speed of light c

[romesh]

I have a query.

 

I want to know that is the restricted speed for any object according to special relativity is the speed of light or it is 299792458 ms^-1

 

The reason for this query is that I have studied that light travels at different speeds in different mediums.

[Purephysics]

The speed of light in a vacuum is 299,792,458 m/s. (300,000 km/s)

 

According to SR; c is the maximum speed at which all matter, energy, and information can travel.

 

The rate at which light can propergate through a transparent material is less than c.

 

The ratio between c and the speed v at which light travels in a material is the 'refractive index' n. Of the material:

 

n=c/v

 

For visible light through glass the value of n is about 1.5. therfore, light through glass travels as c/1.5 = 200,000 km/s.

[elfmotat]

Light can be slowed to less than the speed of sound in some materials. This says nothing about "c" itself, because "c" isn't the speed of light. It's still just called "the speed of light" for historical reasons. "c" is defined as 299,792,458 m/s. It just so happens that light travels at this speed in a vacuum, but indeed any massless particle travels at this speed. "c" is better thought of as "the universal speed limit" rather than "the speed of light."

[romesh]

Light can be slowed to less than the speed of sound in some materials. This says nothing about "c" itself, because "c" isn't the speed of light. It's still just called "the speed of light" for historical reasons. "c" is defined as 299,792,458 m/s. It just so happens that light travels at this speed in a vacuum, but indeed any massless particle travels at this speed. "c" is better thought of as "the universal speed limit" rather than "the speed of light."

 

 

So what you actually said implies that we can travel faster than the speed of light (in which the speed of light i less than c;for example we can travel at a speed >speed of light in water) in some medias.

 

Correct me if i am wrong.

Edited August 27, 2012 by romesh

[timo]

So what you actually said implies that we can travel faster than the speed of light [...] in some media.

 

Correct me if i am wrong.

You're right. But being faster than light in any medium is just not what is meant by saying that you cannot exceed the speed of light. There, "speed of light" refers to the speed of light in vacuum. You did discover an instance of inaccurate/incomplete/ambiguous use of language.

Edited August 27, 2012 by timo

[IM Egdall]

I believe that in a medium such as glass, light is absorbed by an electron in a molecule within the medium and is then reemitted. It is then absorbed by an electron in another molecule and reemitted. Etc. It takes some time for the electron to reemit a photon(actually a new photon) each time. This is why light travels slower in glass.

 

But the photon still travels from electron to electron at the speed of light.

Edited August 27, 2012 by IM Egdall

[timo]

I believe that in a medium such as glass, light is absorbed by an electron in a molecule within the medium and is then reemitted. It is then absorbed by an electron in another molecule and reemitted. Etc. It takes some time for the electron to reemit a photon(actually a new photon) each time. This is why light travels slower in glass.

 

But the photon still travels from electron to electron at the speed of light.

That's a very popular picture. I believe it's wrong: You inherently assume that the vacuum solution of the electromagnetic field is a proper Ansatz for the electromagnetic field in a medium, even for cases where the photon wavelength far exceeds the interatomic distance. In a medium, I wouldn't even speak of "photons" at all (as photons, by the physics I learned, are excitations around a vaccuum state of the electromagnetic field).

[mississippichem]

That's a very popular picture. I believe it's wrong: You inherently assume that the vacuum solution of the electromagnetic field is a proper Ansatz for the electromagnetic field in a medium, even for cases where the photon wavelength far exceeds the interatomic distance. In a medium, I wouldn't even speak of "photons" at all (as photons, by the physics I learned, are excitations around a vaccuum state of the electromagnetic field).

 

I agree.

 

I think that if sequential absorption/emmission was the case, the Stokes-shift that accumulated after several absorptions/emmisions would become large which is obviously not the case we observe for a light shown through clear glass.

 

Do you think Mie and Rayleigh scatteting have something to do with the slowing of light in a medium? I don't know.

[md65536]

That's a very popular picture. I believe it's wrong: You inherently assume that the vacuum solution of the electromagnetic field is a proper Ansatz for the electromagnetic field in a medium, even for cases where the photon wavelength far exceeds the interatomic distance. In a medium, I wouldn't even speak of "photons" at all (as photons, by the physics I learned, are excitations around a vaccuum state of the electromagnetic field).

 

But wouldn't you have to treat this properly wrt. QED, and find that the probability of a photon emerging from a medium at a given location with a given direction, after a photon enters the medium at a given location/direction, is the sum of probabilities of all the possible paths such a photon could take through the medium? That is, every possible sequence of absorption and re-emission by electrons.

 

Then, while you wouldn't speak of specific "photons" in the medium, you also wouldn't speak of specific "electrons" either, because light behaves as if it's doing the average of all specific possibilities. I think that it's fair to speak of the light traveling at a speed of c through the medium, but only in a "sum of probabilities" way. The observed behavior of light traveling at a speed < c is only the average, macroscopic description of what happens at a quantum level.

 

[timo]

But wouldn't you have to treat this properly wrt. QED, and find that the probability of a photon emerging from a medium at a given location with a given direction, after a photon enters the medium at a given location/direction, is the sum of probabilities of all the possible paths such a photon could take through the medium?

You certainly should treat the case properly to arrive at the correct results. What I was trying to express is that I am not convinced that "photon in vacuum that is absorbed and re-emitted by electrons from time to time" is the proper treatment or point of view. Or with respect to what you said: I was wondering if the common folklore about "photons in media" is the proper way to go to calculate the "probabilities of all possible paths".

[md65536]

Or with respect to what you said: I was wondering if the common folklore about "photons in media" is the proper way to go to calculate the "probabilities of all possible paths".

It's not the simplest treatment. I think Feynman explains (in the Douglas Robb Memorial Lectures for example) that considering all possible interior paths of light passing through a lens is equivalent to considering only what happens at the surface points, which is a lot simpler.

 

I'm not sure what I'm trying to say. I think it's that, if there are equivalent treatments (which always arrive at the same answer and have no differing measurable effects), then it doesn't make sense to say that one interpretation is right and the other is wrong. And I'm not sure about this, but I believe there is an interpretation that involves light traveling at c through a series of interactions within a medium, which provides the correct answer given a proper QED treatment.

 

 

If so, then whether light "actually" travels at c through a medium, or is actually slowed, is only a matter of interpretation.

[EMField]

Because mediums possess dielectric properties and dielectrics controls the speed of light.

 

http://en.wikipedia.org/wiki/Dielectric

 

So much so that "whether light "actually" travels at c through a medium, or is actually slowed, is only a matter of interpretation" has already been answered.

 

http://science.nasa.gov/science-news/science-at-nasa/2002/27mar_stoplight/

[swansont]

Because mediums possess dielectric properties and dielectrics controls the speed of light.

 

http://en.wikipedia.org/wiki/Dielectric

 

So much so that "whether light "actually" travels at c through a medium, or is actually slowed, is only a matter of interpretation" has already been answered.

 

http://science.nasa.gov/science-news/science-at-nasa/2002/27mar_stoplight/

You do realize that "stopped light" is just light that has been absorbed and then re-emitted coherently, right? The light doesn't actually stop.