Why Colliding Particles Reveal Reality

By Charlie Wood

Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, physics staff writer Charlie Wood gives an overview of the key technology of 20th-century physics: particle colliders.

Much of what physicists know about the fundamental laws of nature has come from building machines to bash particles together.

Physicists began developing particle colliders in the wake of revelations that there was more to the universe than atoms. Ernest Rutherford glimpsed inside the atom in one of the earliest proto-collider experiments in 1909. He and his student put some radioactive material behind a lead shield with a hole in it, so that a stream of alpha particles (now known to be helium nuclei) could shoot through the hole. When they pelted a thin gold foil with this beam of particles, they observed that one in 20,000 bounced straight backward. Rutherford likened it to an artillery shell reflecting off a sheet of tissue paper. The physicists had discovered that the gold atoms were mostly empty space, but that the alpha particles were occasionally scattering off the atoms’ dense, positively charged nuclei.

Two of Rutherford’s students, John Cockcroft and Ernest Walton, went on to assemble and operate the first proper particle collider in 1932. They used an electric field to speed up protons and slam them into lithium atoms with enough energy to break the lithium atoms in two, splitting the atom for the first time.

In the following decades, physicists built a parade of increasingly capable particle colliders. They increased the density of the projectile particles, added superconducting magnets to steer them better, and bought themselves more runway by designing circular colliders. To produce more violent fireworks, they smashed together beams of particles circulating in opposite directions.

Many of the technological innovations came in pursuit of higher-energy collisions for generating richer varieties of particles. All the matter you’ve ever seen or touched is made up of just three lightweight, fundamental particles: electrons and two types of quarks. Nature allows more than a dozen heavier elementary particles to exist as well, but only for a flash before they transmute into light, stable ones. To learn what massive particles can exist, physicists leverage the interchangeability of matter and energy discovered by Albert Einstein, expressed in his famous equation, E = mc2. By generating more energetic collisions, they see heavier particles pop out.

Another way to think of it is that higher-energy collisions push deeper into the subatomic world. All quantum particles have wavelike properties, such as wavelengths. And their wavelengths determine what they can interact with. Sound waves can get around walls because they’re meters long, for instance, while light waves get stopped by anything larger than their wavelength of a few hundred nanometers. The incredibly tiny waves involved in high-energy collisions are sensitive to equally tiny quantum obstacles. In this way, higher energies let physicists explore the rules of reality at smaller and smaller scales.

Following the lead of Rutherford and his students, researchers continued to explore at a breakneck pace. The energy of particle collisions increased 10 times every six to eight years for the better part of a century, nearly matching the pace of Moore’s law for computer chips. That progress culminated in the construction of the Large Hadron Collider (LHC) in Europe, a circular underground track 27 kilometers in circumference that crashes protons together at energies some 20 million times higher than what Cockcroft and Walton used to split the atom. It was at the LHC in 2012 that physicists discovered the Higgs boson, a heavy particle that gives other fundamental particles mass. The Higgs was the final missing piece of the Standard Model of particle physics, a set of equations that accounts for all known elementary particles and their interactions.

What’s New and Noteworthy

The LHC — which began another six-month run in April — cemented the Standard Model with the discovery of the Higgs boson. But what it hasn’t found has left the field in crisis. For decades, many particle theorists hoped a new “supersymmetry” between particles of matter and particles of force would be observed to solve a puzzle called the hierarchy problem, help connect the quantum forces, and provide a candidate for the “dark matter” particles that hold galaxies together.

But the LHC has seen no sign of the particles predicted by supersymmetry, and in 2016 proponents of the theory conceded a bet , acknowledging that our universe is not supersymmetric in the simple way they had thought. The same year, a hint of a new particle turned out to be a statistical mirage, and physicists had to confront the fact that the LHC probably won’t uncover any new phenomena beyond the particles of the Standard Model — a situation sometimes referred to as the nightmare scenario .

Without hints pointing to the existence of heavier particles that could be conjured up in higher-energy collisions, the case for building another, even bigger multibillion-dollar particle collider is hard to make. Some insist it’s worth doing because there’s still plenty to investigate about the Higgs boson, which might hold clues about any heavier entities beyond the LHC’s reach. But no such clues — or entities — are guaranteed.

A proposal to construct a next-generation collider in Japan has stalled. Europe is mulling a 100-kilometer successor to the LHC, but if approved and funded, it will take so long to build that today’s grad students will be long retired before it switches on.

American particle physicists got some optimistic news last December when a government panel supported studying the prospects of a muon collider . Muons are bulkier versions of electrons that would pack more punch in collisions, while lacking the substructure of protons, so that a relatively small muon collider could achieve clean, high-energy collisions. A bleeding-edge muon collider could fit into the footprint of an existing facility, the Fermi National Accelerator Laboratory in Illinois, and so could conceivably be built more quickly and affordably. The catch is that muons decay in a few microseconds, and the technology required to create and control narrow beams of them doesn’t exist yet. Still, if the project goes forward, proponents hope that such a device could be operational around the time today’s kindergartners start getting their doctorates.

In the meantime, physicists have little choice but to come up with alternative experiments and novel ways of piecing together the clues that colliders have already given them.

Around the Web

• The SLAC National Accelerator Laboratory published a history of the evolution of particle colliders in 1997 that’s still available online as a PDF.
• Knowable Magazine outlines the history of particle colliders and particle physics in a podcast episode .
• Science magazine details the hope — and the challenge — of building a muon collider , the particle physics “dream machine.”