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If photons transmit the electromagnetic force, which is observable: the photon or the electron? Do we ever directly measure a photon, or do we only measure it's effect on electrons. For example suppose I shine a laser at a wall. A red dot will be visible on the wall. We can understand this in terms of the motion of photons; they build up inside the laser, they are focused to move in one direction, then they scatter off the wall and reach my eye. Alternatively, we could understand this in terms of the motion of electrons. As they transition to different energy states, the electrons induce their neighboring electrons to also make certain allowed energy state transitions. This process continues out of the laser, through the air, against the wall, and into my eyes. In this case, the photons were never directly observed. We observed electrons moving in the laser and electrons moving in the wall, and inferred that there must have been photons in between to transfer the energy and momentum. I guess my question is whether photons are an observable thing or are they just used for book-keeping? Do the photons "exist" in between the electrons?

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    $\begingroup$ You haven't said what would make something real, but electromagnetic interaction can transfer both energy and momentum which is real enough for me no matter how you model them (waves or photons). $\endgroup$ – dmckee --- ex-moderator kitten Oct 28 '14 at 1:46
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    $\begingroup$ Physics is a description of reality, for ontological questions please use this forum: philosophy.stackexchange.com $\endgroup$ – Wolphram jonny Oct 28 '14 at 2:08
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If photons transmit the electromagnetic force, which is observable: the photon or the electron? Do we ever directly measure a photon, or do we only measure it's effect on electrons. For example suppose I shine a laser at a wall

Let us clear up that photons ( and also electrons) are quantum mechanical elementary particles, and classical electromagnetic waves (light) emerges from zillions of photons in synergy.

Also the laser is one of the "proofs" of quantum mechanics macroscopically, i.e. if the quantum mechanical particle nature of light were not there we would not have lasers.

I guess my question is whether photons are an observable thing or are they just used for book-keeping? Do the photons "exist" in between the electrons?

This then becomes a philosophical question. "Observable things" are what our five senses tell us individually, out of which we have a consensus of what is reality and real. A system was then evolved of classifying common "observations", to the point of developing mathematical models for these observations that predicted new observations and fitted with our intuitions. When some observations for the microscopic part of matter stopped agreeing with our mathematical models of physical reality quantum mechanics had to be invented, a mathematical model appropriate for the small dimensions. Photons as elementary particles are part of the "language" developed to comprehend experimental data, and electrons and protons etc are other. A photon is as real as an electron or a proton. We observe all these through experimental proxies ( analogues carrying information). A photon is as real as temperature. The proxy for temperature is a thermometer and eventually a program in our brain interpreting all.

Now that we have established what we mean by real in the microcosm, the red laser light is composed of zillions of photons. These are scattered by the electric field of the atoms on the wall without losing energy, since you see them red going and coming, and some of them raise the energy levels in the cones in the retina of your eye to give the signal to the brain of "red", by way of other electromagnetic interactions traveling on neurons etc etc entering the brain model. It is all models, the quantum mechanical one well validated, the brain one still in research, but it is all successful modeling of what we observe.

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  • $\begingroup$ Ok, so if I understood correctly it's all just stories we tell, since we can't observe any of these things with our senses anyways. From what I understand about QFT we need both electrons and photons to describe EM. If it matches experiment who gives a damn what we call it. Thank you this is food for thought. $\endgroup$ – Tetrafold Oct 28 '14 at 5:34
  • $\begingroup$ Yes, yes, stories but written in beautiful mathematical language with predictive power. Yes, models all the way up, to fit "turtles all the way down",( a primitive model of the earth). $\endgroup$ – anna v Oct 28 '14 at 6:42
  • $\begingroup$ Photons are not scattered by the electric field - they are scattered by electrons. $\endgroup$ – Ruslan Oct 28 '14 at 10:29
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    $\begingroup$ @Ruslan atoms are of order of angstrom, red light is about 7000 angstrom. $\endgroup$ – anna v Oct 28 '14 at 11:50
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    $\begingroup$ @garyp and what are polarizable media except media with a polarization of the electric field? I think we are talking at cross purposes here. Light collectively consists of a great number of photons . That the reflection is coherent enough to transmit an image means that the photons keep their phase relations to each other. classical studies are useful but one could go through the song and dance of photons in an ensemble . In metallic surfaces (behind mirrors), where electrons form bands it is easier to see that it is a collective interaction with many electrons shared by the lattice. $\endgroup$ – anna v Oct 28 '14 at 13:35
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Both Photon and electron are real. Electron has rest mass while photon is a form of energy which gains some mass when it travels at speed of light.

"For example suppose I move some electrons around in my laser, these electrons move the electrons around on the wall, and then these electrons move around some electrons in my eyes, so I see a red spot on the wall" ---- what is this ,i cant understand. Pls correct it.

Yes photons are real, quantum mechanics includes theory of photons , so i guess it is real.

Yes photons are but not the only way of determining the length between the particles using concept of diffraction.

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Fields were first invented as a tool for "book-keeping" or keeping track of how a configuration of charges would effect a charge at a distant point, but it was soon realized that the field itself is much more "real" than simply a convenient mathematical tool.

The first reason we treat the field as a separate real thing is the theoretical fact that you don't need to describe any details about the source charges that creates a certain field if you already have a description of the field itself, a description of only the field itself fully describes the mechanical effects/forces on any other charges.

The second reason that we treat fields as independently real is that the a moving charge at one location does not instantaneously affect all other charges, but rather the change must propagate at a finite speed (i.e. the speed of light). So "moving electrons in your laser" will lose energy while electrons in the wall and/or your eye will gain energy, but not at the same time. So after the electrons in the laser have lost energy, but before the other charges have gained the energy, where has the energy gone? The answer is, it's in the field. If you don't believe in the "real" existence of fields, then what you lose is a world where all effects are local and energy is conserved, because if real fields don't carry energy to distant charges then the alternative is energy can just disappear from any location and reappear anywhere else in the universe after an appropriate amount of time.

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