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Can someone explain the weak nuclear force?

aggle-rithm

Ardent Formulist
aggle-rithm
Joined
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At one time I thought I had a pretty good handle on the fundamental forces in physics. There was a nice, orderly hierarchy where the weaker forces had a longer effective range than the stronger ones, but at closer range they stronger forces always won out. Therefore, electromagnetism overwhelms gravity when an object falls to the ground and comes to a sudden stop, and the nuclear forces overwhelm the electromagnetic force when two particles of like charge are squeezed close enough together in nuclear fusion.

The big mistake I had made was assuming that the weak nuclear force was what held the protons and neutrons together in the nucleus, and the strong force is what held quarks together. Now I know that is only partially right (the strong force does hold quarks together, but it is the residual strong force that holds nuclei together). What I have trouble understanding is how exactly the weak nuclear force fits into the grand scheme of things.

I know intellectually that the weak force is responsible for radioactivity. I know that it has been found to be closely related to electromagnetism, so that the two together can be called the "electro-weak" force. But neither of these things really helps me understand how it works.

Is there a simple, metaphorical way to explain this, or can it only be understood through complex mathematics?
 
Tubbythin said:
I would like to be helpful... but I'm not sure exactly what you're asking for.
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Sorry, what I wanted was to understand how the weak nuclear force interacts with the other forces.

You can say that gravity works at long distances but is overwhelmed by the other forces at close proximity. And you can say that the electromagnetic force overwhelms gravity at a specific range, but is itself overwhelmed by the residual strong nuclear force when two particles get sufficiently close together.

So how does the weak nuclear force fit into this model? Or am I not asking the right questions?
 
I guess the notable thing about the weak force isn't that it's a force, but what happens to the particles when there is an interaction mediated by it.

Gravity and electromagnetism don't fundamentally change the particle involved in the interaction. An electron interacting by electromagnetism is an electron both before and after it emits the photon to do it, for example. The strong force can change the colour charge on a particle, but you never see bare colour anyway, so that's not so noticeable either.

The weak force lets a quark change flavours, and in addition the mediating particles can decay into other things too. This means that whereas an electromagnetic interaction will usually involve a pair of particles going in and the same particles going out with some exchange of momentum (ignoring anything like pair production anyway), a weak interaction has some particles going in and a different set going out. This change in what the particles is will be a lot more obvious than any exchange of momentum due to the force. So the 'force' side of it is a lot less important than the kind of interactions it allows, and what options it opens up to the particles involved.

Does that help at all?
 
aggle-rithm said:
Sorry, what I wanted was to understand how the weak nuclear force interacts with the other forces.

You can say that gravity works at long distances but is overwhelmed by the other forces at close proximity. And you can say that the electromagnetic force overwhelms gravity at a specific range, but is itself overwhelmed by the residual strong nuclear force when two particles get sufficiently close together.

So how does the weak nuclear force fit into this model? Or am I not asking the right questions?
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I agree that descriptions of the weak force always seem to be vague.
It is interesting that gravity, electromagnetism and the strong force are described 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 know about the weak force is that it is responsible for some things 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 sometimes repulsive and sometimes attractive like electric charge? Is that why there are two 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?
 
Force assumes force, so step back and ask what's being shoved around?
 
It's also worth clarifying the ranges of the forces. Gravity is long-ranged and weak - it drops off as 1/r2. EM is stronger and has the same range - it also drops off as 1/r2. It's just that because it has charges that can be opposite, charges can shield each other. Gravity can't be shielded like that.
The strong force gets restricted in its range by color confinement rather than being 'innately' short-ranged. Part of the reason it's so short range in practice is actually that it doesn't drop off with distance very fast at all.
The weak force is genuinely short-ranged however - since the W and Z bosons aren't massless like photons and gluons.
 
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aggle-rithm said:
A little! But I think this question is the key:



I am definitely thinking of it, right or wrong, in terms of F = ma. That is difficult to reconcile with any description of the weak force that I've encountered.
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Think of it as a force with side effects? Pretty severe side effects in the case of the weak force. Added to the fact that it's so short range the side effects are basically all you notice.

You could for example notice the neutral current interaction causing an extra force between electrons at high energy, but the photon interaction is much more important, and it's not the most obvious thing the weak force does. Basically two electrons will usually interact by exchanging a virtual photon, but they could use a Z boson from the weak force instead - it's just a lot less likely and more difficult because the Z has a big mass, not zero mass. And it's more obvious the weak force is doing something when the Z decays to something different instead, or the W boson starts changing particles from one thing into something quite different, since that's unique to the weak force.

Proper particle physicists should correct me if I'm wrong - I'm hideously rusty on all this.
 
I have often come away from descriptions of the weak force feeling I've gotten only some partial answer designed for a layman. It seems the real answer requires more of an understanding of particle physics than I have had.
In any case, this thread motivated me to look into this question further. Here is a response I got from someone called The Duck on the Physics forum:
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.
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And to the question concerning what particles are effected:
All of them, except photons and gluons.
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Here is another thought from someone called kurros on the physics forum:


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.
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Perpetual Student said:
I have often come away from descriptions of the weak force feeling I've gotten only some partial answer designed for a layman. It seems the real answer requires more of an understanding of particle physics than I have had.
In any case, this thread motivated me to look into this question further. Here is a response I got from someone called The Duck on the Physics forum:
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I think if I remember to use the word "interaction" rather than "force", that will help me a lot.
 
aggle-rithm said:
There was a nice, orderly hierarchy
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This is really where you're going wrong. It looks a bit like there might be a hierarchy when you just see a general overview, but in reality there isn't one. There are just four separate forces that all do their own thing at all times. Under some conditions one will be much stronger than all the others, but that's not always the case.

The important points are always just charge and range. For example, only quarks and gluons have colour charge, so it doesn't matter how close together two electrons get, the strong force will never dominate their interactions because they don't interact with it at all. Neutrinos don't have electric or colour charge, so they only ever feel gravity and the strong force.

As for range, photons and gluons have zero mass (gravity is a little more complicated, but if gravitons exist and fit in the same framework they'd also be massless), so their range is infinite. The W and Z gauge particles have mass, so the range of the weak force is inherently limited since they travel slower than light (that's the easy way to think of it, of course it's actually a bit more complicated than that). However, gluons also have colour charge which leads to complications that mean the strong force is also very short range (it's essentially similar to how a group of charged particles looks pretty much neutral from a distance, except that the particles that would let you see that the charges don't perfectly cancel also end up helping with the cancelling and make it look even more neutral).

So you can see it's a lot more complicated than a simple linear hierarchy. What forces dominate at what range depends entirely on what you're dealing with. For ordinary matter it often seems as if it just goes gravity->EM->strong with the weak force not really fitting it, but that's just because the conditions that cause this happen to be the ones most familiar from a human scale. But think of these examples. Dark matter makes up the majority of the mass in the observable universe. It doesn't interact with the EM force at all, so clearly that hierarchy can't apply for the majority of the universe. Stars form as a result of the gravitational force being exactly balanced by the EM force, so even for the majority of the visible mass that hierarchy doesn't apply. Even something as simple as a magnet sticking to a fridge shows a situation where the EM force is stronger than gravity at relatively long ranges.

OK, so that doesn't really explain the weak force as such, it's more an explanation of why you shouldn't want an explanation for it any more than you should for any other force. It's one of the four fundamental interactions in the universe. Sometimes it's important (radioactive decay, some nuclear fusion), sometimes the others are more important. Without going a lot further into the details of quantum mechanics, there's not really much more to say about it.
 
Cuddles said:
This is really where you're going wrong. It looks a bit like there might be a hierarchy when you just see a general overview, but in reality there isn't one. There are just four separate forces that all do their own thing at all times. Under some conditions one will be much stronger than all the others, but that's not always the case.

The important points are always just charge and range. For example, only quarks and gluons have colour charge, so it doesn't matter how close together two electrons get, the strong force will never dominate their interactions because they don't interact with it at all. Neutrinos don't have electric or colour charge, so they only ever feel gravity and the strong force.

As for range, photons and gluons have zero mass (gravity is a little more complicated, but if gravitons exist and fit in the same framework they'd also be massless), so their range is infinite. The W and Z gauge particles have mass, so the range of the weak force is inherently limited since they travel slower than light (that's the easy way to think of it, of course it's actually a bit more complicated than that). However, gluons also have colour charge which leads to complications that mean the strong force is also very short range (it's essentially similar to how a group of charged particles looks pretty much neutral from a distance, except that the particles that would let you see that the charges don't perfectly cancel also end up helping with the cancelling and make it look even more neutral).

So you can see it's a lot more complicated than a simple linear hierarchy. What forces dominate at what range depends entirely on what you're dealing with. For ordinary matter it often seems as if it just goes gravity->EM->strong with the weak force not really fitting it, but that's just because the conditions that cause this happen to be the ones most familiar from a human scale. But think of these examples. Dark matter makes up the majority of the mass in the observable universe. It doesn't interact with the EM force at all, so clearly that hierarchy can't apply for the majority of the universe. Stars form as a result of the gravitational force being exactly balanced by the EM force, so even for the majority of the visible mass that hierarchy doesn't apply. Even something as simple as a magnet sticking to a fridge shows a situation where the EM force is stronger than gravity at relatively long ranges.

OK, so that doesn't really explain the weak force as such, it's more an explanation of why you shouldn't want an explanation for it any more than you should for any other force. It's one of the four fundamental interactions in the universe. Sometimes it's important (radioactive decay, some nuclear fusion), sometimes the others are more important. Without going a lot further into the details of quantum mechanics, there's not really much more to say about it.
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How does the highlighted bit work?
 
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