Science › Fields & particles
The weak force and radioactivity
1968· Theory
ProvenTested many times; part of working physics.
The force that changes one type of particle into another, responsible for some radioactivity and for the first step of the Sun's fusion.
See it on the Science map →How it works
The weak force can turn a neutron into a proton, releasing an electron and a neutrino; that is beta radioactivity. It acts through heavy carrier particles called W and Z bosons, so it only works over distances smaller than a proton. It is 'weak' because those carriers are so heavy, not because it is unimportant. In the 1960s it was unified with electromagnetism: at very high energies the two act as one 'electroweak' force.
Why it matters
Without it the Sun could not burn, and many elements could not form. It is the only force besides gravity that affects neutrinos.
How we know
Fermi's theory (1933); electroweak unification by Glashow, Weinberg and Salam (Nobel 1979); W and Z bosons found at CERN in 1983 with the predicted masses.
Still a mystery
It treats left- and right-handed particles differently, a lopsidedness nobody can explain. It may also relate to why matter outnumbers antimatter.
Connections (4)
▸The weak force and radioactivity is one part of The Standard ModelProven
The relationship
The weak force is described in the Standard Model through the electroweak theory, combined with electromagnetism. Its W and Z carriers are heavy, which is why it is weak at low energies. It is the only part of the model that changes one particle type into another, like a neutron into a proton.
How the link was found
Glashow, Weinberg and Salam, 1961-68. Gerard 't Hooft proved in 1971 the theory gives finite answers.
Why it's proven
W and Z bosons discovered at CERN in 1983 (Nobel 1984 to Carlo Rubbia and Simon van der Meer), with properties since measured very precisely.
Open questions and mysteries
Neutrinos turned out to have mass (1998), which the original model did not include. How they get it is unknown.
▸The weak force and radioactivity lets the Sun burn in Stars forge the heavier elementsProven
The relationship
The Sun's fusion starts when two protons meet. To make stable deuterium, one proton must turn into a neutron, which only the weak force can do. Because the weak force acts so rarely, an average proton in the Sun waits billions of years to fuse. That slowness is why the Sun burns steadily for about 10 billion years instead of exploding.
How the link was found
Hans Bethe and Charles Critchfield described this first step, the proton-proton chain, in 1938.
Why it's proven
Each such reaction releases a neutrino. Ray Davis detected solar neutrinos from 1968; the Borexino experiment detected those from this first step directly in 2014.
Open questions and mysteries
The physics is settled. Precise solar neutrino measurements are now used to probe the Sun's interior composition, where some disagreements with models remain.
▸The Higgs field and boson (2012) explains the short reach of The weak force and radioactivityProven
The relationship
The weak force's carriers, the W and Z bosons, gain large masses, about 85 and 97 times a proton, by interacting with the Higgs field. Heavy carriers can only travel a tiny distance before vanishing, so the weak force reaches only about a thousandth of a proton's width. The photon ignores the Higgs, stays massless, and so electromagnetism reaches across the universe.
How the link was found
Built into the electroweak theory by Steven Weinberg in 1967 and Abdus Salam in 1968, using the 1964 Higgs mechanism.
Why it's proven
The theory predicted the W and Z masses before their 1983 discovery. The Higgs boson's measured coupling to W and Z, since 2012, matches.
Open questions and mysteries
Why the Higgs field settled at the strength it did, setting the W and Z masses, is unexplained.
▸The weak force and radioactivity splits from electromagnetism in The Higgs switches on (electroweak epoch)Proven
The relationship
Above about 10¹⁵ degrees, the weak and electromagnetic forces were one electroweak force with four massless carriers. When the Higgs field settled, three carriers became the heavy W⁺, W⁻ and Z, and the weak force shrank to subatomic range; the fourth stayed massless as the photon. From then on the two forces behaved as separate. At very high collision energies they still act alike.
How the link was found
The unified theory came from Glashow (1961), Weinberg (1967) and Salam (1968). Its claim that the forces act alike at high energy was tested directly at Germany's HERA collider, 1992–2007.
Why it's proven
At HERA's highest energies, weak and electromagnetic interaction rates became equal, as predicted. The W and Z masses also match the theory.
Open questions and mysteries
Did the split leave relics? Some propose it generated tiny magnetic fields that later seeded the magnetism seen in galaxies; this is unconfirmed.