Quite often on my socials, I see a T-Shirt designed by the ever-so awesome, Jim’ll Paint It, which says on it “Large Hadron Collider. What does it do? No one knows -> WTF? I once saw it in the wild on a night out with mates, and very drunkenly tried to explain it to the wearer who wasn’t in the slightest bit interested in anything a drunk physics nerd had to say. I do that a lot, try and explain things to people after I’ve had a couple of beers, when they don’t actually care and wish the boring person excitedly gesticulating before them would please shut up so they could get on with their night out. Which is one of the reasons I started writing about this stuff here.
Anyway, enough of my reminiscing on great nights out with friends. You don’t want to hear any of that, you’re here to hear all about the LHC.
Somewhere beneath the gentle vineyards and villages that straddle the French-Swiss border, a stream of protons is being whipped around a 27-kilometre underground ring at a whopping 99.9999991% the speed of light. The machine doing this is so cold that parts of it are colder than outer space. It consumes enough electricity to power a small city. It contains magnets so powerful they could tear everyday metal objects across a room. And the entire purpose of this absurdly expensive, ridiculously complicated contraption is to smash tiny particles together and stare very carefully at the wreckage.
Although this sounds less like science and more like the opening chapter of a hard science-fiction novel. It is, in fact, very real. And it is something you have all heard of.
The Large Hadron Collider, or LHC, is arguably the most ambitious scientific instrument humanity has ever built. It is our attempt to reverse-engineer reality itself. If the Universe is a giant cosmic puzzle, the LHC is the world’s most expensive method of throwing the pieces into the air and watching where they land.
In 2012, it succeeded in finding the Higgs boson, the last missing piece of the Standard Model of particle physics. Today it continues searching for dark matter, probing conditions that existed fractions of a second after the Big Bang, and hunting signs that our current understanding of nature is incomplete.
Which, if we’re being honest, once it’s sussed those out, would be the most exciting outcome of all.
But what actually is the LHC? At its simplest, the Large Hadron Collider is a gigantic circular particle accelerator operated by CERN, the European particle physics laboratory. The ring is buried roughly 100 metres underground and stretches for 27 (just over 16.5 miles) kilometres, crossing back and forth underneath France and Switzerland.
But calling it an accelerator is only half the story, as the clue to its true purpose is in the name. It’s a collider.
Many particle accelerators simply speed particles up and fire them at a target. The LHC does something far more dramatic. It accelerates two separate beams of protons in opposite directions around the ring and then deliberately smashes them into each other.
This turns out to be much more efficient. If one moving proton hits a stationary target, much of the energy goes into pushing the target backwards. But if two equally energetic beams collide head-on, almost all of the energy is available to create new particles.
It’s the particle-physics equivalent of two Formula One cars driving into each other at full speed rather than one car hitting a parked vehicle.
Keeping these proton beams under control is astonishingly difficult. Protons naturally want to continue in a straight line, so the LHC uses more than a thousand enormous superconducting magnets to bend them around the circular tunnel.
These magnets are kept very, very cold, and are cooled to about 1.9 Kelvin, which is roughly a very chilly -271°C. And that, folks, is colder than the average temperature of deep space.
The reason for this is simple but weird. Electricity flowing through ordinary wires encounters resistance, producing heat and wasting energy. At extremely low temperatures certain materials become superconductors, allowing electricity to flow without resistance. The LHC’s magnets need immense electric currents to generate the magnetic fields required to steer particles that are moving at nearly light-speed. Without superconductivity, the whole endeavour would be impractical.
OK, I hear you, you get that, but why is the machine so huge?
And that is because making particles faster isn’t simply a matter of pressing a bigger accelerator pedal. The faster a particle goes, the harder it becomes to bend its path. To keep ultra-relativistic protons moving in a circle, you either need stronger magnets or a larger ring, and the LHC uses both.
Let’s take a step by step breakdown and see how it actually works, and the journey itself begins with something surprisingly ordinary: Hydrogen.
A hydrogen atom consists of one proton orbited by one electron. Scientists strip away the electron, leaving behind a lone proton ready for acceleration, before setting it off on its grand tour of CERN.
First, it enters a linear accelerator before then passing through a series of progressively larger booster rings. Each stage adds more energy, like pushing a swing higher and higher, until, eventually the proton beam arrives in the LHC itself. Which is where the real fun starts.
The beams circle the ring over 11,000 times per second, accelerating to 99.9999991% of light speed. At these velocities Einstein’s relativity becomes unavoidable. The proton’s speed can barely increase further, so additional energy instead increases its momentum enormously.
The two beams race around in opposite directions until they meet at four major collision points, where there are some of the most sophisticated scientific instruments ever built waiting for them.
These are ATLAS, CMS, ALICE, and LHCb.
ATLAS and CMS are the giant all-purpose detectors responsible for the Higgs discovery. ALICE specialises in studying quark-gluon plasma. LHCb focuses on subtle differences between matter and antimatter.
When the beams cross, protons collide, which is where many popular descriptions get the story slightly wrong as the point isn’t really to smash particles apart to see what’s inside them. The real goal is to convert energy into entirely new particles.
Einstein’s famous equation, E = mc², tells us that mass and energy are different forms of the same thing. Sufficient collision energy can literally become matter, and as the protons collide, for a fleeting instant, particles that haven’t existed naturally since the early Universe can pop into existence. Most of them survive for merely tiny fractions of a second before decaying into other particles, and the detectors capture these decay products.
Imagine investigators arriving after a firework factory explosion. They never saw the explosion itself. They only have scattered debris, burn marks, smoke patterns and fragments. From that evidence they reconstruct what happened, which is essentially what ATLAS and CMS do, but they do it millions of times every second.
And here’s a neat callback to the quantum measurement problem.
Before detection, quantum mechanics often describes multiple possible outcomes existing in superposition. Once the detector records an actual event, one specific outcome becomes reality’s official entry in the ledger. Every collision is therefore an example of a quantum process producing a definite measurable result. The detectors don’t merely observe the aftermath. They convert quantum possibilities into cold, hard, recorded facts.
No article on CERN or the LHC would be complete without mentioning the big one. The Higgs Boson. If particle physics has a celebrity, the Higgs boson is the most famous by a country mile.
To understand why though, we need to talk about the Higgs field.
According to modern physics, fields exist throughout the Universe. The electromagnetic field exists everywhere. So do the fields associated with electrons and quarks. The Higgs field is another such field. It fills all of space.
Particles moving through it interact with it to differing degrees. Some interact strongly and acquire large masses. Others interact weakly and remain relatively light. An imperfect but useful analogy would be to imagine walking through a crowded room.
A celebrity trying to cross the room gets constantly surrounded and slowed by admirers, whereas a complete unknown person, such as you or I, could walk through almost unhindered.
The Higgs field acts a little like that crowd and the Higgs boson is a ripple in this field, just as a photon is a ripple in the electromagnetic field.
By the late twentieth century the Standard Model had become an astonishingly successful theory. It explained quarks, leptons, gluons, photons, the weak force and an enormous range of experimental results, but there was still one piece missing.
The Higgs.
Without it, the mathematical framework simply didn’t work correctly. The Standard Model was like a jigsaw puzzle with a hole in the middle, and physicists spent decades searching for it.
Multiple generations of accelerators looked for clues, and found…
Nothing.
Then the LHC arrived on the scene, and on the 4th July 2012, CERN announced that the ATLAS and CMS experiments had independently observed a new particle consistent with the long-predicted Higgs boson. The statistical confidence even exceeded the famous five-sigma threshold, meaning the probability of the result being a random fluke was roughly one in several million. After nearly fifty years of theoretical expectation, the particle had finally been found. We had the Higgs Boson.
One of the most memorable images from the announcement was theoretical physicist Peter Higgs sitting quietly in the audience as decades of speculation became experimental reality. A truly remarkable sight, that pulls at the heart strings and to this day still lubricates my tear ducts with the memory. And as the significance of the result became clear, he reportedly wiped away his own tears. Just imagine it. Your life’s theoretical work becoming a fact of science. Wow. Just WOW!
A year later, in 2013, Peter Higgs and François Englert (sadly, Robert Brout had passed away in 2011 and Nobel doesn’t award posthumously), very much deservedly, received the Nobel Prize in Physics.
The discovery was monumental not because it opened a new chapter but because it completed an old one. For the first time, every fundamental particle predicted by the Standard Model had been observed and humanity had found the final missing actor in nature’s best-tested theory.
After such a triumphant discovery, you may be wondering what the LHC did next, and is still doing as I type this article. The Higgs may be the headline act, but it isn’t the whole show, as those clever dudes at CERN have been recreating the early universe.
The ALICE experiment studies something called quark-gluon plasma. Now then, ordinarily quarks are permanently confined inside protons and neutrons. But at the unimaginably high temperatures that existed shortly after the Big Bang, quarks and gluons formed a superheated fluid.
Heavy-ion collisions at the LHC briefly recreate these ancient conditions and for an instant, the Universe is returned to a state it last experienced nearly 14 billion years ago.
As we discussed in the dark matter article, most of the matter in the Universe appears to be invisible, and the LHC is very much hunting for this dark matter. Although it can’t directly see dark matter particles, if collisions create them, they may escape the detector unnoticed, so scientists are searching for the telltale signature of missing energy and momentum. Something leaves the collision carrying energy away, but nothing visible accounts for it. Thus far, no confirmed dark matter particle has emerged, but the search very much continues.
Physicists once hoped the LHC might discover evidence for supersymmetry, often shortened to SUSY. This idea predicts that every known particle has a heavier partner and supersymmetry could help explain dark matter, unify forces and solve various theoretical problems. Indeed, it was one of the most anticipated possibilities before the LHC began operations, and so far it has found…
Nothing.
Which is itself a scientific result. Nature is under no obligation to reward our favourite theories. Sometimes experiments confirm ideas, and sometimes they eliminate them, and both outcomes move science forward.
Then we have the matter-antimatter mystery. The LHCb experiment investigates why the Universe contains far more matter than antimatter. According to current theory, the Big Bang should have created them in nearly equal quantities, yet a Universe made of equal matter and antimatter would mostly have annihilated itself, an eventuality which quite clearly did not happen, and somewhere in the laws of physics, matter gained a slight advantage. LHCb is hunting for clues.
As we look at the bigger picture, the most remarkable thing about the Large Hadron Collider is not that it found the Higgs boson.
It’s that humanity built a 27-kilometre machine colder than outer space, accelerated particles to within a hair’s breadth of light speed, recreated conditions from the first moments after the Big Bang, and then used the resulting subatomic shrapnel to test ideas about the deepest structure of reality. All the Higgs really did was to complete the Standard Model.
But the Standard Model is not the end of the story. It still cannot explain dark matter. It says nothing about dark energy. It does not incorporate gravity. It leaves major questions unanswered. Which, in turn, means the LHC stands in a curious position, as it is both humanity’s triumph and an admission of ignorance.
We built it to answer questions. Instead, it gave us something scientists secretly like even more.
It gave us better questions!
P.S. I’ll do a post about the current upgrades the LHC is getting at the moment when it is up and running again.
