of c, but one can make a similar statement about everything in physics.

See section 3 of

Experimental Basis of Special Relativity

Of course, tomorrow somebody might publish the results of an experiment indicating a variable value of c, but one can make a similar statement about everything in physics.
I still have trouble dealing with the speed of light is supposed to change in the precense of gravity, but haven't heard anything explaining why or how other than it is just accepted.

In SR, the constant speed of light causes spacetime dialation. IDK why general relavity doesn't need it to be constant to get the same effects..

...As light travels through a substance that approaches absolute zero, its speed goes down. This suggests light is influenced by radiation (heat)...
Where did you see that - can you cite a source? (Edit: you are probably referring to a BEC (Bose-Einstein Condensate), which is kind of exotic quantum mechanical in nature. It cannot be reversed to give superluminal speeds.) In general about the only appreciable effect would be a somewhat reduction in the absorption of light in the medium - velocity in say glass/plastic/crystal being negligibly effected by temperature.
...My question revolves around what might happen to light on passing by black holes. Bending light is easy to do but suggests a part of the frequency is faster then the rest...
By contrast with a material medium, diffraction of light by gravity is totally dispersion free (independent of frequency components). As for getting a 'boost' from passing near a large gravitating mass, try following through this thread: https://www.physicsforums.com/showthread.php?t=462147, - the opposite is true as already pointed out.

Last edited:
Rap
We all know the constant which stands for the speed of light in a vacuum.
However the speed of light is not constant, we can slow its speed to near zero right here on Earth (Scientific American). My question is, if the speed of light can be slowed, can it not also be increased? Supposed we could formulate an exotic material that could increase the speed of light when it passes through or near the material.
It is by the speed of light that we know the age of our Universe, just find the boundry and calculate the distance in light years it had to travel. But if light speed were variable, the Universe could be an infinite number of shapes.
There are two velocities - phase velocity and group velocity. (See wikipedia articles). The phase velocity of light can be less than, equal to, or greater than the speed of light. The group velocity is always the speed of light or less. If you shine a light into a medium, the front part of the beam will never travel faster than the speed of light, its propagates at the group velocity. In that beam, the peaks of the waves may travel faster than the speed of light, or equal, or slower.

There are two velocities - phase velocity and group velocity. (See wikipedia articles). The phase velocity of light can be less than, equal to, or greater than the speed of light. The group velocity is always the speed of light or less. If you shine a light into a medium, the front part of the beam will never travel faster than the speed of light, its propagates at the group velocity. In that beam, the peaks of the waves may travel faster than the speed of light, or equal, or slower.
Remember that that relationship is due to wave mechanics. If, for some reason, the effective speed of light were to be larger than c (interpreted as a constant) then the group velocity times the phase velocity would necessarily equal the square of the higher effective speed of light.

ZapperZ
Staff Emeritus
We all know the constant which stands for the speed of light in a vacuum.
However the speed of light is not constant, we can slow its speed to near zero right here on Earth (Scientific American).

You should also learn about how such speed are measured (i.e. what is "group velocity"?).

It is only when you understand the CORRECT reason can you extrapolate that to something else, not before. Extrapolating something based on faulty knowledge is speculation without valid foundation, which is not allowed in this forum.

Zz.

Remember that that relationship is due to wave mechanics. If, for some reason, the effective speed of light were to be larger than c (interpreted as a constant) then the group velocity times the phase velocity would necessarily equal the square of the higher effective speed of light.
This can formally happen 'in practice' for propagation through media within a region of anomalous dispersion, but still won't allow superluminal communication. The concept of group velocity loses precise definition under such circumstances, so therefore does c2 = vpvg. The Wikipedia article:http://en.wikipedia.org/wiki/Faster-than-light pretty well covers all bases re 'superluminal speed' (including 'superluminal quantum mechanical tunneling'), and the sub-heading "Group velocities above c" is particularly relevant here.

jtbell
Mentor
I still have trouble dealing with the speed of light is supposed to change in the precense of gravity,
In general relativity, the speed of light in vacuum is always c when measured locally. That is, even if spacetime is curved at your location, you will always measure the speed of a light pulse to be c as it whizzes past you, right next to you.

If you measure the speed as (total distance)/(total elapsed time) over a large enough distance that the effects of spacetime curvature are significant, you may get something different from c.

There are two velocities - phase velocity and group velocity. (See wikipedia articles). The phase velocity of light can be less than, equal to, or greater than the speed of light. The group velocity is always the speed of light or less. If you shine a light into a medium, the front part of the beam will never travel faster than the speed of light, its propagates at the group velocity. In that beam, the peaks of the waves may travel faster than the speed of light, or equal, or slower.

fyi
The general physics faq does not contain "group velocity", "wave velocity" nor "signal velocity".

ZapperZ
Staff Emeritus
fyi
The general physics faq does not contain "group velocity", "wave velocity" nor "signal velocity".
That is why I said "You should ALSO learn.... " The FAQ simply tries, in a naive way, to argue why photons, even in a medium, still move at c.

Zz.

Dale
Mentor
If you measure the speed as (total distance)/(total elapsed time) over a large enough distance that the effects of spacetime curvature are significant, you may get something different from c.
To further what jtbell said, over distances where the curvature of spacetime is significant the very meaning of relative velocity becomes ambiguous (parallel transport). So in some sense, only the local measurements of light speed have any physical meaning.

HallsofIvy
Homework Helper
I still have trouble dealing with the speed of light is supposed to change in the precense of gravity
Then you are misunderstanding. The velocity of light changes in that the gravity can change the direction (and velocity is a vector) but NOT its speed.

, but haven't heard anything explaining why or how other than it is just accepted.
Really? Einstein calculated the change in direction very accurately in 1918.[/quote]

In SR, the constant speed of light causes spacetime dialation. IDK why general relavity doesn't need it to be constant to get the same effects..[/QUOTE]
Once again- it does. Speed is constant, not velocity.

In general relativity, the speed of light in vacuum is always c when measured locally. That is, even if spacetime is curved at your location, you will always measure the speed of a light pulse to be c as it whizzes past you, right next to you.

If you measure the speed as (total distance)/(total elapsed time) over a large enough distance that the effects of spacetime curvature are significant, you may get something different from c.
So then the speed of light is different only if you don't take into account the curvature of spacetime or its curved pathway, but is the same along that curvature?

DrGreg
Gold Member
So then the speed of light is different only if you don't take into account the curvature of spacetime or its curved pathway, but is the same along that curvature?
Yes.

Here's an analogy. If you try to draw a map of the Earth's surface on a flat piece of paper, you can do it quite accurately for a small area (say, 10 miles wide), but for a large area (say 1000 miles wide), there's a problem. The map may be accurately to scale in the centre of the map, but it's bound to be distorted near the edges, with either distance or angle or both incorrect relative to the centre scale.

In the same way, spacetime diagrams in general relativity can be accurate over small regions but over larger regions they can get distorted away from the centre (where the observer is). In the centre we can accurately measure the speed of light from the slope of a graph, but further away the slope may be incorrect on the graph. Of course you can draw a map centred anywhere you like which is correct at the centre, but inevitably incorrect somewhere else, because you can't plot-to-scale (a 2-D slice of) curved spacetime on a flat piece of paper.

Ah, so then it only maintains the constant speed of light in higher dimensions, but not in our 4 dimensions of spactime. I was thinking, something as simple as, point A to B just being longer curve on a paper so then the average speed moveing to A to B would be slower, before. That is interesting.

DrGreg
Gold Member
Ah, so then it only maintains the constant speed of light in higher dimensions, but not in our 4 dimensions of spactime. I was thinking, something as simple as, point A to B just being longer curve on a paper so then the average speed moveing to A to B would be slower, before. That is interesting.
You don't really need any extra dimensions than the 4 of spacetime. To continue my analogy, although the surface of the Earth occupies 3 dimensions, its geometrical structure can be described purely in terms of two dimensions (e.g. latitude & longitude). So we think of the surface as 2D with non-Euclidean geometry, no need for a third dimension. Of course the 3rd dimension helps us picture it in our minds but there's no mathematical need for it.

To take my analogy further, suppose distance-in-the-north-south-direction is analogous to distance in spacetime and distance-in-the-east-west-direction is analogous to time in spacetime. Then a flash of light in spacetime could be represented by a line drawn in the northeast direction on the Earth. Any local observer would accurately measure the local direction as northeast (using a local grid of easting metres and northing metres, dividing one by the other). And someone on equator dividing latitude by longitude would also get the right answer as the line crossed the equator, but would get the wrong answer further north because the relationship between degrees-longitude and metres-east varies with latitude.

By the way, I'm not suggesting the geometry of spacetime resembles a sphere. That was just an example. In fact a more realistic analogy, near a star or black hole, would be the flared end of a trumpet. See www.relativitet.se/spacetime1.html

You guys really had me going here, okay, so then the speed of light changeing in a gravitational field is just a lack of considering its newly curved path?

Sounds like now your saying that it actually doesn't change in presence of gravity. It gets confusing because spacetime is curved in our 4 dimensions, but gravity is also caused by curvature in a 5th dimension. Got me thinking that Einstein makeing the speed of light a constant in 5 dimensions wouldn't allow for it to remain constant in 4 dimensions, that would cause GR to predict that it does change in the precense of gravity. We know spacetime is curved in the 4 dimension because of the observable curvature it has near a source of gravity. But, then saying that the space is actually flat in 4 dimensions makes it only appear to be slower, so then it doesn't actually change?

Got myself confused about how many dimension light could be constant in, since an extra dimension would allow extra distance of travel. Then a velocity in one dimension would be different in less dimensions that doesn't consider the extra distance traveled.