Physics questions about light

[bennyboy]

I guess there should be an "Alex answers. . . " section, since I can guess who will answer this one.


1.  About the passage of light across boundaries of different density and the subsequent bending.  Does a photon's path actually change, or is it the absorption/re-ejection of the photon by atoms that somehow changes the angle?

2.  In double slit experiment involving prisms and semi-reflective mirrors and entangled photons, doesn't the photons' interactions with those matter?  In diagrams of the experiments, it's just said that 1/2 of photons "pass through" the material, and 1/2 do not.  Is that really an accurate description of what happens? Or are photons getting absorbed and re-transmitted a gazillion times before they reach their final destination on a photographic plate or whatever?

[vorlon13]

I don't think a 'given' photon could be absorbed and then 'instantly' re-emitted (recreated, reconstituted, reborn) at the same energy and traveling in the same direction as the original

[ignoramus]

Are we talking photons travelling straight through gravitationally bent space or the natural unpinpointable path of a wave of photons?

[Alex K]

Not an easy question at all, bennyboy!

Imagine you set up an interferometer involving a beam splitting mirror. In principle, one should be able to tell from the movement of the mirror whether the photon passed or was diverted, bc in the latter case the mirror absorbs the change in photon momentum. Quantum uncertainty in the mirror position and momentum must somehow conspire to render this unobservable, but I don't have a satisfactory full answer yet.

As to the other one, yes, from the point of view of feynman diagrams, the change in dispersion relation must come from interactions with the material, and in QED that always means obsorption and reemission. Again, this must happen in a coherent fashion...

[brewer]

Is this behavior (absorb/emit) particle or wave description? Seems like describing the phenom in the OP as a wave gives the best answer.

[Alex K]

Is this behavior (absorb/emit) particle or wave description? Seems like describing the phenom in the OP as a wave gives the best answer.

The full quantum theoretic description does not, I would say, introduce this dichotomy all that explicitly, but it employs what looks more like a wave picture up until the particles are detected using a position measurement, at which point the distribution of the quantum wave simply gives you the probability distribution of your position measurement for the particle. As soon as this position measurement has passed, the particle, if it still exists, is again described by a wave.

The incoming particles of, say, photon-electron scattering, are described as a two-particle wave of a certain size, details depending on the quantum uncertainty of their initial positions, momenta and whether they are entangled.

[brewer]

Is this behavior (absorb/emit) particle or wave description? Seems like describing the phenom in the OP as a wave gives the best answer.

The full quantum theoretic description does not, I would say, introduce this dichotomy all that explicitly, but it employs what looks more like a wave picture up until the particles are detected using a position measurement, at which point the distribution of the quantum wave simply gives you the probability distribution of your position measurement for the particle. As soon as this position measurement has passed, the particle, if it still exists, is again described by a wave.

The incoming particles of, say, photon-electron scattering, are described as a two-particle wave of a certain size, details depending on the quantum uncertainty of their initial positions, momenta and whether they are entangled.

Sorry Alex. You'll need to dumb that down a little for this physics tard.

[Alex K]

Sorry Alex. You'll need to dumb that down a little for this physics tard.

Biiiig particle crush like club, make lightning!

No, kidding.

I was going to say, the maths at almost all stages describes "particles" using waves, where the amplitude of the wave at each point specifies the probability to find the particle there if you look. Only when experiments are described where the position is actually measured, does one treat the particle like a point particle. Whenever it does anything by itself though, it behaves wavey.

[bennyboy]

As to the other one, yes, from the point of view of feynman diagrams, the change in dispersion relation must come from interactions with the material, and in QED that always means obsorption and reemission. Again, this must happen in a coherent fashion...

Are photons considered to have anything like an identity? If, for example, a photon is absorbed by an atom and then reemitted, is it a "new" photon, or the "same" photon, or is the question meaningless in this context? For example, will the reemitted photon, if previously entangled with another, keep that entangled property, or will it kind of "reset"?

I'm sorry, I'm sure these are all first-year n00b questions that I'm asking, but I find it hard to continue on with my philosophical discussions about the nature of reality when I don't understand the best ideas physicists have right now. Your help so far has been VERY educational and very much appreciated, I'm sure not only by me.

[vorlon13]

you don't get the original photon back when an atom absorbs it and later emits one