I'm trying to explain in simple terms what the weak interaction does, but I'm having trouble since it doesn't resemble other forces he's familiar with and I haven't been able to come up (or find on the web) with a good, simple visualization for it.
The weak force "looks" different because in the first (and still most important) reincarnation we have encountered it - namely beta-decay (including the decay of the neutron) - the force seems to be a contact interaction: it has an extremely short range, essentially zero.
However, any phenomenon that differs from the indefinite existence of an object that moves in the same direction by the same speed forever requires a force to be explained. The force required for the beta-decay is the weak nuclear force.
While the decay seems to "directly" transform a neutron into a proton, electron, and antineutrion, a closer investigation of the force that began in the 1960s has demonstrated that this force is actually analogous to other forces, including electromagnetism, because its range is finite (nonzero). It's only limited because it's mediated by the W and Z bosons which are, unlike photons, massive. So the force doesn't get "too far".
However, in our modern description of the forces, electromagnetism and the weak force have to be described by a unified "electroweak" theory and they mix with one another. At distances much shorter than the range of the W/Z bosons, the electromagnetic and weak forces become equally strong and, in some proper sense, indistinguishable.
I would actually emphasize the difference between the forces, rather than the similarity. Although we (as theorists) like to bundle the whole shebang into a "neat" $U(1)\times SU(2) \times SU(3)$ gauge structure (and possible some gauge version of gravity), it doesn't mean that reality has to be that neat (e.g. chirality of electroweak, neutrino masses, etc.)
And for gravity, say whatever your favourite quantum gravity picture say it is :-)
I like the history oriented approach, when explaining something to a layman. In this case you could start by briefly explaining the fundamental forces and how we needed a new force to model beta decay.
The weak force is like why water stays together in a system.
The strong force is why when the atoms in the bottoms of your feet don't go through the ground when you walk.
protected by Qmechanic♦ Jul 20 '14 at 20:30
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