Is the weak force really a force ?

  • #1

Main Question or Discussion Point

Is the weak force really a "force"?

I have seen that gravity, electromagnetism and the strong force are described by physicists in detail in the sense that specific things can be said about what is attracted and/or repulsed and under what circumstances these forces are manifested. All we ever seem to see about the weak force is that it is responsible for some phenomena like beta decay and it is mediated by the W and Z bosons. But what is it? Is it attractive? Is it repulsive? Is it sideways? Is it sometimes repulsive and sometimes attractive like electric charge? Is that why it is mediated by two types of bosons? What particles are effected by this force? Is it really a "force" in the sense that particles can be caused to move as in F = ma?
 

Answers and Replies

  • #2
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All we ever seem to see about the weak force is that it is responsible for some phenomena like beta decay and it is mediated by the W and Z bosons. But what is it? Is it attractive? Is it repulsive? Is it sideways? Is it sometimes repulsive and sometimes attractive like electric charge? Is that why it is mediated by two types of bosons? ... Is it really a "force" in the sense that particles can be caused to move as in F = ma?
The idea of force as used in F = ma isn't really applicable to the weak force. The weak force is only noticeable on tiny length scales that are dominated by quantum mechanics, and quantum mechanics doesn't contain the classical idea of a force. So by "force" particle physicists mean something different and more general than the idea you have in your head. To avoid confusion in particle physics oftentimes we replace the word "force" with the word "interaction," and speak of the electromagnetic, strong, and weak interactions, instead of the EM, strong, and weak forces.

What particles are effected by this force?
All of them, except photons and gluons.
 
  • #3


Thanks for your response. Is there some intuitive way to understand the mechanism of the weak [STRIKE]force[/STRIKE] interaction? Does it act counter to the strong force to allow for radioactive decay in the sense that it's repulsive action might overcome the attractive action of ther strong force? Is the strong force also better described as an interaction rather than a force? It seems to be appropriate to call the strong interaction a "force" since it does account for holding the nucleus together.
 
  • #4
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Thanks for your response. Is there some intuitive way to understand the mechanism of the weak [STRIKE]force[/STRIKE] interaction? Does it act counter to the strong force to allow for radioactive decay in the sense that it's repulsive action might overcome the attractive action of ther strong force?
I think the best way to imagine it is just as an extra mechanism through which particles can exchange energy/momentum and be created or annihilated. And that it only works when the particles are in basically exactly the same position as each other, i.e. the range is pretty much nothing. So it is a way that particles can collide with each other, just as electromagnetism is, except electromagnetic collisions are sort of "soft" since they occur over long distances "gradually", while a weak interaction is really "hard" and point-like. This sort of explains its weakness too, since the colliding particles have to get extremely close to each other before anything happens, so most often they just "miss" each other. This is a rather classical picture, but I think it nevertheless captures some of the intuition.

As for acting counter to the strong force, well no it doesn't really do that. It just allows there to be some probability of neutrons turning into protons or vice versa, which can provide extra ways for a nucleus to lower its energy.

Is the strong force also better described as an interaction rather than a force? It seems to be appropriate to call the strong interaction a "force" since it does account for holding the nucleus together.
People often use the analogy of the strong force being like a rubber band between two coloured particles, i.e. there is not really any force exerted when they are really close to each other, but as they move apart the force goes up, sucking them back together again (or snapping and creating new particles out of the energy stored in the "tension"). So some kind of classical idea of force can make a certain amount of sense for the strong interaction.
 
  • #5
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I'm curious about the category of field and conserved quality expressed by the Weak Force/Interaction, and I'm hoping someone can educate me. My understanding is that the EM force produces changes in the EM field in which charge is conserved. Like wise, "color" is conserved in the chromodynamic (color) field with the Strong Force. What field is involved in the Weak Force activity, and what is conserved, or is this even an appropriate question?
 
  • #6
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I suspect that the answer will be that the EM and Weak fields are actually the same field, thus transmitting the electroweak unified interaction, but that's what I'm really curious about. Is there a good reference source for the mathematically challenged individual?
 
  • #7
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I'm curious about the category of field and conserved quality expressed by the Weak Force/Interaction, and I'm hoping someone can educate me. My understanding is that the EM force produces changes in the EM field in which charge is conserved. Like wise, "color" is conserved in the chromodynamic (color) field with the Strong Force. What field is involved in the Weak Force activity, and what is conserved, or is this even an appropriate question?
Well the quantum version of what you are saying is that the EM force is mediated by photons, which are the quanta of the electromagnetic field. The equivalent carriers for the weak force are the W+, W- and Z bosons. However, since the force is so short range there is no equivalent of a classical EM field.

As for the conserved charge, well, it is the third component of weak isospin. This is a little bit more challenging to understand than electric or even colour charge since you need to know more group theory, but here is an analogy that may help:

First, consider photon exchange between two electrons. Photons are uncharged, so the electron gets to keep its charge during this interaction.

Next, consider a gluon exchanged between a red quark and a green quark. The red quark emits a red/antigreen quark, so the red quark turns green, and the green quark turns red. I.e., the has been a flow of colour through the gluon (since the gluon has colour charge), but overall colour is conserved.

Finally, consider a W- boson exchanged between an electron and an up quark. This one is more complicated so hang on. The electron has [itex]T^3[/itex] (third component of weak isospin) -1/2. It spits out a W- boson, which has [itex]T^3=-1[/itex], which leaves the 'electron' with [itex]T^3=+1/2[/itex], turning it into an electron neutrino! Next, the up quark (with [itex]T^3=+1/2[/itex]) absorbs the W-, turning it into a down quark with [itex]T^3=-1/2[/itex]!
The W- obviously carries electric charge also, and this too is conserved in the interaction. The weak force is a little trippy due to the electroweak symmetry breaking, and is indeed somewhat mixed up with electromagnetism as you suspect. Anyway, what I just said will make more sense if you also realise that (left handed) electrons and neutrinos are partnered together into a 'weak isospin doublet' (as are up and down quarks), which just means that things carrying [itex]T^3[/itex] can switch them from one to the other, very similar to flipping around colours like we did with the strong force. The analogy between weak isospin and ordinary spin is very strong, it is very much like flipping a spin 1/2 particle from the +1/2 spin state to the -1/2 spin state.
 
  • #8
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Thanks much Kurros. That gives me some info to try to wrap my head around. Do you know of a good web source that shows this interaction diagrammatically?
 
  • #9
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Err, ok here: http://en.wikipedia.org/wiki/Weak_interaction
There are two feynman diagrams on that page and they both show the process I described, although one involves muons instead of electrons but it is the same. You can just mentally rotate them to get the right process :). Actually the process I describe basically never happens so it was probably a bad example (actually I suppose it is what happens in electron-capture nuclear decays), but with some twisting you can turn it into the second diagram on the wikipedia page, which happens all the time.

Just remember that W-, W+ and Z have T3 = -1, +1 and 0 respectively (conveniently coinciding with their electric charge), electrons and neutrinos have T3 = -1/2 and +1/2 (and their antiparticles have the opposite!), and up-type and down-type quarks have T3 = 1/2 and -1/2 respectively (and again vice versa for antiparticles).

Usually one doesn't really have to keep track of T3 when drawing feynman diagrams, since it basically takes care of itself if electric charge is conserved.
 
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  • #10
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Thanks for the effort to respond. You've helped a lot.
 

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