Q: How is the “Weak nuclear force” a force? What does it do?

The original question was: I keep reading on the internet about how it’s the electromagnetic force which causes positive and negative charges and their reactions, it is gravity which pulls all matter together, and it’s the strong nuclear force which pulls quarks together. What is it that the weak force does though? I have read that it “governs” radioactivity, but what does that mean? What is its purpose?

Physicist: The short answer is that the W and Z bosons (the Weak interaction carriers) really don’t “fit” anywhere else.  And, although it’s not the most important thing it does, the W and Z bosons do carry momentum, which is all that a force really has to do at the end of the day.
The big difference is that the other force carriers; photons, gluons, and (if they exist) gravitons, leave the involved particles alone (other than energy and momentum).  They’re very “clean” interactions.  While the Weak force can mediate interactions between particles without changing them (this is a result that helped point the way to the discovery of the “electroweak force”), it can also carry stuff with it, like charge.  By taking charge from one thing, and giving it to another, it’s changed the particles involved.  That last bit is important during “beta decay”, which is the type radioactive decay that the Weak force is involved in.

A muon and an electron neutrino exchange a W boson (carrying a negative charge, and some momentum), which changes the muon into a muon-neutrino and the electron-neutrino into an electron. This interaction is extremely rare. Far more common is muon decay, but that doesn't demonstrate the point.

Gluons can also cause the involved particles to change, specifically switching protons and neutrons.  However, if you start with one neutron and one proton, and you end with one neutron and one proton, what’s the difference?  The Weak force is the alchemist of the forces.

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7 Responses to Q: How is the “Weak nuclear force” a force? What does it do?

  1. Scott says:

    It will be the alchemist of forces when someone figures out how to use it to turn lead into gold.

  2. Greg Johnson says:

    The other three forces are attractive forces, electromagnetic being attractive and repulsive. The weak force appears on the surface to be more of an intermediary particle than a force. What is the force? Is it quantum jitters or something else?

  3. edward says:

    How would removing the Weak nuclear force from the universe affect life if at all?

  4. newtspeare says:

    Talking about ‘the weak nuclear force’, is a bit like describing friction as a force. Somebody who really understood friction, would not call it a force; rather they would explain it in terms of movement and electrical forces between the electrons and protons that make up atoms.

    A neutron outside a nucleus will spontaneously split into a proton and an electron. Saying that this is caused by the weak nuclear force mediated by bosons, is about as much use as saying that friction is caused by friction worms.

    Neutron decay must involve at least 2 forces. Obviously one must provide the energy to spring the electron and proton apart, whilst another must hold them together until the neutron decides to decay. A comparable situation involves nuclear fission, where the spring force in a uranium atom is provided by the repulsion between the 92 protons in the nucleus, and the force that holds it together until it chooses to decay is a surface force between nucleons, termed the strong nuclear force.

    A neutron is heavier than the combined mass of a proton and an electron, by 1.5 electron masses. Sometimes in neutron decay all this extra mass gets converted to kinetic energy, and the electron comes flying out at 0.9 lightspeed; but on average only half this mass gets turned into kinetic energy, and half into neutrinos. The question then is, how is this extra energy stored inside a neutron? The best analogy would be to think of the electron stuck to the proton by an explosive device which picks its own time to go off. A similar such device is a muon, which has a half life of about a millionth of a second before converting its mass to energy such as neutrinos. I cannot explain nuclear forces any further without employing Squish Theory.

    Usually physicists give informative answers, but sometimes they cannot because there are always gaps in our knowledge of the universe. Unfortunately human pride seems to prevent people from acknowledging their ignorance; so when physicists do not understand what is going on, they tend to invent imaginary beings like crystal spheres, epicycles, phlogiston, quarks, gluons and the weak nuclear force; and when anybody criticises these imaginary beings they get branded a heretic.

  5. The weak force is the only force that can create, destroy, or transform a single leptonic elementary particle (rather than a particle-antiparticle pair). To ensure that a newly created electron (for example) is exactly the same as every other electron ever created, the weak force IVBs must recreate the primordial state of energy density in which these leptons were originally created – the electroweak unified force energy state. This is why the weak force IVBs are so massive even though they are only creating a single electron and neutrino. Google-search “weak force IVBs” for a full discussion.

  6. The Weak Nuclear Force is supposed to be responsible for radioactive decay; that is, a neutron suddenly turning into a proton and emitting an electron, changing the element of the atom. We believe that very heavy particles cause this; we call them called W+, W- and Z0 (that’s a zero). Let’s say a neutrino gets close to a neutron. If a W+ particle leaves the neutrino and hits the neutron, the neutrino will become an electron (it becomes negative), and one of the neutrons quarks gets changed so it is positively charged, making the neutron a proton. At least, this is an explanation for radioactive decay and one of the things the weak nuclear force is responsible for. All the weak nuclear force can do is change quarks or leptons.

    Why do we define it as a separate force? The thing is, none of the fundamental forces were able to explain radioactivity, so they invented a new force. Later, they were able to get evidence for the force carriers of this force in particle accelerators, so it became a thing.

  7. Ray says:

    I’ve been trying to understand the weak force all my life and I still don’t see how it is a ‘force’, it does not push or pull, it transforms one thing into another. How do you compare, say, the strength of a magnet to the strength of frying an egg? We have these Ws and Zs, fine, but where do they come from? What makes a neutron decide to decay?

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