LOOK closely enough and almost everything we know of in the universe boils down to a handful of elementary particles. These entities constitute individual threads of the scientific masterpiece that is the standard model of particle physics, our current best picture of matter and its workings.
Its roots lay in the quantum revolution early in the 20th century, where the classical, common-sense notion that everything is predictable was unceremoniously thrown out. By contrast, the development of the standard model was anything but a revolution. Instead, it was more like the gradual forming of a new order, constructed piece by piece by dozens of physicists across decades.
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Many expected the new order to fail. But it didn’t. In fact, the standard model has survived every test we have thrown at it, including attempts to create new particles or to find new forces that it doesn’t predict (see “Six ways we could finally find new physics beyond the standard model“). So how, exactly, did physicists working throughout the 20th century come up with such an unbreakable framework? This is the story of the most successful theory we have ever devised.
Life was simpler in the 1920s. As far as anyone knew, the only elementary particles were photons, which made up light; protons, found in the centres of atoms; and electrons, which orbited around atomic nuclei. It was a simple picture, but a troublingly immutable one.
According to quantum physics, which arose in that decade, there was no way for these particles to be created or destroyed. Yet, for example, when you shine a torch, electrons in the torch seemingly create and spew out photons. It was mathematician Paul Dirac who realised that the potential for light exists everywhere, in the form of an underlying field. This field is so weak as to be invisible, but given the right energy – if it interacts with an electron, say – it will develop a spike, or excitation, which we observe as a photon.
Dirac called this theory quantum electrodynamics (QED). It was the first quantum field theory – one in which invisible fields, not particles, are the ultimate fundamental entities. Such theories would come to be the bread and butter of particle physics; the standard model itself is merely a more elaborate one. Successful as they have been, however, they are hard to stomach. When a field isn’t excited into particles proper, it is frustratingly unobservable – a background murmuring of not-quite-somethings that physicists have come to call “virtual particles”.
Within a few years of Dirac’s QED, other theorists had extended the field concept to electrons and protons. This allowed physicist Enrico Fermi to explain how radioactive materials can emit electrons when their atomic nuclei don’t actually contain any. The unchanging picture of subatomic matter was gone. Building on work by theorist Wolfgang Pauli, Fermi showed that electrons could be created, so long as sprightly, neutral particles called neutrinos are created simultaneously. The journal Nature rejected Fermi’s paper on this, saying “it contained speculations too remote from reality to be of interest”. Yet the neutrino, experimentally discovered in the 1950s, was only one of many strange new particles that physicists would have to come to terms with.
New subatomic particles
The first addition, courtesy of British experimentalist James Chadwick in 1932, was the neutron, a neutral particle around the mass of the proton. Across the Atlantic, a young US physicist called Carl Anderson had spotted some backwards-looking tracks in photographs of electron ionisations. These turned out to be something Dirac had predicted: examples of the first antimatter particle.
At this point, the world of particle physics still just about made sense. There were atoms, made of protons, neutrons and electrons; there was light, or photons; and there were positrons, or antimatter. But in 1936, in partnership with his colleague Seth Neddermeyer, Anderson found something no one expected: a seemingly heavier version of the electron called the muon.
This unexpected muon was the least of theorists’ worries, though. A few years earlier, J. Robert Oppenheimer, an intense, straight-shooting physicist, had unearthed a rather large problem with QED. He realised there were infinitely many ways an electron could emit and reabsorb virtual photons – meaning its energy ought to be infinite, which made no sense. Soon, the infinities were cropping up everywhere. The theory was “in a hell of a way”, said Oppenheimer. The arrival of the second world war didn’t help matters.
When that conflict was over, three different methods to constrain the infinities came to light: two in the US, devised by Julian Schwinger and Richard Feynman, and one in Japan, conceived by Sin-Itiro Tomonaga. In the end, mathematician Freeman Dyson realised they all amounted to the same thing. “It came bursting into my consciousness, like an explosion,” he later recalled.
Dyson’s singular method was called renormalisation. It works by ignoring the lion’s share of the electron’s energy, which isn’t measurable in an experiment. Only calculate what can be measured and the total energy is no longer infinite. It was a ridiculously simple idea, but it worked for a time.
Forces under scrutiny
By the 1950s, two other forces of nature beyond the electromagnetic force at the heart of QED were coming under increasing scrutiny: the weak force, which is behind Fermi’s radioactive decay and neutrinos, and the strong force, which holds protons and neutrons together in atomic nuclei. For these forces, renormalisation just didn’t work.
This time, the crucial insight came from Chen-Ning “Frank” Yang, a mild-mannered Chinese theorist who had relocated to the US. He had been dwelling on symmetries in particle interactions, a property that implies certain values are preserved during a transformation of some sort (see “Upside down, back to front”). For instance, when photons interact with electrons, the equations naturally balance without needing the electron charge to vary. In this sense, the raison d’être of photons is to conserve the electron charge. We call this charge symmetry.
Working with his colleague Robert Mills, Yang introduced the idea that force-carrying particles similar to photons must exist in order to conserve other particle qualities. The trouble was, he couldn’t make any definite predictions – much to the annoyance of Pauli, who heckled Yang so much during a presentation that the latter fell silent and nervously sat down. Oppenheimer, chairing the seminar, came to his defence, declaring “we should let Frank proceed”. It was a good job they did. By showing how the concept of symmetry in QED (a renormalisable theory) can, in principle, be applied elsewhere, Yang had laid the groundwork for a new class of theories that would form the backbone of the standard model.
In the 1960s, Steven Weinberg, together with fellow physicists Sheldon Glashow and Abdus Salam, created a renormalisable field theory that encompassed both QED and the weak force. It predicted that certain qualities of neutrinos are set by three new particles, known as the W+, W- and Z bosons. These are what we call carriers of the weak force.
Meanwhile, physicists including Murray Gell-Mann were fashioning a new theory of the strong force that holds protons and neutrons together in atomic nuclei. This theory, known as quantum chromodynamics, showed that protons and neutrons are made up of smaller entities known as quarks, the qualities of which are set by yet more new particles called gluons, the carrier particles of the strong force.
By now, with particle colliders becoming a standard tool for discovery, the list of particles being found was rapidly proliferating. Most of these were composite, formed of quarks in different combinations. Although quarks in protons and neutrons come in either “up” or “down” flavours, more flavours were needed to account for the diversity of other composites.
Physicists eventually settled on there being six quarks across three generations of elementary matter particles. In the first generation were the up and down quarks, along with the electron and the electron’s associated neutrino. The second and third-generation particles appeared to be progressively heavier copies of the first.
To this day, no one knows why, when it comes to elementary particles, nature is so profligate. “If I could remember the names of all these particles,” said Fermi, “I would have been a botanist.” There was just one more to complete the set: the Higgs boson, the particle that fixes the mass of all others. Hypothesised back in 1964 by theorist Peter Higgs, among others, it was only in the early 1970s that it came to be taken seriously. It was discovered another 40 years after that. “It is nice to be right about something sometimes,” said Higgs after it was found in 2012.
With the theorised Higgs admitted to the stable, Weinberg delivered his 1973 talk introducing the name for the collection of entities that govern the most fundamental aspects of reality: the standard model of particle physics. The various patches of research had been stitched into a complete account of all the elementary particles, which, to this day, has never been contradicted by any experimental data. “The theory we now have is an integral work of art,” Glashow wrote a few years later, “the patchwork quilt has become a tapestry.”
This article is part of a special series on the standard model, in which we explore:
WHO INVENTED THE STANDARD MODEL?
There was no single moment when the standard model came into being, so the date for its anniversary is up for debate. A meeting in 2018 celebrating 50 years of the standard model took its genesis as physicist Steven Weinberg's 1967 paper "A model of leptons". The term was introduced in scientific literature in 1975, where it was mentioned in passing, suggesting it was already in circulation.
Weinberg said he first used the term at a talk in 1973 in Aix-en-Provence, France. But even he expressed doubts about its origin. "I think I'm the one who gave it that name," he said in 2010, before swiftly adding: "I've never been quite sure about that."
UPSIDE DOWN, BACK TO FRONT
The standard model was guided by a principle shown by mathematician Emmy Noether in 1918: that every symmetry implies a conservation law. The symmetry of physics in time implies energy is conserved, for instance, and symmetry in space implies momentum is conserved.
This proved indispensable to early particle physicists (see main story) who spotted plenty of new symmetries. From these, they inferred conservation laws that revealed which interactions were possible. A symmetry in the behaviour of electrons and neutrinos, for instance, led to electromagnetism and the weak nuclear force coming together into electroweak theory.
Taking symmetry much further, we get "supersymmetry", an extension to the standard model that says every particle has a heavier superpartner. So far, there is no evidence for this.
