Physics To The Rescue

[Lorizael]

Calling KrazyHorse, Jon Miller, and hey, why not Rogan Josh too. I was talking to this friend of mine the other day, and the conversation found its way to light and its speed.

This friend of mine began to talk about how it was possible to slow down light, making it actually travel at a velocity that was less than the "speed of light".

I was under the impression that when scientists slowed down light, they were simply causing the photons to be absorbed and emitted by whatever medium the light is passing through so much that the photons essentially lose time. The photons are still moving at the speed of light, but they get sidetracked whenever they hit something. This is what I thought.

He thinks otherwise, and then says that light can also be sped up (which I know can happen under special circumstances). To illustrate this point he tells me that this is why nuclear reactors glow... because the light they give off is going faster than the speed of light. I give him a blank stare, and change the subject.

Is he talking out of his ass, is he grossly distorting some little factoid he picked up along the way, or is he completely and totally accurate and I don't know a thing about physics?

[Jon Miller]

Option A.

JM
(He is talking out of his ass)

[Eli]

To illustrate this point he tells me that this is why nuclear reactors glow... because the light they give off is going faster than the speed of light. I give him a blank stare, and change the subject.

OK.

[Eli]

Seriously though, why does light travel slower in a material? Lorizael's photon colliding explanation makes no sense. What's the real one? How does the epsilon-r gets into the Maxwell equations?

[Lorizael]

Damn, then we were both wrong?

[Eli]

Originally posted by Lorizael
Damn, then we were both wrong?

Well, you can never guarantee 100% absorption, so what happens to the photons that do not get absorbed? From your explanation it follows that the photons lose speed in a non-uniform way and that's plain wrong.

[Lorizael]

True. Well. I'm just glad the other guy's wrong.

But yes. I would like to know how light can change speed too.

[germanos]

IIRC the speed of light is dependent on the medium it's travelling in, nothing special about that.

generally speeking, the speed of light is that in vacuum (and for a specific wavelength as well, some shade of red IIRC )

[Combat Ingrid]

Re: Physics To The Rescue

Originally posted by Lorizael
He thinks otherwise, and then says that light can also be sped up (which I know can happen under special circumstances). To illustrate this point he tells me that this is why nuclear reactors glow... because the light they give off is going faster than the speed of light. I give him a blank stare, and change the subject.

That glow comes from electrons traveling faster than light through the water in the reactor: http://en.wikipedia.org/wiki/Cherenkov_radiation

[Kuciwalker]

Originally posted by Eli
Seriously though, why does light travel slower in a material? Lorizael's photon colliding explanation makes no sense. What's the real one? How does the epsilon-r gets into the Maxwell equations?

Classically, it's because the electrons start moving with the changing electromagnetic field, which absorbs some of the energy and then re-releases it (because the electrons are an accelerating charge). There's a phase shift that accompanies this that essentially slows down the light as it's done repeatedly.

IANAP.

[Kuciwalker]

Re: Re: Physics To The Rescue

Originally posted by Combat Ingrid
That glow comes from electrons traveling faster than light through the water in the reactor: http://en.wikipedia.org/wiki/Cherenkov_radiation

To clarify: the electrons going faster than the speed of light in water. They don't go faster than the speed of light in vacuum.

(Not correcting you Combat Ingrid, it's just your statement is ambiguous.)

[BlackCat]

It could be a a mix of option B and C.

http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html

[Lorizael]

Originally posted by Kuciwalker
Classically, it's because the electrons start moving with the changing electromagnetic field, which absorbs some of the energy and then re-releases it (because the electrons are an accelerating charge). There's a phase shift that accompanies this that essentially slows down the light as it's done repeatedly.

So I wasn't entirely off my rocker. Why do different mediums slow down light more or less?

IANAP.

You are not a physicist?

[Kuciwalker]

Originally posted by Lorizael
So I wasn't entirely off my rocker. Why do different mediums slow down light more or less?

IIRC it's a combination of the density of medium and the freedom of electrons to move within the medium.

You are not a physicist?

No.

[Lorizael]

No you're not a physicist, or no that's not what that means? I know you're not a physicist.