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I'm trying to explain in simple terms what the weak interaction does, but I'm having trouble since it doesn't resemble other forces he's familiar with and I haven't been able to come up (or find on the web) with a good, simple visualization for it.

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Is there a specific reason why the modern picture of the weak force is harder to explain than the other forces? –  Tim van Beek Feb 2 '11 at 7:48
@Tim: well, massive bosons decay, so that's something completely different from EM interaction (even if the person is already acquainted with role of bosons in explanation of forces). –  Marek Feb 2 '11 at 9:03
I don't know that many "laymen" who are familiar with the picture that EM forces come from the exchange of virtual photons :-) –  Tim van Beek Feb 2 '11 at 11:09
@Tim: neither do I. But I meant that most people are familiar with the fact that EM is actually the same thing as light which is the same as photons. So the connection between electric force and photons is easy to make. Weak force is much more removed from the basic knowledge. –  Marek Feb 18 '11 at 18:07
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3 Answers

up vote 9 down vote accepted

The weak force "looks" different because in the first (and still most important) reincarnation we have encountered it - namely beta-decay (including the decay of the neutron) - the force seems to be a contact interaction: it has an extremely short range, essentially zero.

However, any phenomenon that differs from the indefinite existence of an object that moves in the same direction by the same speed forever requires a force to be explained. The force required for the beta-decay is the weak nuclear force.

While the decay seems to "directly" transform a neutron into a proton, electron, and antineutrion, a closer investigation of the force that began in the 1960s has demonstrated that this force is actually analogous to other forces, including electromagnetism, because its range is finite (nonzero). It's only limited because it's mediated by the W and Z bosons which are, unlike photons, massive. So the force doesn't get "too far".

However, in our modern description of the forces, electromagnetism and the weak force have to be described by a unified "electroweak" theory and they mix with one another. At distances much shorter than the range of the W/Z bosons, the electromagnetic and weak forces become equally strong and, in some proper sense, indistinguishable.

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Great, I really enjoy your explanations. –  Killercam Jun 29 '13 at 17:35
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I would actually emphasize the difference between the forces, rather than the similarity. Although we (as theorists) like to bundle the whole shebang into a "neat" $U(1)\times SU(2) \times SU(3)$ gauge structure (and possible some gauge version of gravity), it doesn't mean that reality has to be that neat (e.g. chirality of electroweak, neutrino masses, etc.)


  • Electromagnetism is long ranged, and drops off in strength with distance.
  • Strong force is actually also long-ranged, but gets stronger with distance! This causes the side effect that trying to separate a pair of opposite charges causes pair creation, and so we always see neutral composite particles.
  • Weak force is intrinsically short ranged (order $1/M_W$), and primarily it does not transmit a force --- but transmutates particles. Electrons go to neutrinos, quarks mix, etc.

And for gravity, say whatever your favourite quantum gravity picture say it is :-)

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Sorry, but the claim "anything goes" about gravity is totally off here. At the level of accuracy you pursued in the case of other forces, the claim about gravity is equally indisputable and it is that gravity is a long-range force that drops off in strength with distance, much like electromagnetism. In fact, the room for confinement and/or higgsing is much more constrained in the case of gravity so this statement is more solid, and not less solid, in the case of gravity. Every viable theory of quantum gravity has to respect the long-range character of gravity and other general features, too. –  Luboš Motl Feb 27 '13 at 6:36
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I like the history oriented approach, when explaining something to a layman. In this case you could start by briefly explaining the fundamental forces and how we needed a new force to model beta decay.

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