Of the four fundamental forces, the weak force is the most mysterious. It is the only one with no obvious role in the world we know: The strong force builds protons and nuclei, electromagnetism is responsible for nearly every macroscopic phenomenon, and gravity, though weaker than the rest, is noticeable because of our close proximity to a reasonably large planet.
The only observable phenomenon due to the weak force is the radioactivity of certain substances (not all). I sometimes wonder if this major aspect of nature might have gone unnoticed if Henri Becquerel hadn’t kept his unexposed photographic film and his uranium samples in the same drawer. Early 20th-century physicists wondered why some rocks emit strange rays—it turns out that there’s a new force that transforms particles so that they are no longer bound to the nucleus. Mid-20th-century physicists wondered why this force is so weak—it turns out that its intermediary force carrier, its analog of the photon in electromagnetism, is very massive and therefore rarely produced. Physicists today wonder why the weak force carrier is so massive—it may be that there’s an omnipresent Higgs field binding to it, slowing it down and giving it effective mass. The particulate form of that Higgs field may have been discovered last year, at long last.
The weak force is the most eclectic of the four—it violates most of the conservation rules that the others uphold. The strong force, electromagnetism and gravity all act on antimatter the same way with the same strength as on equivalent samples of ordinary matter; the weak force does not. The same is true of mirror-flipped and time-reversed configurations; the weak force uniquely distinguishes between clockwise and counter-clockwise, between forward and backward.
In fact, interactions through the weak force change the identity of all particles involved. In previous articles, we showed how forces push and pull by exchanging an intermediary. In the case of electromagnetism, two charged particles repel by throwing a photon from one to the other, like a heavy sack thrown between two boats. For the weak force, this is either a charged W boson or a neutral Z boson. When a quark emits a W boson, however, it becomes a new type of quark. Charmed quarks turn into strange quarks, and muons become electrons. In addition to carrying the momentum of the force, the W boson takes some of the strangeness or the charmness out of one quark and into another.
In its unique role as rule-breaker, the weak force may be responsible for the matter-antimatter asymmetry in the universe. Its weakness may be hiding dark matter. The weak force seems to be tied to so many fundamental mysteries, it’s amusing to think that the whole thing might have been overlooked if Victorian physicists hadn’t been so curious about strangely warm rocks.
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