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I’ve been researching this and can’t find a straight answer.

I’ve heard that the direction of a photon, when it’s emitted, is random. But I’ve also heard that if it’s emitted from point a to b, it takes all paths simultaneously. Other people say that it doesn’t take all paths, but that it’s only a probability. But with a polarizer, doesn’t a photon shift in order to pass(or not shift to not pass)?

I guess to clarify: if a photon is emitted from a light bulb and reaches my eye, will it go straight, or could it kind of swerve(or does it take all possible paths at once)? I keep getting conflicting answers.

Please elaborate on this for me. I’m a noob to this stuff. Plain English would be appreciated! Thanks!

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  • $\begingroup$ Read QED by Richard Feynman, or, better, watch the lecture series from which the book was transcribed. Feynman's quantum electrodynamics is the modern theory of light and it indeed can be interpreted as taking all paths from a source to a receiver. $\endgroup$
    – Ben H
    Commented Nov 29, 2022 at 22:38
  • $\begingroup$ Here is a question about wave functions. The answer is about the wave function of an electron. It describes how an electron propagates. Photons have the same behavior. Does the collapse of the wave function happen immediately everywhere? $\endgroup$
    – mmesser314
    Commented Nov 30, 2022 at 1:51
  • $\begingroup$ to get a feeling how the quantum mechanical particle physics photons build up the classical light wave behavior see this answer of mine physics.stackexchange.com/questions/285142/… $\endgroup$
    – anna v
    Commented Nov 30, 2022 at 6:12

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Photons, to the extent that it makes sense to model them as particles, follow the trajectories predicted by classical ray optics. On the other hand, for phenomena like diffraction, you must model electromagnetic radiation as waves regardless of whether your model is classical or quantum.

The classical version of the "all paths" idea is the Huygens–Fresnel principle. Quantum electrodynamics elaborates on this when interactions with other quantum fields are involved.

The difference between the classical and quantum views is that the intensity at the detector in the classical model is the energy that arrives at the detector, while in a quantum model it is the probability that a photon arrives.

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  • $\begingroup$ Thanks for your reply. Would a single photon spread out like a wave in all directions(some people say it does). Is it in a constant state of superposition with all locations? Like if a photon is emitted in your bedroom, would it actually travel from point a to b in a straight line like how it’s classically depicted? Or is propagating straight just the most likely probability? $\endgroup$ Commented Nov 30, 2022 at 0:38
  • $\begingroup$ What does the light output of your source do? If it spreads, it spreads. If it's a beam, it's a beam. In most cases, it makes little sense to model light as photons until it hits a detector. $\endgroup$
    – John Doty
    Commented Nov 30, 2022 at 1:03
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The photon takes all possible paths from the light source to your eye. Coherently. That last little "coherently" part is a big deal: it means the photon goes in a straight line from the light source to your eye because the straight line is a stationary point in the phase from start to finish. Now if there are mirrors (or any surface, really) or refractive media in between, the photon can bounce or bend, but it is taking the path of least time (Fermat's Principle).

The question is: how straight is straight? Really straight. You can borrow some analysis from the communication industry, the Fresnel Zone (https://en.wikipedia.org/wiki/Fresnel_zone), in which a radio wave propagates from Tx to Rx in a straight line, but with some lateral influence being relevant. Thought radio is classical, the phase and propagation considerations don't differ from quantum mechanics. (If you want to know how a classically coherent wave is made from photons, see "The Glauber State").

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  • $\begingroup$ Thanks so much for your reply. So you’re saying that if a photon is emitted from a flashlight(point A) to my eye(point B), it would go all possible directions simultaneously? If so, does it take the straightest and shortest path due to the high probability of it doing so? Like with the double slit experiment, a single photon can go through two slits. Is that a good example of what I’m asking? Thanks again. $\endgroup$ Commented Nov 30, 2022 at 3:27
  • $\begingroup$ @theguineapigking "it would go all possible directions simultaneously" no, it has a probability at a given time to follow any possible direction (given by the wavefunction describing it which depends on the particular set up boundaries). The actual path looks random for individual photons . It is the accumulation of photons that builds up the classical light ray , so its actual path is not random but weighted with the quantum mechanical probability . see physics.stackexchange.com/questions/285142/… $\endgroup$
    – anna v
    Commented Nov 30, 2022 at 6:18
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A light source can emit single photons or radiate many photons, a light source can emit in a confined direction (laser, flashlight) or many directions (bulb). Photons are best considered as being waves in the EM field and the EM field can handle many many photons due to superposition of waves .... but energy is never lost in the medium (EM field).

Only electron activity puts waves (photons) in the EM field as quanta (one at a time), only electrons (in atoms) eventually absorb photons (as quanta) .... there are likely many photons in space that are travelling energy in the EM field that will never get absorbed.

Nobody knows what a photon actually does in space, we can only absorb them to observe. We can also generate, for example we can bounce a laser off the moon. Based on c and the time taken we determine they travel in a straight line.

An excited electron in an atom (even before real photon emission) is already interacting with the EM field (virtually, forces only) ... thus we can say it is considering all directions .... but it does not mean the real the photon is traveling all paths .... the EM field with the electron considers all paths. The real photon direction likely results from the EM field/electron interaction. In a laser cavity we heavily confine the EM field ... which gives the eventual photons a confined direction.

Maxwell proved that the E field and the M field were tied together and proposed that light was a unified concept of E and M, he also derived a propagation equation for light which would support straight line travel of a confined wave packet .... this accomplishment is as astounding as Einstein's E=mc^2.

So swerving is not likely ..... but since we can't observe the EM field directly we will never know .... we can only observe energy coming out of the EM field as quanta/photons.

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