# Simply Explain the "Weak Force"

I am not looking for anything detailed, but I still am having trouble with the weak force. I get the other three forces, I mean basically get then

Gravity - Throw something in the air it comes down

Electromagnetism - xerts a force on particles that possess the property of electric charge

Strong Force - binds the quarks inside an atom together. It effects those particles that have a color charge

But the weak force I still am not getting. I’ve read a bit on beta decay where a neutron is converted into a proton while emiting an electron. But is that all there is to it.

I am not looking for any detailed explination, everything I read is either way too technical or it doesn’t add up in my mind

Grossly oversimplified, the weak force was originally a bookkeeping device postulated to explain the behavior of the zoo of subatomic particles produced by accelerators beginning in the 1950s. Primarily it was invoked when dealing with conservation laws governing such properties as spin.

BTW: is it a “force” in the sense that it involves exchanging energy between particles? My references prefer to call it the “weak interaction”.

The weak interaction acts like a force too; the problem is that it’s almost always too, well, weak to be noticeable when we consider it as a force between particles. It acts on all left-handed fermions, but almost every left-handed fermion is charged, and the quarks are all coloured as well; so any particle interactions from the weak force are simply too small compared to their interaction via the electromagnetic and strong forces. As a result, the only practical way to observe the effects of the weak interaction is to look at other processes like beta decay (or even more exotic ones, like flavour-changing interactions) that don’t have classical action-at-a-distance analogs.

The exception to the above is neutrinos. Neutrinos are the only particles that don’t interact via the strong or electromagnetic forces, so if somehow we could bounce neutrinos off of each other, we could potentially see the weak interaction really act like a “force”. The problem is that, well, neutrinos don’t interact via the strong or electromagnetic forces, so it’s near impossible for us to “grab onto” them in order to manipulate & study them, and just as near impossible to detect what happens to them after they interact.

To expand a bit on what MikeS said, when we think about forces, we mostly think about attraction and repulsion – electromagnetism: like charges repel, gravity: masses attract, strong force: neutrons and protons attract each other. However, that’s not really the whole story – in the case of the strong force, for instance, things only look that easy because the particles involved happen to be colour neutral to an outside observer; internally, a lot more than simple attraction and repulsion happens.

Forces, or interactions between particles, are themselves transmitted via particles. Those force carrying particles can transmit a number of properties according to their own properties – in the simplest case, they transmit merely momentum, leading two particles to either attract or repel each other.

However, that’s only possible if the force-carrying particle itself carries no other charge – take electromagnetism: without going into the subtleties of virtual particles and such, an electron more or less emits a photon that meets with another electron and tells it where to go, and vice versa, leading to a net repulsion between the two. This is only possible in that way because the photon is electrically neutral; if it weren’t, an electron emitting a photon would have to alter its own charge, since charge is conserved (i.e. it would have to give a bit of charge to the photon it sends off), thus becoming a different particle.

That’s basically what happens in strong interactions: quarks carry a type of charge, called colour, and so do the force carrying particles they exchange, the gluons. Thus, whenever quarks interact via the strong force, they change their own colour according to certain rules, causing this nice and tidy Feinman-diagram of nucleons interacting via the exchange of a (virtual) pion to become this considerably more tangly and messy representation when one looks at the internal processes going on.

Yet, quarks of a different colour are still quarks; the weak interaction is mediated by particles which may carry electric charge, +1 in the case of the W[sup]+[/sup], and -1 for the W[sup]-[/sup]. This leads to a wholly different kind of change, and here’s where beta decay comes into play: beta decay, or rather beta-minus decay, is a neutron decaying into a proton, an electron, and an electron-antineutrino, n -> p + e + 'v[sub]e[/sub].
But, looking at the messy insides, what actually happens is the conversion of a down-quark (which has the electric charge -1/3) into an up-quark (charge 2/3) via the emission of a W[sup]-[/sup], which then decays into electron and neutrino, corresponding to a Feynman diagram like this; you can see how the (electric) charges add up: 2/3 - 1 = -1/3.
Thus, in this case, there appears to be no attraction/repulsion going on, since the particles in question change; however, that’s due to the particulars of the force-carrying particles, otherwise there’s really no distinction from the other forces.

But, as MikeS already pointed out, there is a way in which the weak force behaves exactly like our more everyday notions of ‘force’ would have us expect. This is due to the fact that, besides W[sup]+[/sup] and W[sup]-[/sup], there’s another force-carrying particle associated with it: the electrically neutral Z (sometimes called Z[sup]0[/sup], but I’ll spare myself the coding).

Now, this thing is essentially a heavy photon – really heavy, in fact; all the weak force carrying particles are basically the lard asses of the particle zoo, weighing more than individual atoms, which is also why they don’t usually get very far: mass is correlated with half-life, and those little things decay really quickly – and thus, a bit hard to observe: all interactions between electrically charged particles that can be accomplished via (virtual) Z boson exchange can be more easily accomplished via photons, so, one needs to look at electrically neutral particles, i.e. neutrinos. There, Z boson exchange leads to elastic scattering, which is pretty much the transfer of momentum I have earlier equated to our everyday attraction/repulsion notion of force; the existence of this ‘neutral weak current’ has been experimentally confirmed in 1974 at the Gargamelle bubble chamber at CERN.

So, put simply, the confusion about the weak force behaving strangely doesn’t really originate with the particulars of said interaction, but with our everyday notion of force being rather incomplete when it comes to particle physics – beta decay is as much the result of a force, or rather interaction, as two electrons repelling each other is.

So what’s the connection between the W bosons and the Z boson? It sounds like two completely different forces the way you explained it. And in the case of beta decay, what is the neutron interacting with? In the case of all the other forces, particles are exchanged between fermions, but in the case of the weak force, the W particle is just emitted and then produces two new particles. And (this may be getting too complicated for me to understand, if it hasn’t already) where does the mass of the W come from and where does it go when the boson decays?

The decaying neutron is interacting with the particles it emits when it decays. And the mass of the W doesn’t come from everywhere: You’re allowed to have a virtual particle (that is, one that terminates at some other Feynman vertex before it’s observed) even if you don’t have enough energy to make up its mass (though the process becomes a lot easier if you do indeed have enough energy).