How do particle colliders work?

Particle accelerators, also known as particle colliders or atom smashers, have been responsible for some of the most exciting physics findings over the past century, including the discovery of the elusive Higgs boson, the fundamental force-carrying particle of the Higgs field, which gives other particles their mass.

But how do particle colliders work?

As the name suggests, particle accelerators involve accelerating subatomic particles to incredibly high speeds and smashing them into tiny targets, usually atomic nuclei, to achieve a desired effect.

To get an idea of the scales at which particle accelerators work, consider one of the basic units used in their applications: the "barn." It's equal to a square just 10 femtometers — 10 quadrillionths of a meter — on each side. And yes, the term, coined in the 1940s by physicists at Purdue University in the heart of the Midwest, is meant to invoke such sayings as "That's as big as a barn" and "You couldn't hit the broad side of a barn."

However, the simplest and earliest accelerators were relatively straightforward devices. There was a source of electrons, and then you launched those electrons through a cavity filled with electric fields. The electrons hit something on the other side of the cavity. Done.

For a few decades, most people in the U.S. had such a particle collider in their home: a CRT television. CRT stands for "cathode ray tube," and cathode rays are an old name for electrons (before scientists realized that electrons are particles). The electrons accelerated and smashed into a phosphorescent screen that lit up for our viewing pleasure.

Naturally, these kinds of colliders have limits on the size of the cavity and the strength of the electric field you can put in that cavity. So the next step in the evolution of colliders was known as a linear collider. The largest one operating today is the SLAC National Accelerator Laboratory, a 2-mile-long (3.2 kilometers) device outside San Francisco.

The idea behind a linear collider is to repeat the basic operation of simpler colliders. Most importantly, electric fields can either push or pull on electric charges, depending on their direction. So, as a charged particle enters a chamber, the electric field pulls on it to accelerate it. Then, once it gets halfway through, the electric field switches into "push" mode, which continues the acceleration.

Next, the charge exits that cavity and enters another with the same setup — and then another, and then another, repeated as long as you can get away with it (or until funding runs out).

Small linear accelerators power a variety of applications around the world. Need an X-ray at the dentist? There's an accelerator that fires electrons at a piece of metal to generate those X-rays. Need a tumor removed? A proton accelerator makes a great device for targeting cancer cells without harming surrounding tissue. Need a new semiconductor? An ion implanter paints microscopic transistors to create circuits. Need new tires? An accelerator has cross-linked those polymers to make the plastics and synthetic rubber more durable.

But what if you could keep the charge accelerating for an infinite length? The easiest way to make a finite line infinite is to bend it into a circle. The charge can then keep looping around and around, without reaching a stopping point.

But one of the main challenges with this is a bottleneck from relativity. As the particle speeds up, it gains kinetic energy — and energy equals mass, so effectively, the particle gets heavier and heavier. This isn't so much from a pure acceleration standpoint; it's kind of easy to just keep pushing subatomic particles harder. Rather, to keep them moving in a circle, we need to employ magnetic fields. At a heavier mass, the magnetic field can't keep up, and the particle starts to drift and slam into the side of the circular chamber.

So the magnetic field has to stay synchronized with the increasing mass of the particle, ramping up in strength as the particle whips around. Thus, we call these kinds of accelerators synchrotrons.

The flagship synchrotron is the Large Hadron Collider (LHC), operated by CERN (the European Organization for Nuclear Research). It features a ring with a circumference of 16.8 miles (27 km) holding 36,000 tons of magnets and chilled to minus 459.58 degrees Fahrenheit (minus 273.1 degrees Celsius) — colder than outer space — and it can accelerate protons to 99.9997828% the speed of light.

The LHC works in both directions at once. Then, at the last minute — right when the two particle beams have reached their peak energy — they slam into each other head-on at a total energy of 14 tera electron volts.

That's less than a billionth of the energy of a thrown baseball. But considering all that energy is crammed into an incredibly tiny area, the energy densities reach conditions not seen in the universe since the earliest moments of the Big Bang. At those energies, short-lived particles appear from the vacuum, giving physicists a glimpse of the most basic operations of nature.