Monday, 4 May 2015

particle physics - Does the weak force have an attractive/repulsive force observable in everyday life like the other forces?


After the correct comments, this question is not here to compare gravity's and EM's long range forces' energetics and amplitudes to microscopic scattering amplitudes of such forces as weak and strong. I am basically trying to figure out if there is any observable effect in everyday life of the weak force. From the answers it is obvious that the weak force can be attractive or repulsive too. I am just trying to figure out if there is an observable effect of this (for the weak force) that we can see somehow in everyday life. Maybe not obvious, maybe we see it, experience it every day, we just do not know (that it is because of the weak force) it until it is explained in detail (like the strong force).


Maybe my question can be asked as simply as, 1. can the weak force pull/push (attr/rep) particles? 2. does it push/pull (attr/repl) particles in everyday life observable or is it just a rare thing, like decay? Does it hold something (particles) together or keep something apart in the everyday matter that we live in/around?


I have read these questions:


Weak force: attractive or repulsive?


Do strong and weak interactions have classical force fields as their limits?


Has the weak force ever been measured as a force?


As it is currently known,





  1. EM force is mediated by virtual photons, and can either be attractive or repulsive, and in everyday life it is easily observable, just hold a magnet. You can see the same thing with electricity. Then there is the covalent bond that makes molecules out of atoms. It is observable too, that the EM force is stronger on the short distance then gravity




  2. gravity, just let something go, and you see it is always attractive, there are obviously observable effects in everyday life, and it is observable that gravity on the short distance is weaker then the EM force or the strong force




  3. even the strong force, that keeps quarks confined, inside a nucleon, a neutron or proton, and the residual strong force that keeps neutrons and protons inside a nucleus, has an observable effect in everyday life, since without it, nuclei would not exist, they would fall apart. It is attractive on certain distances (between 0.8 fm and 2.5 fm), but it becomes repulsive at short distances (less then 0.7 fm), and that makes sure neutrons and protons do not get too close. This effect, though not commonly known, is responsible in part for giving material volume. It is observable too, that the strong force is stronger then gravity and EM on the short scale.





But what about the weak force? I know it can be repulsive or attractive, see here:


Weak force: attractive or repulsive?


So:




For weak isospin, there are two isospin charges (or flavors), up and down, and their associated anti-charges, anti-up and anti-down.




up repels up (anti-up repels anti-up)

down repels down (anti-down repels anti-down)
up attracts down (anti-up attracts anti-down)
up attracts anti-up (down attracts anti-down)
up repels anti-down (down repels anti-up)



For weak hypercharge, there is just one type of charge and its associated anti-charge.





hypercharge repels hypercharge (anti-hypercharge repels anti-hypercharge)
hypercharge attracts anti-hypercharge



Note that electric charge is a certain mixture of weak isospin and weak hypercharge.




OK, so I know that the weak force can be either attractive or repulsive. But the answers say too, that the weak or strong force does not have a classical field theory. Still, the strong force does have observable (in everyday life) attractive or repulsive effects.


Question:




  1. But what about the weak force, are there any effects that are in everyday life observable where the weak force is attractive or repulsive?



Answer



In everyday life? Like in your kitchen? No. Or if yes, totally not in the way that you're thinking.


If you insist on thinking of the fundamental interactions in terms of attraction and repulsion, one way to do that is to describe them all in terms of the Yukawa potential energy,


$$ U = \pm \alpha \frac{\hbar c}{r} e^{-r/r_0} $$


where the sign comes from the relative signs of the charges involved and distinguishes attractive from repulsive potentials, the coupling constant $\alpha$ is determined experimentally, and the range parameter


$$ r_0 = \frac{\hbar c}{mc^2} $$


depends on the mass $m$ of the field which mediates the interaction. For gravitation, electromagnetism, and the QCD color force, the this field (graviton, photon, gluon) is massless, so those forces in principle have infinite range. However, in the strong case, the coupling constant $\alpha$ is so large that multi-gluon exchanges are more important than single-gluon exchanges. This strong coupling means that color charges effectively can't be separated from each other, which is known as "color confinement." At low energies and long distances, the effective strong interaction is mediated by a spectrum of massive meson fields, whose own Yukawa potentials conspire to give the nuclei the structure that they have. An attractive force, mediated by pions, acts between nucleons that are separated by a few femtometers, but a repulsive force mediated by heavier mesons makes it expensive for nucleons to approach each other closer than about one femtometer.



For the weak interaction, the charged- and neutral-current bosons both have masses of nearly $100\,\mathrm{GeV}/c^2$. That's three orders of magnitude larger than the pion mass $140\,\mathrm{MeV}/c^2$, which is what mostly defines the size of a nucleon. So in order for nucleons to feel any attraction or repulsion due to the weak force, they would have be substantially "overlapping" in a way that's forbidden by the hard-core repulsion of the residual strong force. The effects of the strong force are much larger than the effects of the weak force --- partially because the coupling constants are different, but partially because the strong force prevents particles from approaching each other close enough that the weak force can affect them very much directly.


This same feature that makes the weak force mostly-irrelevant in nuclei (and more so in electromagnetically-bound systems, where the length scales are longer than in nuclei, and even more so in the even-larger gravitationally-bound systems) also makes the weak interaction harder to measure. In fact, measurements of the weak interaction would be impossible in strongly-interacting systems if the strong and weak interactions had the same set of symmetries, and we would be limited to patiently waiting for weak decays. However, we can take advantage the fact that the weak interaction is the only one of the fundamental forces which changes under mirror reflection.


If there's a way that the weak interaction affects life in your kitchen, it's because the weak interaction is parity-violating and the other fundamental interactions aren't. The Vester-Ulbricht hypothesis suggests a way that parity violation may have been important historically. But it's a much more subtle situation than "X is attracted to Y," because in contests of attraction and repulsion the weak interaction always loses to electromagnetism and the strong force.


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