# Do photons age in a medium?

According to special relativity, time starts to slow down as we increase our speed and eventually stops once we get to the speed of light. By that logic, photons don't age in a vacuum state as, to us, the time stops for them. However, in a medium, their speed decreases, that means time is 'stationary'. Does that mean they start to age in a medium?

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The proper time of a photon is meanigless. –  jinawee Nov 21 '13 at 10:42
Aging is an ability to undergo some reaction. Such reactions for photons/electrons/protons are not known. Hence even if the time goes they are still infinitely young (not changing) :) –  Asphir Dom Nov 21 '13 at 10:44
Unfortunately I'm a layman. Could you please elaborate? –  Biruman Nov 21 '13 at 10:44
Related: physics.stackexchange.com/q/27794 and physics.stackexchange.com/q/83919, though these do not address the issue of propagation in a medium of refractive index > 1. –  John Rennie Nov 21 '13 at 10:46
@Biruman Photons always travel at the speed of light. They never travel at a different speed –  David H Nov 21 '13 at 10:55

Photons don't have a rest frame, since in all inertial frames they must go at the speed of light.

So the following statement:

By that logic, photons don't age in a vacuum state as, to us, the time stops for them.

is meaningless because one really can't talk about proper time for a photon.

However, in a medium, their speed decreases,

Nope. The net speed of field propagation decreases. However, the photons still move at $c$. When light travels through a medium, it induces electric field vibrations that emit photons in other directions, changing the net field velocity without changing the velocity of the photons.

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If the photons move from the medium back into the vacuum - have they lost energy (relative to their passage though the first vacuum)? –  Rob Nov 21 '13 at 14:26
@Rob It's probably not the same photon. A single photon can lose energy while moving through a medium, but of course that energy is not "lost", it is absorbed by the medium and possibly emitted in the form of another photon –  Manishearth Nov 21 '13 at 14:28
Another way to think about the "age" of a photon is to ask a photon if it's married or single; it's a question that doesn't make sense to ask about a photon. "What's the age of the photon" is a nonsensical question like "What's the marital status of the photon." –  EtherDragon Nov 21 '13 at 17:57
@EtherDragon Not quite: one can ask whether there is an internal state that evolves through different values and which could thus mark or "stand for" an "age". Some particles, although not elementary ones (almost by definition), do indeed age see physics.stackexchange.com/a/69534/26076 –  WetSavannaAnimal aka Rod Vance Nov 23 '13 at 2:35
What is the difference between field velocity and photon velocity? If you quantize a field that propagates with velocity $v<c$, would you still get photons propagating with velocity $c$? How can we see this mathematically? –  Tarek Nov 23 '13 at 6:24
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I will turn my comment into an answer, because the question in the header:

Do photons age?

is very anthropomorphic , and physics is a discipline that discourages interpreting data by use of the anthropic principle.

The photon is an elementary particle. Aging is not a verb to be used with elementary particles in general because

a) they have no identification other than their quantum numbers and thus cannot be tagged to be checked,

and

b) the quantum numbers associated with them like spin, parity, intrinsic angular momentum are invariant in time

as also

c) is their mass.

So whether one is talking of photons or the other elementary particles in the Standard Model, the answer is the same:no meaning can be attached to the verb "age" with respect to elementary particles

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