Is relativity remain unchanged?

[sanjibghosh]

I was a little lost about what you were trying to say. Can you have a go at rephrasing?

cheers,

neopolitan

I have a problem about the speed of light in refractive medium. if the speed of light in that medium is less than 'c'. then the observer who standing outside the medium will realize that the rest mass of photon (moving in the medium) is non-zero. because the speed of the photon is less than 'c' with respect to the outside-observer.
so i conclude that the speed of photon in that medium is not less than 'c' and introduce the idea of absorption and then emission by the atoms in the medium .within this a-e(absorption-emission) there is a little time delay ,and therefore when we calculate the speed of light within the medium we see that it is less than 'c'.
but actually the speed of light is just 'c', microscopically .because when we will see the photon within medium in between two a-e, a-e proses we will still observe that speed is 'c'. during a-e proses the photon is no longer a photon, it was just absorbed by atom.

 

[Ich]

ZapperZ promotes a https://www.physicsforums.com/showpost.php?p=899393&postcount=4" . I'd say it's a bit more complicated even in the case of weak coupling, but I'm not an expert.

 

[neopolitan]

I have a problem about the speed of light in refractive medium. if the speed of light in that medium is less than 'c'. then the observer who standing outside the medium will realize that the rest mass of photon (moving in the medium) is non-zero. because the speed of the photon is less than 'c' with respect to the outside-observer.
so i conclude that the speed of photon in that medium is not less than 'c' and introduce the idea of absorption and then emission by the atoms in the medium .within this a-e(absorption-emission) there is a little time delay ,and therefore when we calculate the speed of light within the medium we see that it is less than 'c'.
but actually the speed of light is just 'c', microscopically .because when we will see the photon within medium in between two a-e, a-e proses we will still observe that speed is 'c'. during a-e proses the photon is no longer a photon, it was just absorbed by atom.

That's certainly what I understand to be the case. In a vacuum the photon travels at c, even in the micro vacuum between the atoms in a crystal lattice, liquid or gas (and even some solids, we can see through paper after all). But yes, when you get absorption of a photon, all bets are off. The photon emitted is not the same one which was absorbed.

You could say "the speed of a photon is c" and then no longer need to say "in a vaccuum".

I tried to find a nice authoritative reference which explicitly discussed this process, but Google pretty much just pointed me back to physics forums (there are a few other threads on the topic, such as https://www.physicsforums.com/archive/index.php/t-104317.html").

cheers,

neopolitan

 

[JesseM]

It's possible this was posted already, but post #4 on the Physics Forum FAQ sticky thread has a good explanation of what's going on in QM terms when photons slow down traveling through a medium.

 

[neopolitan]

It's possible this was posted already, but post #4 on the Physics Forum FAQ sticky thread has a good explanation of what's going on in QM terms when photons slow down traveling through a medium.

If I am reading that right, the photon is not actually absorbed, but "almost absorbed" and re-emitted, with a delay.

the lattice does not absorb this photon and it is re-emitted but with a very slight delay

Is it the same photon? I guess the question may seem a bit non-sensical because there is nothing to distinguish one photon from any other photon, except to allow the informal statement "photons travel at c" and if you could meaningfully say there is photon prior to prior to interaction with lattice ions, photon during interaction with lattice ions and photon after interaction with lattice ions.

An alternative might be to say that "light travels at c when not interacting with matter", which is the same as "in a vaccuum" but indicates that it is the interaction which slows light down, rather than "not being in a vaccuum". (In other words, if the photon did not interact at all with some hypothetical medium, then it could maintain a speed of c even if it was not in a vaccuum.)

cheers,

neopolitan

 

[sanjibghosh]

to,
neopolitan
is it a meaningful question that "if the speed of light in that medium is less than 'c'. then the observer who standing outside the medium will realize that the rest mass of photon (moving in the medium) is non-zero."

 

[neopolitan]

to,
neopolitan
is it a meaningful question that "if the speed of light in that medium is less than 'c'. then the observer who standing outside the medium will realize that the rest mass of photon (moving in the medium) is non-zero."

No, I don't think so. When it is actually travelling, the photon travels at c and you observer will be happy that the photon has zero rest mass. The average speed of the photon across the medium is a different thing if it is being stopped every now and then and semi-absorbed into lattices.

I can see that your observer might make the assumption of a non-zero rest mass for the photon, if ignorant of what was happening in the medium. But this does not amount to realising a "fact" that that the photon has a non-zero rest mass.

cheers,

neopolitan

 

[JesseM]

If I am reading that right, the photon is not actually absorbed, but "almost absorbed" and re-emitted, with a delay.

The language is slightly confusing, but I think the article is saying that the vibrational modes of the lattice do absorb photons, but in some cases they are permanently absorbed and their energy converted to heat, in other cases they are re-emitted in short order.

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

It may also be that the word "absorb" has the connotation of permanent absorption, I don't know. If not, maybe that last bolded sentence should be rewritten to say "So the lattice does not permanently absorb this photon and it is re-emitted but with a very slight delay."

 

[neopolitan]

The language is slightly confusing, but I think the article is saying that the vibrational modes of the lattice do absorb photons, but in some cases they are permanently absorbed and their energy converted to heat, in other cases they are re-emitted in short order.

It may also be that the word "absorb" has the connotation of permanent absorption, I don't know. If not, maybe that last bolded sentence should be rewritten to say "So the lattice does not permanently absorb this photon and it is re-emitted but with a very slight delay."

I guess you would have to confer with ZapperZ.

The bottom line seems to be that there is a significant difference between absorption and reemission by an atom and absorption and reemission by a lattice. I could think that there is a process where capture of a photon is required before it can be absorbed, so it would be either capture-absorb, if the frequency is right, or capture-release, if it isn't. The "checking" process would not be instantaneous (although it may be so damn quick it is close enough for government work, like a Planck time), so delays would thus be added.

If neither lattice nor photon have memory, then the photon would be repeatedly captured and released while in the lattices area of influence. That would result in a significant overall delay. (For this to work the medium could not be nothing but lattice, but rather mostly vaccuum.)

cheers,

neopolitan

 

[v2kkim]

I prefer the wording almost captured to absorb/re-emit, because in case absorb/re-emit it is very hard to guarantee the light keep the same direction, but light keep exactly same direction in a crystal.