I Why do we still say “the speed of light” instead of “c”?

[katakuri]

I get it, it sounds cool. But it is a very misleading and sometimes confusing way to represent c.

The speed of light is not constant. When I say that, I’m talking about the speed of LIGHT. Not c. Cherenkov radiation is a result of particles moving through some material (usually water) faster than light does. The particles are moving faster than light in this scenario, but that doesn’t mean that they’re moving faster than c. Anything with mass will always move slower than c, but under certain circumstances can move faster than light.

It just so happens that light travels at c when in a vacuum. But so do x-rays, gamma rays, magnetic fields, gravitational waves, etc…

 

[malawi_glenn]

"the speed of light in vacuum"
x-ray and gamma rays are light
"magnetic fields"
light is electromagnetic wave.

Well we could say that ligth in vacuum travels with the speed of gravitational waves! That would sound dope!

 

[Dale]

Anything with mass will always move slower than c, but under certain circumstances can move faster than light.

This isn't a universally true statement. For instance, in a rotating reference frame distant objects may move very much faster than c, but slower than light.

A better naming would be "the invariant speed", or even better would be "null path". However, although those would better address the issue that you are raising, they would be unfamiliar to many people. So there is no one-size-fits-all solution.

 

[PeroK]

I get it, it sounds cool. But it is a very misleading and sometimes confusing way to represent c.

$c$ is the speed of light (electromagnetic radiation) in a vacuum.

The speed of light is not constant.

The important thing about $c$ is that it is the invariant speed in Special Relativity. That means the speed of light in vacuum is measured to be the same in all inertial reference frames.

 

[bob012345]

I get it, it sounds cool. But it is a very misleading and sometimes confusing way to represent c.

The speed of light is not constant. When I say that, I’m talking about the speed of LIGHT. Not c. Cherenkov radiation is a result of particles moving through some material (usually water) faster than light does. The particles are moving faster than light in this scenario, but that doesn’t mean that they’re moving faster than c. Anything with mass will always move slower than c, but under certain circumstances can move faster than light.

It just so happens that light travels at c when in a vacuum. But so do x-rays, gamma rays, magnetic fields, gravitational waves, etc…

In a medium light is effectively slowed as it is absorbed and remitted by atoms but in the vacuum between interactions it travels at $c$. So when light is light, it travels at $c$. This is how I have always interpreted it but if it is wrong please correct me.

 

[Lord Jestocost]

In a medium light is effectively slowed as it is absorbed and remitted by atoms ...

That's the wrong picture. When light travels through a medium like water or glass, it appears to slow down. The apparent "slower speed" is the result of the superposition of two radiative electric fields: the incoming light, traveling at speed $c$, and the light re-radiated by the atoms in the medium (oscillating charges driven by the incoming light) in the forward direction, traveling at speed $c$, too. The superposition merely shifts the phase of the resulting radiation in a way that would occur as if the light were to go slower than $c$ in medium.

To understand how the apparent or effective speed of light in media comes about, I recommend to read chapter 31 “The Origin of the Refractive Index” in “The Feynman Lectures on Physics, Volume I". On Bruce Sherwood’s homepage (https://brucesherwood.net/) you find an article “Refraction and the speed of light” dealing with this question, too.

 

[phinds]

It just so happens that light travels at c when in a vacuum. But so do x-rays, gamma rays,

Yes, because those are all the same thing ... electromagnetic radiation. What we call "light" is just that portion of the EM spectrum that is directly visible to humans.

 

[PeroK]

In a medium light is effectively slowed as it is absorbed and remitted by atoms but in the vacuum between interactions it travels at $c$. So when light is light, it travels at $c$. This is how I have always interpreted it but if it is wrong please correct me.

If that was happening then the light would be scattered by all those collisions. Like a pinball machine.

 

[malawi_glenn]

@bob012345

 

[bob012345]

That's the wrong picture. When light travels through a medium like water or glass, it appears to slow down. The apparent "slower speed" is the result of the superposition of two radiative electric fields: the incoming light, traveling at speed $c$, and the light re-radiated by the atoms in the medium (oscillating charges driven by the incoming light) in the forward direction, traveling at speed $c$, too. The superposition merely shifts the phase of the resulting radiation in a way that would occur as if the light were to go slower than $c$ in medium.

To understand how the apparent or effective speed of light in media comes about, I recommend to read chapter 31 “The Origin of the Refractive Index” in “The Feynman Lectures on Physics, Volume I". On Bruce Sherwood’s homepage (https://brucesherwood.net/) you find an article “Refraction and the speed of light” dealing with this question, too.

So, the correct answer is light is not slowed by a medium, it only appears like it is.

 

[berkeman]

Thread closed briefly for Moderation...

 

[berkeman]

Thread reopened. Thanks for your patience.

 

[vanhees71]

It's indeed true that the statement that the speed of light in vacuo is identical with the "limiting speed" of relativity is an empirical fact. It boils down to the measurement of the photon mass. This is possible with very high precision and the current upper limit is $10^{-18} \; \text{MeV}/c^2$.