5 ms·

doesn't refraction also require the speed of light to change? I'm guessing you mean something subtler.

danbruc 11y ago
In case of refraction it is a macroscopic illusion. If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later. The macroscopic speed of light is just the result of absorption and emission probabilities and the microscopic geometry of the material. But there is another layer below but I am less sure about that, it may be (slightly) wrong. In quantum electro dynamics photon take, figuratively spoken, all possible paths from source to detector, also paths that require going slower and faster than the speed of light. All those possible paths interfere with each other and the net result is that the photon seems to move with the speed of light with very high probability. But really take that with a large grain of salt.
- cperciva 11y ago
  If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later. That's a story we tell kids who ask too many awkward questions. If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. My understanding is that the real answer involves photons gaining inertia via coupling with particles in the material they are passing through (cf. Higgs) ... but I'm probably mangling that explanation horribly.
  - danbruc 11y ago
    I shouldn't have written »absorbed by atoms and reemitted later«, that is really a different thing, interacting with the atoms is better. The superposition of all possible paths and interactions results in a wave function that effectively travels slower than the speed of light.
  - wnoise 11y ago
    > If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. Uh, we do. The imaginary portion of the index of refraction near a resonance generally looks roughly like a gaussian, and the real portion does have jumps that look a bit like tanh.
  - deleted 11y ago
    [deleted]
- mcguire 11y ago
  Do photons absorbed and reemitted follow the trajectory of the original photon?
  - danbruc 11y ago
    Just to be clear, I am not a physicist, only a interested layman. As I understand it currently, photons don't travel any path. They are emitted somewhere at some point in time and detected somewhere else at another point in time, what they did in-between is not a meaningful question, at least in the classical sense of asking for a path the photon took. When we want to calculate the probability of observing the photon at a specific place at a specific point int time, then we consider all possible paths - including the photon traveling to the edge of the universe and returning from there, including the photon going into the future and coming back - and sum them up. There is a relatively simple formula for every possible path and those possibilities interfere when we sum them up, some path amplify each other, some cancel out, in the end you have a probability distribution in space and time telling you where it is likely and where and when it is unlikely to find the photon in case you tried to observe it there. What the photon did in-between, we don't know, is a meaningless question, ... at least I can't tell.
  - marcosdumay 11y ago
    They often do. Momentum must be conserved (despite this article...)
- TrevorJ 11y ago
  IS this what is meant when people say the speed of light in x or y material is less than in a vacuum?
  - danbruc 11y ago
    Yes, due to the interaction of the photon with the atoms of the material the photon takes longer to move through certain distance of a material than through the same distance in vacuum.
zodiac 11y ago
"speed of light" normally means "speed of light in a vacuum" (eg see https://en.wikipedia.org/wiki/Speed_of_light https://en.wikipedia.org/wiki/Speed_of_light)
- lunchTime42 11y ago
  vacuum is not empty? So its speed of light in quantum foam?
  - zodiac 11y ago
    ¯\_(ツ)_/¯
kaybe 11y ago
Fun fact: Cherenkov light is emitted when particles move faster than the speed of light in a given medium. It can be seen in nuclear reactors in blue. Tho is of course, in line with current theory, only possible because the speed of light in a medium is lower than the speed of light in vacuum, which particles cannot reach.

In case of refraction it is a macroscopic illusion. If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later. The macroscopic speed of light is just the result of absorption and emission probabilities and the microscopic geometry of the material. But there is another layer below but I am less sure about that, it may be (slightly) wrong. In quantum electro dynamics photon take, figuratively spoken, all possible paths from source to detector, also paths that require going slower and faster than the speed of light. All those possible paths interfere with each other and the net result is that the photon seems to move with the speed of light with very high probability. But really take that with a large grain of salt.

cperciva 11y ago
If you look closely enough, you will see photons traveling at the speed of light between atoms, getting absorbed by atoms and reemitted later. That's a story we tell kids who ask too many awkward questions. If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. My understanding is that the real answer involves photons gaining inertia via coupling with particles in the material they are passing through (cf. Higgs) ... but I'm probably mangling that explanation horribly.
- danbruc 11y ago
  I shouldn't have written »absorbed by atoms and reemitted later«, that is really a different thing, interacting with the atoms is better. The superposition of all possible paths and interactions results in a wave function that effectively travels slower than the speed of light.
- wnoise 11y ago
  > If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. Uh, we do. The imaginary portion of the index of refraction near a resonance generally looks roughly like a gaussian, and the real portion does have jumps that look a bit like tanh.
- deleted 11y ago
  [deleted]
mcguire 11y ago
Do photons absorbed and reemitted follow the trajectory of the original photon?
- danbruc 11y ago
  Just to be clear, I am not a physicist, only a interested layman. As I understand it currently, photons don't travel any path. They are emitted somewhere at some point in time and detected somewhere else at another point in time, what they did in-between is not a meaningful question, at least in the classical sense of asking for a path the photon took. When we want to calculate the probability of observing the photon at a specific place at a specific point int time, then we consider all possible paths - including the photon traveling to the edge of the universe and returning from there, including the photon going into the future and coming back - and sum them up. There is a relatively simple formula for every possible path and those possibilities interfere when we sum them up, some path amplify each other, some cancel out, in the end you have a probability distribution in space and time telling you where it is likely and where and when it is unlikely to find the photon in case you tried to observe it there. What the photon did in-between, we don't know, is a meaningless question, ... at least I can't tell.
- marcosdumay 11y ago
  They often do. Momentum must be conserved (despite this article...)
TrevorJ 11y ago
IS this what is meant when people say the speed of light in x or y material is less than in a vacuum?
- danbruc 11y ago
  Yes, due to the interaction of the photon with the atoms of the material the photon takes longer to move through certain distance of a material than through the same distance in vacuum.

zodiac 11y ago

"speed of light" normally means "speed of light in a vacuum" (eg see https://en.wikipedia.org/wiki/Speed_of_light https://en.wikipedia.org/wiki/Speed_of_light)

lunchTime42 11y ago
vacuum is not empty? So its speed of light in quantum foam?
- zodiac 11y ago
  ¯\_(ツ)_/¯

kaybe 11y ago

Fun fact: Cherenkov light is emitted when particles move faster than the speed of light in a given medium. It can be seen in nuclear reactors in blue. Tho is of course, in line with current theory, only possible because the speed of light in a medium is lower than the speed of light in vacuum, which particles cannot reach.