The weak nuclear interaction: the enigmatic fundamental force that makes life possible © Getty Images

The weak nuclear interaction: the enigmatic fundamental force that makes life possible

Without the weak nuclear interaction, stars couldn't turn hydrogen into the vast array of elements necessary for life.

The existence of all elements heavier than hydrogen is a marvel, their formation only possible in the centre of stars. The abundance of oxygen, carbon and silicon on our planet is thanks to the least well-known of the four fundamental interactions of physics, the weak nuclear interaction. Chad Orzel explains this unusual process in this extract from his book, Breakfast With Einstein.

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There’s some hefty science in this piece, so here are five things you’ll need to know before you get started.

Five things you’ll need to know about particle physics:

  1. The Standard Model of particle physics describes the four fundamental forces (gravity, electromagnetism, the strong nuclear force and the weak nuclear interaction) and the elementary particles (quarks, leptons and bosons).
  2. An atom has a nucleus made up of protons and neutrons, surrounded by electrons. The protons and neutrons are made up of different groups of quarks. Electrons are a type of lepton.
  3. The quarks in protons and neutrons are either ‘up’ or ‘down’ quarks. In beta decay, a down quark changes to an up quark, turning a neutron into a proton.
  4. Particle physics is full of ‘conservation laws’: when any particles interact with each other, quantities like the total energy and momentum must be the same before and after.
  5. Stars release energy by fusing atomic nuclei into progressively heavier and heavier elements. 


The weak nuclear interaction occupies an unusual position in the Standard Model, being arguably the least obvious fundamental interaction, while also being one of the best understood.

The mathematical theory of the weak interaction and its close relationship with electromagnetism was developed through the 1960s and early 1970s, and the experimental confirmation of that theory’s predictions, culminating in the discovery of the “Higgs boson” in 2012, ranks among the greatest triumphs of the Standard Model.

The strong nuclear interaction, meanwhile, continues to pose problems for theorists computing properties of matter, while gravity is famously mathematically incompatible with the other three.

At the same time, however, it’s very difficult to point to exactly what the weak nuclear interaction does. What makes the weak interaction especially tricky to explain to non-physicists is that, unlike the other interactions, it doesn’t manifest as a tangible force in the usual sense.

The pull of gravity is a central element of our everyday experience, and electromagnetic forces between charges and magnets are something you can feel. And while the strong interaction operates at an extremely remote scale, it’s still easy to understand as a force holding the nucleus together against electromagnetic repulsion.

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The weak interaction, on the other hand, isn’t used to stick anything together, or to push anything apart. This is why most physicists have dropped the pleasingly alliterative term “fundamental forces” in favour of “fundamental interactions”.

Instead of pushing or pulling on particles, the weak nuclear interaction’s important function is to cause particle transformations: more specifically, it turns particles from the quark family into particles from the lepton family.

This lets a down quark (which has a negative charge) transform into an up quark (which has a positive charge) by emitting an electron and a third particle known as a neutrino—or an up quark transform into a down quark by absorbing an electron and emitting an antineutrino. These transformations enable neutrons to turn into protons, and vice versa.

The process taking place in the Sun involves the latter, and is the inverse of the better-known phenomenon of “beta decay”, in which a neutron in the nucleus of an atom spits out an electron and changes into a proton.

Beta decay has been known of since the early days of research into radioactivity, but explaining it posed a vexing challenge in the early days of quantum theory, leading to one of the more colourful anecdotes of 20th Century physics.

The problem with beta decay is that the electrons spat out by decaying nuclei emerge with a wide range of energies (up to some maximum value).

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This shouldn’t be possible for a reaction involving only two particles—the laws of conservation of energy and conservation of momentum should dictate only a single possible energy for the departing electron (as is the case for the process of “alpha decay”, in which a heavy nucleus decays by spitting out a helium nucleus: two protons and two neutrons stuck together).

Explaining the range of energies seen in beta decay stymied physicists for a long time and led some to propose drastic measures—like abandoning the idea of conservation of energy as a fundamental principle of physics.

The solution was found by the young Austrian physicist Wolfgang Pauli, who in 1930 suggested (in a letter sent to a conference he was skipping to attend a ball in Zurich) that beta decay didn’t involve two particles, but three—the neutron-turned-proton, the electron, and a third, undetected particle with a very tiny mass.

The new particle, quickly dubbed the “neutrino” (loosely “little neutral one” in Italian), carries away some energy, with the precise amount depending on the exact momentum of the electron and neutrino when they leave the nucleus.

Introducing the neutrino initially didn’t seem much less desperate than ditching conservation of energy—Pauli himself wrote to a friend, “I have done something terrible. I have postulated a particle that cannot be detected. That is something a theorist should never do.”

Within a few years, though, the great Italian physicist Enrico Fermi developed Pauli’s rough suggestion into a complete and remarkably successful mathematical theory of beta decay, and the idea was quickly adopted.

Pauli’s original neutrino turns out to be one of three (the original electron neutrino, plus “muon”, and “tau” varieties), and despite his initial lament, neutrinos can, in fact, be detected, and were experimentally confirmed by Clyde Cowan and Frederick Reines in 1956.

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What does all this have to do with the Sun? The answer is subtle. The Sun is powered by fusing hydrogen nuclei, which are single protons, into helium nuclei consisting of two protons and two neutrons stuck together.

Somewhere in this process, two protons need to turn into neutrons, which is possible thanks to the weak nuclear reaction and the process of “inverse beta decay” mentioned above, in which a proton turns into a neutron, emitting a neutrino in the process.

As a result, the Sun produces incredible numbers of neutrinos, which have been detected on Earth, and measurements of these solar neutrinos provide information both about nuclear reactions in the core of the Sun, and also about the properties of neutrinos themselves.

The conversion of protons to neutrons inside stars is essential for the existence of the enormous range of elements we interact with on a daily basis—the oxygen in the air we breathe and water we drink, the carbon in the food we eat, the silicon in the ground beneath us.

When a very heavy star burns through most of the hydrogen in its core, it begins to fuse helium into even heavier elements; when the helium runs low, these extremely heavy stars begin to burn carbon, and on up through the periodic table. At each step of the process, though, the strong-interaction energy released by fusion decreases, until silicon is fused into iron.

The fusion of iron does not release any energy, cutting off the flow of heat that’s propping up the core of the star. At that point, the outer layers of the star come crashing inward, and bounce off the core to produce a supernova explosion, releasing enough energy that the exploding star often temporarily outshines the rest of its home galaxy.

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In a supernova, much of the mass of the star is blasted outward in an expanding cloud of gas, carrying with it the heavier elements produced in the core during the later stages of fusion. These gas clouds expand and cool and interact with other gas in the neighbourhood, forming the raw material for the next generation of stars—and also rocky, Earth-like planets, which are largely made up of the heavy elements created in the core of the dying star.

The enormous variety of substances we see on Earth— rocks and minerals, breathable air, plants and animals—are all built from the ashes of dead stars, created through all four fundamental interactions.

Starting with simple clouds of hydrogen formed shortly after the Big Bang, gravity pulls gas together, electromagnetism resists the collapse and heats the gas, and the strong nuclear interaction releases vast amounts of energy in nuclear fusion.

And, finally, the weak nuclear interaction enables the particle transformations that turn hydrogen into heavier and more interesting elements.

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Take any one of these fundamental interactions away, and our everyday existence would be impossible.

Breakfast with Einstein: The Exotic Physics of Everyday Objects by Chad Orzel is out now (£9.99, Oneworld).

Breakfast with Einstein: The Exotic Physics of Everyday Objects by Chad Orzel is out now (£9.99, Oneworld)