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JJRittenhouse
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Since we know the precise velocity of any photon, does that mean it's location is always undeterminable?
JJRittenhouse said:Since we know the precise velocity of any photon, does that mean it's location is always undeterminable?
jostpuur said:The position of photon is more mysterious than the mere uncertainty principle might suggest, however.
Upisoft said:We know precise speed(in vacuum), not velocity. And yes we cannot know exactly where the photon is.
AFAIK, the photons of visible light do not have enough energy to excite the electrons of a transparent material. The photons just pass through having lower speed.JJRittenhouse said:Isn't the speed still the same in another medium, just interrupted by electron interaction?
Upisoft said:AFAIK, the photons of visible light do not have enough energy to excite the electrons of a transparent material. The photons just pass through having lower speed.
The uncertainty relation is derived from the commutation relation for the position and momentum operators (see this post), but there is no position operator for photons. There are mathematical theorems that tell us that it's impossible to define one.JJRittenhouse said:explain?
What did Feynman think about that?JJRittenhouse said:Well then I guess Feynman was mistaken.
Upisoft said:What did Feynman think about that?
Anyway it is impossible to express transparency as absorption/re-emission, because there will be discrete spectrum of the possible state transitions of the electrons. And we clearly have continuous spectrum.
Fredrik said:The uncertainty relation is derived from the commutation relation for the position and momentum operators (see this post), but there is no position operator for photons. There are mathematical theorems that tell us that it's impossible to define one.
I'm not so sure about that. Can you provide a reference?Tac-Tics said:There is actually no guarantee that light moves at the speed of light. It's more LIKELY to do so, but it's not a rule.
That can be said about massive particles too. To be honest I don't fully understand what the result that there's no position operator for massless particles really means.JJRittenhouse said:So can it be said photons aren't anywhere until they are absorbed?
If the term "photon" is defined by QED, then they are massless by definition, and the claim that they might have mass doesn't make sense. You can however consider the quantum theory of a massive spin-1 field in Minkowski spacetime, and use it to make predictions about results of experiments. There's one such theory for each value of the mass (and of course for each choice of interactions). Experiments that test those predictions to see which values of the mass gives us the best predictions can be thought of as measurements of the mass of the photon, if the term "photon" is now defined by that class of theories, instead of specifically by QED. I think I've read somewhere that if the mass is small enough, the results are predictions are practically indistinguishable from the predictions of QED. (No, I don't know where). So measurements that tell us that the predictions of QED are "at least this accurate" also give us an upper bound on "photon" mass.diazona said:I'm not so sure about that. Can you provide a reference?
No, this can not be said.JJRittenhouse said:So can it be said photons aren't anywhere until they are absorbed?
No, photon direction is uncertain after position measurement.JJRittenhouse said:Since we know the precise velocity of any photon, does that mean it's location is always undeterminable?
Hm, the negation of "isn't anywhere", is "is somewhere", right? So to answer "no" to the question asked, is to answer "yes" to the question of whether the photon "is somewhere" before detection. I would interpret that as having a specific well-defined position, not as being spread out over a smaller region, like the insides of an optical fiber. That's why my answer is "yes".zonde said:No, this can not be said.
JJRittenhouse said:doesn't matter, my question is about the position of photons as they travel.
diazona said:I'm not so sure about that. Can you provide a reference?
Tac-Tics said:Light gets absorbed and emitted over and over ...
Tac-Tics said:Light gets absorbed and emitted over and over (and over and over). You can think of it like a busy man walking to work from the train station. If a pan-handler asks him for money, it'll slow him down, even though when he's walking, he's always walking as quickly as he can without appearing to be in a hurry :)
Originally Posted by JJRittenhouse
Isn't the speed still the same in another medium, just interrupted by electron interaction?
AFAIK, the photons of visible light do not have enough energy to excite the electrons of a transparent material. The photons just pass through having lower speed.
Upisoft said:AFAIK, the photons of visible light do not have enough energy to excite the electrons of a transparent material.
DrChinese said:The controlling rule is in fact the Uncertainty Principle. So answering your question with anything else ends up stretching the language in a fashion which leads to either contradiction or confusion. (Which is why Fredrik is correct.)
It is probably easiest to say that when a photon has a known velocity (momentum actually), it's position is essentially undefined.
DrChinese said: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.
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JJRittenhouse said:Was this discovered after Feynman was writing QED, or was he simply mistaken or using obsolete information?
DrChinese said:This is not accurate, and I probably should have commented earlier. The excitation energy of electrons does not determine whether a material is transparent or not. It is more closely related to field effects of the atomic structure. I.e. the arrangement and type of atoms/molecules. They create a virtual field and this leads to the effects of color we see.
Upisoft said:Is this effect (absorption/re-emission by the field effect of the atomic structure) ever observed? It is hypothesized that light will travel faster than c in Casimir vacuum. Couldn't the field effect of the atomic structure effect the speed of light?
Explanations like that are only speculations.JJRittenhouse said:So the idea of absorption and emission is correct, but not with the electrons within atoms, but the collective nature of the material in which is passes.
Was this discovered after Feynman was writing QED, or was he simply mistaken or using obsolete information?
zonde said:Explanations like that are only speculations.
So I can propose different speculation.
When you go down to length scales of photon wavelength photon is traveling as a wave i.e. it travels many available paths. In material these paths are not straight and when you average over all the paths the photon travels within one wavelength it is shorter then straight path would be. So while you can still say that it travels at c average speed will be less than c.