Why Do Weaker Forces Couple to More Particles?

[Lapidus]

(Forgive the long title of this thread, but I was reminded to be descriptive with the title. Hope that's ok now...)

Anyway, may question:

Are there any speculations or even explanations why weaker forces couple to increasingly more particles?

1. strong force only couples to hadrons and gluons (i.e. to itself)

2. em force couples to all massive particles but the neutrinos

3. weak force couples to all massive particles

4. gravity couples to everything

Or just some coincidence?

 

[marcus]

excellent clear title and a beautiful question
I really hope some knowledgeable people respond, even if the answers at this point might only be conjectural

 

[euquila]

heh I don't claim to be knowledgeable but here goes:

Our current understanding of spacetime is incomplete (thus "beyond the standard model")

That said, I think we've conveniently setup "fundamental forces" that encapsulate physical systems that we observe. However, I do not think for a second that it's accurate; a more elegant realization of spacetime exists but we just don't understand this yet.

Back to your question: these "fundamental forces" operate at different length scales (the strong nuclear force being of the smallest we know of). Therefore it makes sense that its reach or interaction with other systems is generally less. Take this with a grain of salt because this view is narrow in light of the fact that we do not really understand spacetime yet.

I think your observation is an important piece of the puzzle and theories show that at high enough temperatures, all the fundamental forces actually tend to manifest as a single unified force.

Edit: you might want to read about string theory and extra dimensions... I think that this (at least in part) the road to a better understanding of how spacetime works.

 

[ohwilleke]

I totally agree with Marcus. It is an interesting observation.

A parallel observation is that all particles that couple to the weak force have rest mass (fermions, W and Z bosons and the Higgs boson), while all particles that do not couple to the weak force do not have rest mass (photons and gluons), which is suggestive of the possibility that rest mass arises out of weak force interactions generally and not so much the Higgs boson in particular.

Another observation is that the sets of things that are involved in different fundamental forces are not nested sets. Gravity couples to everything, weak force couples to everything but photons and gluons. But, while fundamental particles with electrical charge are a subset of fundamental particles that interact with the weak force, the photon, which lacks electrical charge and does not interact via the weak force, is obviously a key part of electromagnetic interactions. Likewise, gluons, which do not interact via either the weak force or the electromagnetic force, do interact via the strong force. The lack of nesting is suggestive of the notion that the link between the number of particle that interact via a force and its strength is not a very direct one.

I also have issues with the conventional language that describes one force as "weaker" or "stonger" than another when they aren't really apples to apples comparisons: mass is not color charge is not electromagnetic charge is not weak isospin.

In top quarks, the "weak" force always overcomes the "strong" force. At the scale of a hundred angstroms or more, the electromagnetic force (and probably gravity as well), are going to swamp the short range weak and strong nuclear forces. At terrestrial and astronomy scales, electromagnetic forces are typically much weaker than gravity. The "strong" force is asymptotically free at small scales, and even at it strongest is no stronger than gravity near the event horizon of a black hole. The nuclear binding force which is derivative of the strong force is excellent at holding together elements like iron, but isn't so successful at holding plutonium together. Comparing force strengths only at the scale of atomic nuclei and in units associated with fundamental particles is more of a bias of perspective on the part of the kind of people asking the questions than it is fundamental truth, particularly in light of the face that the three Standard Model force coupling constants are themselves energy scale dependent quantities that "run" and it isn't self-evident that any particular energy scale is the most fundamental one at which to measure a coupling constant.

 

[atyy]

Gravity couples to everything because it is massless spin 2 on flat spacetime.
http://arxiv.org/abs/1105.3735 (p3)
http://arxiv.org/abs/1007.0435 (section 2.2.2)

 

[euquila]

Gravity couples to everything because it is massless spin 2 on flat spacetime.

That's assuming that the graviton even exists... :)

 

[friend]

Could it be because the gauge groups that various particles belong to are subgroups of other particles? The Standard Model has symmetry SU(3)XSU(2)XU(1). The strong force is SU(3), the weak force is SU(2)XU(1), and the electromagnetic force is U(1). Did I get that right? And it may be that particles that belong to smaller groups interact with particles of a larger group, but not visa versa.

In other words, the symmetry groups can only break into subsymmetries. Or something like that.

I'm not real sure. This is just a guess to spark conversation.

Edit: But if my perspective is correct, then the question becomes why do some symmetries result in weaker forces.