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?
[Physics] How is a photon measured
electromagnetic-radiationelectromagnetismelectronsphotonsquantum-field-theory
Related Solutions
You have to realize that when we are speaking of photons, we are speaking of elementary particles and their interactions are dominated by quantum mechanics, not classical mechanics, and in addition special relativity is necessary to calculate anything about them.
In general, we know about elementary particles because we observe their traces in detectors for almost a hundred years. We never see an electron, or a proton in the way we see a particle of dust.
This is the most visual detector, a bubble chamber photo of electromagnetic events.
Here we see some electromagnetic events such as pair creation or materialization of high energy photon into an electron-positron pair (green tracks), the Compton effect (red tracks), the emission of electromagnetic radiation by accelerating charges (violet tracks) (bremsstrahlung) and the knock-on electrons or delta ray (blue tracks)
Now lets see about your questions:
1) How did we arrive at "electrons exchange virtual photons and that's the cause of the electromagnetic force between them" from merely observing electrons absorbing or emitting photons?
That is not the way we arrived at this conclusion. A very large number of controlled scatterings, which is what this picture shows, of electrons on matter have been studied over the years and the theoretical framework of calculating the probability of the scatter and the angular distributions has been very well developed for years. This involves mathematics which cannot be handwaved. To start with, the crossection of an electron scattering on an electron can be written in a series of convoluted integrals which can be pictorially represented by Feynman diagrams. In those Feynman diagrams, the propagators of the interaction between the incoming and outgoing particles can be thought as virtual photons because they carry the quantum numbers of the photon but are off mass shell. So it is a convenient mathematical identification which defines virtual photons.
Anything between the incoming vertices and the outgoing vertices is virtual, and their reality depends on the correct representation of the quantum numbers for the exchanged particle, in this case photon quantum numbers.
2) If electrons throw photons at each other doesn't that mean that they should only scatter (repel)? If that is so why do magnets attract?
Virtual photons are not like balls, they are off mass shell, they are useful a mathematical construct .There is an interesting analog though where two boats throwing balls at each other represent the repulsive forces, and boomerangs the attractive.
Do these two phenomena happen here on earth naturally (no LHC or other particle accelerators), in the upper atmosphere, or only in the deep dark space?
There exist cosmic rays of all energies, the cosmic accelerator, and elementary particles were first seen in emulsions exposed to cosmic rays in high altitudes, for example the pion was thus discovered. So any process seen in accelerators can be found if looking hard enough in cosmic rays. Accelerators allow detail and exact measurements of crossections and branching ratios etc. because of the high statistics possible.
Best Answer
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.
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.