A once-fantastical collider could answer physics’ biggest mysteries

Kyle Ellingson When it comes to particle physics, Tova Holmes has been there, done that and got the T-shirt – in fact, she designed the T-shirt herself. It all started back in 2022, when she and a few colleagues arrived at a meeting of particle physicists determined to make the case for developing an entirely new kind of particle-smashing machine. They did so by sporting tops emblazoned with a motif representing a circular particle accelerator and a single word: BUILD. “We wanted to find a way for people to visibly show how excited they were about a muon collider,” says Holmes, who is based at the University of Tennessee, Knoxville. To its advocates, this newfangled collider would be exactly the shot in the arm that particle physics so desperately needs. The famous Large Hadron Collider (LHC) at the CERN particle physics laboratory near Geneva, Switzerland, wonderful as it is, simply hasn’t delivered any truly new discoveries in years. The answer, say Holmes and her ilk, isn’t to build ever-more powerful successors to the LHC, as some would like, but to change the game entirely. They want to collide together a strange type of particle known as the muon. We've discovered a door to a hidden part of reality – what's inside? Physicists would dearly love to find new particles, but there's no sign of them in colliders like the LHC. Now we have found a new way of accessing a tiny slice of reality where they might be hiding To many, though, the proposal has long seemed fanciful at best. After all, muons live for only a fraction of a second. But technological developments are now starting to make the idea more feasible – and funding organisations are eyeing it with serious interest. All of which makes it worth asking: what would it take to build this magnificent muon machine and, if we did, what secrets of reality might it reveal? In 2012, the LHC confirmed the existence of the Higgs boson , a particle proposed nearly half a century earlier to explain how the fundamental forces of nature first split in the early universe. The boson is produced by an excitation in the Higgs field, which endows certain particles with mass – including the W and Z bosons that carry the weak force – while leaving others, such as the photon, untouched. Subscriber-only newsletter Sign up to Lost in Space-Time Untangle mind-bending physics, maths and the weirdness of reality with our monthly, special-guest-written newsletter. Sign up to newsletter It was a spectacular vindication of physicists’ theories about the world of particles. But it was also unsettling. The Higgs boson’s own mass is puzzlingly small . Quantum field theory suggests it should be far larger, yet it perches, unnaturally balanced, at precisely the level required to keep the vacuum of space-time stable. Why so perfectly poised? “People talk about the Higgs discovery as the completion of particle physics,” says Patrick Meade at Stony Brook University in New York state. “But it was really the most confusing answer