How to find the photon's path?

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Discussion Overview

The discussion centers around the trajectory of photons in the context of electromagnetic fields described by Maxwell's equations and their treatment in quantum mechanics. Participants explore the classical versus quantum perspectives on photon behavior and the implications of quantization.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that photons can be considered as moving along the Poynting vector, defined as ##\vec{S}=\vec{E}\times\vec{B}##, at the speed of light.
  • Others argue that photons do not have classical trajectories and cannot be described in classical electromagnetic terms, suggesting a need for a quantum treatment.
  • It is noted that photons lack a position operator, leading to the question of whether they can be said to exist at specific locations.
  • Some participants suggest that the initial electromagnetic field should be quantized and that averages could be computed based on this quantization.
  • There is a discussion about the implications of the photon creation and annihilation operators affecting the entire electromagnetic field, with considerations of finite spatial constraints on photon wavelengths.
  • One participant emphasizes that photons cannot be located from first principles and that their behavior is best described through relativistic quantum field theory, where observables relate to detection probabilities rather than classical positions.

Areas of Agreement / Disagreement

Participants generally agree that a quantum treatment is necessary for discussing photons, but there is disagreement on how to conceptualize their trajectories and existence, with multiple competing views on the implications of quantization and the nature of photons.

Contextual Notes

The discussion reveals limitations in understanding photon behavior, particularly regarding the absence of a position operator and the implications of treating photons as quantum fields rather than classical particles.

jk22
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Let be given the ##\vec{E}(r,\theta,\phi,t)## and ##\vec{B}(r,\theta,\phi,t)## fields that are solution of Maxwell equations in vacuum.

How to find the trajectory of the photons ?

Is the photon moving along the Poynting vector, like ##\vec{S}=\vec{E}\times\vec{B}## at the speed of light ?
 
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Photon? What photon?
 
jk22 said:
Let be given the ##\vec{E}(r,\theta,\phi,t)## and ##\vec{B}(r,\theta,\phi,t)## fields that are solution of Maxwell equations in vacuum.

How to find the trajectory of the photons ?

Is the photon moving along the Poynting vector, like ##\vec{S}=\vec{E}\times\vec{B}## at the speed of light ?

Photons appear only after treating those as quantum fields, and they don't have a classical trajectory. Actually, they don't even have a position operator, unlike electrons.
 
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So this question would need a quantum treatment.

There is no frame attached to the photon but could this question be asked in the lab frame ?
 
jk22 said:
So this question would need a quantum treatment.
Yes, the photon is a QM particle. There is no photon in classic EM.
 
Ok. So basically what would the protocol be : one should take the initial electromagnetic field and quantize this by projecting onto the number operator basis ? (Like a wave-packet)

Then computing an average ?

Or does no position operator mean that the photon is everywhere ?
 
jk22 said:
Or does no position operator mean that the photon is everywhere ?

The photon creation and annihilation operators of QED really do affect the whole EM field everywhere in space. Unless you think of the field as contained in some finite box, but then only a discrete set of photon wavelengths are allowed.
 
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jk22 said:
Ok. So basically what would the protocol be : one should take the initial electromagnetic field and quantize this by projecting onto the number operator basis ? (Like a wave-packet)

Then computing an average ?

Or does no position operator mean that the photon is everywhere ?
A photon is not a particle in any sense. It does not even allow to introduce a position observable. In other words it cannot be located at all from first principles. The only way to describe photons is relativistic quantum field theory and the only observables are defined as photon detections by some measurement device. Usually photons are detected using the photoelectric effect. So all you can calculate in theory and observe in nature are the probabilities to detect a photon given the state it's prepared in.
 

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