· Fundamentals · 2 min read
What Is the Weak Force?
The weak force is one of the four fundamental forces of nature. It governs beta decay and is the only force besides gravity that neutrinos feel.
The weak force, also called the weak interaction, is one of the four fundamental forces of nature, alongside gravity, electromagnetism, and the strong force. It is responsible for radioactive beta decay and for the nuclear reactions that power the Sun. For neutrinos it is especially important: apart from gravity, the weak force is the only way a neutrino can interact with anything at all.
Why is the weak force called “weak”?
The name reflects its feeble strength at everyday energies, not any lack of importance. At the scale of an atomic nucleus, weak interactions are far less likely than electromagnetic or strong ones, which is why beta decay can take minutes, years, or even billions of years to occur. A neutrino can sail through a light-year of lead with only a modest chance of being stopped.
This weakness has a precise origin. The weak force is carried by very heavy particles, and a heavy carrier gives the force an extremely short range, around a thousandth of the diameter of a proton.
What carries the weak force?
The weak interaction is mediated by three force-carrying particles: the electrically charged W⁺ and W⁻ bosons, and the neutral Z boson. Each is roughly eighty to ninety times as massive as a proton. The W bosons change one type of particle into another, for example turning a neutron into a proton during beta decay. The Z boson allows particles to scatter without changing identity, the channel through which many neutrino detectors register a signal.
How does the weak force affect neutrinos?
Because a neutrino has no electric charge and feels neither electromagnetism nor the strong force, the weak interaction is its near-exclusive means of touching ordinary matter. When a neutrino does interact, it exchanges a W or Z boson with a quark or an electron. These rare encounters are exactly what every neutrino detector is built to catch, and they are the reason the particles were so hard to find in the first place.
The weak force is also unique in another way: it is the only interaction that distinguishes left from right, treating mirror-image processes differently. This property, known as parity violation, is woven deeply into the behaviour of neutrinos.
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References
- 1.S. Navas et al. (Particle Data Group), Review of Particle Physics (gauge and Higgs bosons), Phys. Rev. D (2024). DOI: 10.1103/PhysRevD.110.030001.