OK, I'll post this now rather than wait until tomorrow, as my interlocutor is so eager.
If I had to explain the mechanics behind pair production without assuming too much mathematical ability or knowledge of physics, I think I'd try the following. Before continuing, though, I will emphasise that this is nothing more than a very crude analogy, and that it assumes you know what (classical) fields are.
Let's see how it goes.
To start with, imagine there are two fields, let's call them the A field and the B field, which permeate all of spacetime. These two fields contain excitations which we call A and B particles, somewhat like (but also rather unlike) ripples propagating on a lake, or vibrations through the interior of a very large, springy mattress. If you like, you can picture the fields as being either scalars or vectors at each point in spacetime. In fact it's a little more complicated than that, particularly in the case of fermions, but it doesn't matter for now.
Now, suppose the two fields are
coupled. This means that at each point, each field is locally, somewhat weakly linked to the other. Thus, at any given point an excitation in one field can "tug" on the other field at the same point, and, if it tugs in the right way, actually produce new excitations in the other. (If you like, imagine two magically interleaved mattresses with soft springs coupling them at regular intervals. However, macroscopic analogies can be misleading so don't take the picture
too seriously.)
With this set-up, it may be that you could start with excitations in the A-field, and have them interact in just the right way to transfer all their energy to the B-field:
A-particles ---> B-particles.
If so, then the reverse process must also be possible:
B-particles ---> A-particles.
There may also be more complex interactions, with mixtures of As and Bs on each side. If you know the precise nature of the fields and the coupling between them (and in QED, for example, we do), you can figure out every possible outcome of making a bunch of A and B particles interact, in exquisite detail, and calculate the chances for each outcome to occur to as much precision as you please.
In
ridiculously simple terms, that is how processes involving annihilation and creation of particles work. Energy in one field (say, the electron/positron field) is transferred into another (say, the photon field) via their coupling. From the experimenter's point of view, particles of one type go in and particles of another type come out.
The mechanics described in outline above not only underpin the γγ --> e
+e
- process you originally asked about, but also
all the other QED processes. Vague though it is (of necessity, given the deliberate avoidance of mathematics), the same basic picture works for Compton scattering, electron-positron annihilation, and bremsstrahlung. It also works for Møller scattering and Bhabha scattering. Perhaps, if your imagination is good, it also gives you an idea of why an electron is surrounded by a cloud of virtual particles (a constant, noisy dance of energy between fields), and hence why things like vacuum polarisation occur. It also applies beyond QED, to the electroweak theory and strong interactions, and indeed QFTs in general. It's not all bad, as analogies go.
On the minus side, of course this analogy is imprecise and qualitative. It also has the horrible defect of obscuring the great
naturalness of QED to which ben m alluded earlier. It turns out, for instance, that the electron/positron field is described by one of the simplest possible models that can realistically represent a "matter" particle (i.e. one that obeys the Pauli exclusion principle). It also turns out the photon field
must exist and couple to the electron/positron in the way it does, once you demand that the electron/positron field has a certain very natural internal symmetry. The analogy also spectacularly fails to explain the quantised nature of the fields - excitations at a given frequency come in discrete amounts, unlike with classical fields, giving rise to their particle-like nature.
However, to really understand, you need to go beyond the type of explanation I've struggled to formulate above. You have to study QFT properly. It's as simple and as difficult as that.
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and ignorance as is the rest of your post, Farsight.
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