speed of light

[bwanaaa]

The other thread was winding and looping so I hope the mod fprgives me for this new one.

Why does light slow down in certain media?

This i think is conected to the fact that certain media can also polarize light. Does anyone know the speed of light in a polarizing filter vs. the speed of light in the same but not polarizing material?

 

[rjain]

Basically, it has to do with the light being absorbed and reemitted by the free electrons in the medium. There are also issues to do with interference and wave group velocity, which you should take a course in waves and optics to understand, IMO.

 

[Sahakiel]

Or, you can just take a drive along I-5 during rush hour.

 

[anxman69]

rjain sums it up well without getting into the real nitty-gritty of Lorentz atoms and driven damped oscillators.

If you are really really interested in knowing the physics behind it, I can send you the lecture notes from my Optical Engineering class: Electromagnetic Theory. I warn you ... this is the really "challenging" stuff they warned you about when starting an engineering curriculum

-Ankur

 

[PowerMacG5]

Some substances can actually stop light in its tracks, and then re-emit it when we want.

 

[Fencer128]

Originally posted by: KraziKid
Some substances can actually stop light in its tracks, and then re-emit it when we want.

Hi,

Could you elaborate on your statement a little please. I'm aware of theoretical 3D photonic crystal models that predict light confinement, but I was not aware of a substance (as opposed to structure) that could indefinitely "trap" light.

Cheers,

Andy

 

[PowerMacG5]

Originally posted by: Fencer128

Originally posted by: KraziKid Some substances can actually stop light in its tracks, and then re-emit it when we want.

Hi, Could you elaborate on your statement a little please. I'm aware of theoretical 3D photonic crystal models that predict light confinement, but I was not aware of a substance (as opposed to structure) that could indefinitely "trap" light. Cheers, Andy

Here is a link about a crystal stopping light.

 

[Shalmanese]

The fallacy is that when we talk about "light", we actually mean two different things. PHOTONS, will always travel at c regardless of medium. However, a Light WAVE can have different speeds. In short, when light goes through something other than a vacuum, the photons get absorbed and new photons are created which means that the aggregate waveform slows down. Similarly, with "stopping light" examples, what really happens is that the information from the waveform is captured by the atoms in the gas and then emitted again some interval afterwards.

 

[Fencer128]

Originally posted by: KraziKid

Originally posted by: Fencer128

Originally posted by: KraziKid Some substances can actually stop light in its tracks, and then re-emit it when we want.

Hi, Could you elaborate on your statement a little please. I'm aware of theoretical 3D photonic crystal models that predict light confinement, but I was not aware of a substance (as opposed to structure) that could indefinitely "trap" light. Cheers, Andy

<a class=ftalternatingbarlinklarge href="http://www.nature.com/nsu/020107/020107-2.html" target=blank>Here</A> is a link about a crystal stopping light.

OK, I see now. Basically they've found a way of indefinitely and coherently maintaining an excited population of atoms. When they let the atoms relax the light is emitted coherently with respect to how it was absorbed. I'm not sure on the definition of "stopping light". This seems more like destroying and then creating photons coherently.

Cheers,

Andy

 

[rjain]

Here's an analogy that may help:
Photons are like water molecules
Light waves are like waves in water

They move at different speeds. It's the aggregate motion that determines the speed of the wave, no matter how fast the individual particles move.

Actually, individual photons are themselves also waves, and there is evidence that the force they carry (EM) must travel far faster than the speed at which a photon's wavefunction can propagate. I've proposed that it's the decoherence (which enforces some of the most fundamental conservation laws) that travels "instantaneously" (as determined by the local gravity field?). This would imply that changes in the strength of an EM field propagate at light speed, but changes in the direction propagate, at least, many millions of times faster.

 

[rjain]

As far as polarization goes, that's just a tendency of a medium to absorb the photons that are oscillating in one direction over the same tendency in the perpendicular direction.

 

[User1001]

I guess there's no theoretical medium that can speed up light. Darn.

 

[silverpig]

Originally posted by: User1001
I guess there's no theoretical medium that can speed up light. Darn.

Actually, there is. The ionosphere sort of speeds up radio waves. The catch is that you can't use these superluminal waves to transmit information, so they're more of a curiosity than a useful effect. Again, it has to do with group velocities and such, but the gist is that the signal hits the ionosphere and refracts. When it refracts it undergoes a "rippling" akin to the ripples you see when throwing a rock into a pond. The initial ripple travels at c, but the waves behind it caused by the addition and subtraction of consequential waves travel at speeds greater than c (look into the pond, you will see the outermost ring travelling outwards, and the inner rings seem to travel faster and catch up to the outer one, but they can never pass it).

So light waves can and do travel faster than c, but it's not breaking the speed limit.

 

[rjain]

it all depends how you define "light waves". As I said before, there are two things that are moving here: the composite waves (which are groups of photons) and the photons. The individual photons can't travel faster than c, but the composite waves can have their peaks travel faster. However, their leading edges are limited to travelling at c. The significant distinction here is peak vs. leading edge. A medium can distort or amplify a waveform so that the peak moves forward relative to the leading edge.

 

[PowerMacG5]

Originally posted by: User1001
I guess there's no theoretical medium that can speed up light. Darn.

Well, you can use quantum tunnels through materials, and we have achieved greater than 4 times the speed of light. The major controversy is some scientists believe you can transmit information through this tunnel, and others believe you cannot.

 

[rjain]

Originally posted by: KraziKid

Originally posted by: User1001
I guess there's no theoretical medium that can speed up light. Darn.

Well, you can use quantum tunnels through materials, and we have achieved greater than 4 times the speed of light. The major controversy is some scientists believe you can transmit information through this tunnel, and others believe you cannot.

I keep hearing that quantum tunneling occurs faster than c, but I don't see how the wave front can progress faster than c. This would indicate that quantum tunneling is not actually a "normal" quantum phenomenon.

 

[f95toli]

There is really no such thing as "quantum tunneling" of a particle, what I mean by that is that you have to remember that in order to understand tunneling you have to remember that it is the wavelike nature of the "particle" that makes tunneling possible. If the wavefunction extends through the barrier there will always be a finite probability to find the particle on the "other side" of the barrier.
I know this sounds weird but the point is that until you actually measure on which side of the barrier the particle is nothing is happening (in the static case), the wavefunction just stays where it is so the "particle" (better described as a wavepacket) can be on both side of the barrier, when you measure the wavefunction collapses and you find the particle on either side of the barrier.

It takes a certain amount of time to change the shape (which is related to the probability distribution) and this change must happen at a speed slower than c, this means that if you want to change the tunneling rate or send a message across the barrier you are still limited to speeds <c.

 

[rjain]

f95toli: Yes, that's how I understood quantum tunneling. However, it's been claimed a few times in this forum (the only place I've seen it), that tunneling makes the particle move faster than c.