Peter Ward asks
1. What physically is a photon? The standard answer is a photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation. This is more a concept than a description of what a photon is physically. Is it simply a massless oscillation in space?Eli goes to the Google: A photon is the vector boson which carries the electromagnetic force. It is a massless particle of spin one and zero charge. Single photons are labelled by energy, momentum and polarization where energy, E = hν and momentum k = 2π/λ
Eli might ask what is an electron? A sufficient answer is that it is a particle of spin 1/2, unit charge and a mass of 9.10938291 x 10-31 kg.
Quantum electrodynamics provides rules for calculating the probability of photons interacting in various ways with charged particles including electrons. As with any quantum anything, QED provides an instruction sheet on what to do, but the epistemology oft is lacking. As John Bell once wrote there is both speakable and unspeakable in quantum mechanics and venturing into the latter is brain threatening. Ce la vie.
It is worth mentioning that in QED charged particles do not interact directly with each other but do so by exchange of photons. Using QED one can calculate at least in principle the probability of an electron scattering off an electron, changing the motion (or quantum state) of the electron or charged nucleus in space and time, etc.
Quantum behavior is difficult, but tractable. Playing the game of why in the quantum realm is not recommended unless you shrink down a bit and acquire some practice. Assembling the machinery on the blackboard scale takes some time, and anybunny who wants to see the bottom line first might go to minute 37 in the video below where Feynman calculates the interaction of two electrons and then go back and view the entire lecture.
So no, photons do not beat their kids, and asking when they stopped is not going to lead to a fruitful discussion. They simply will not allow themselves to be forced into your theory of knowledge of choice, but humans can figure out how they will and do behave.
4. Do the photons interact with each other in space? If not, why not? If yes, how?See the lecture. Since photons can decay into electron-positron pairs (or other beast pairs at super high energies) and the other photons can interact with the charged particles before they recombine, yes in principle, in practice not damn much in labs with budgets under the price of unicorns. Without virtual pair production uncharged massless particles like photons do not interact. No gravitational attraction either, they are massless.
What about interference? Well from the QED point of view this is a function of the interaction of photons with the charged particles at the detector, that is the interference does not exist until it is mediated by the interaction at the detector. That also answers the question of where the photon is, it is where the detector detects it. Some, not Eli to be sure, may not like that but that's the engineering level report.
7. We talk of an electromagnetic field that can be mapped out in three dimensions and time with a suitable sensor. What is the physical relationship of such a field to photons?
In the interest of getting to bed and the comforts of Ms. Rabett, Eli will hand this one off to Lubos and return to the other questions tomorrow.