Weak and Strong Forces

[Anjruu]

As stereotypical as this may seem, I have recently read Brian Greene's "Elegant Universe." Concerning the weak and strong forces, Greene says on page 10, "the strong and weak forces are less familiar because their strength diminishes over all but subatomic scales." However, on page 177, he talks about how the strength of the weak and strong forces diminishes over short distances, because "the quantum cloud of particles eruptions and annihilations amplifies the strength of the strong and weak forces...and so the strengths of these forces get weaker when they are probed on shorter distances." This seems like a non-sequitur to me, wouldn't the forces INCREASE over non-sub-atomic scales, since there is a huge amount of atomic cloud? Where do the forces' strength peak?

[5614]

Both the weak and strong force occur on a sub atomic scale... as far as I was concerned they got drastically weaker as distance increases, I'm not familiar with the "quantum cloud amplification" you mention.

[Severian]

The strong forces has two properties: confinement and asymptotic freedom.

 

Asymptotic freedom happens when you look at the strong force at very short distance scales (or very high energies) and is saying that the force is becoming weaker and weaker as you go to shorter and shorter distances. This is as Greene says and actually won the Nobel prize a couple of years ago. It is somewhat counterintuative, since it is the other way around from quantum electromagnetism (QED) (where the physical picture of a cloud of particle anti-particle pairs around the charge works nicely). In fact, it is the opposite way around from QED because the force carrier of the strong force (the gluon) is has a 'color charge' and therefore interacts with other gluons, whereas the photon is neutral and doesn't interact with other photons.

 

However, although the strength of the force increases as you separate color charges, eg a quark and a color matched anti-quark, it is still not manifest over macroscopic distances. This is because as you pull two color charges apart, you need to put an increasing amount of energy into the system (to overcome the ever increasing force). Eventually you will have put enough energy into the system to spontaneously create a quark-anti-quark pair (of the same color but not necessarily the same type) out of the vacuum in between the two charges. The anti-quark will shoot off towards the original color-charge while the quark will shoot off to the original anti-color charge, and form color neutral systems (mesons in this case, but more complicated things can happen forming baryons too).

 

The anology which is often used is of a peice of elastic. The tension in the elastic increases as you pull its ends apart, until the energy you are putting in is enough to eventually break the strin, forming two new ends which snap back releasing the tension again.

 

Therefore we can never have free quarks - they will always pair up as neutral hadrons (mesons and baryons) and this process is known as hadronization. The overall property is confinement, because quarks are always confined within hadrons. Since the macroscopic objects are neutral, the strong force is only observed at close range.

 

 

The weak force is a bit different. Its force carriers are also charged (under the weak force) so it naively has the same properties as the strong force. So it decreases with decreasing distances too. However, there is an added complication in that (due to the Higgs mechanism) its force carriers are very heavy. They are so difficult to make and move about that doing anything with the weak force at range is basically impossible. This is also why the weak force is 'weak'.

[Anjruu]

Ah, that makes sense. Thank you.