To visit CERN is to go down a rabbit hole. This is not because to reach the organisation’s atom-smashing Large Hadron Collider (LHC), in a ring-shaped tunnel 27km in circumference below the Swiss-French border, requires burrowing down at least 80 metres. Nor is it because visiting one of the most complex machines ever built, using technology that’s at the limits of invention, involves a short trip from Geneva by quaint, lo-tech tram. To accept CERN’s invitation to turn away from mundane everyday concerns to look at fundamental questions – such as what there is and where it came from, and why it exists at all – is to encounter the unexpected and counter-intuitive. Just as we look out at the night sky into mostly empty space, so if we look inward there’s mostly empty space there, too. “I know it sounds terrible,” says particle physicist Steven Goldfarb, who is taking me underground to visit his ATLAS experiment. “But our universe is like that.” The scientists at CERN are using their machines to provide us with ever more detailed knowledge about just how little we know, and their findings are often helping us to reach ever higher levels of ignorance. CERN physicists release Covid-19 ventilator design to ease shortage CERN welcomed its first non-scientific visitors just two years after it opened in 1954. Its main campus is now by far Geneva’s most popular attraction. “The DNA of CERN is to share whatever we do, not only among scientists, but also with the public,” says physicist and science communication specialist Antonella Del Rosso. “This is part of our mission.” You may well be using a version of one of CERN’s ideas to read this article. CERN is where the World Wide Web was invented. Covid-19 may have reduced visitor numbers, but it hasn’t slowed the pace of research; work continued even while the LHC underwent a three-year shutdown for a substantial upgrade. “The whole purpose of CERN has always been, ‘What are the building blocks of the universe? What are the Lego bricks that you can no longer break any more?’” says Goldfarb. So by the end of my visit am I going to understand the origins and the meaning of the universe? “Oh yes,” says Goldfarb, cheerfully. “We can get that done in the first five minutes.” We’re waiting for an industrial lift to take us down to the cavern holding the ATLAS experiment’s giant drum-like detector, 25 metres in diameter and 45 metres long, around which instruments attempt to capture the results of collisions between two beams of protons accelerated in opposite directions to 99.999999 per cent of the speed of light. So he is taking the particles we know about, and smashing them up to see what they are made of? We’re doing this to answer one of the most fundamental questions you can have about the universe – Why is there something rather than nothing? Michael Doser, senior research physicist working with antimatter at CERN “Well, that’s one way to look at it,” he says. But, of course, it’s the wrong way. “Really what we’re trying to do is to pass protons through each other. This is not easy to do. We take 100 billion against 100 billion, and every 25 nanoseconds we do this. And, at least during the [experiment’s] second run, about 60 actually passed through each other.” Even then only one in a million interactions have interesting results. But when a new particle seems to appear, he can’t just say, “Here’s a Higgs boson.” “We never say that. We say ‘That’s not a Higgs boson, it’s a Higgs boson candidate,’ because there are other things that look like that.” The ATLAS and the CMS (compact muon solenoid) experiment, another of the four stationed around the LHC’s rim, pooled data to produce statistically significant evidence and announce in 2012 the discovery of the Higgs boson – a particle or field that gives mass to other particles, and a discovery of major importance to the Standard Model of particle physics. Almost everything of interest to Goldfarb took place in the universe’s first second, 13.8 billion years ago, when a range of heavier particles decayed into lighter ones plus energy, although apparently “decay” is also the wrong description. It has all been a bit dull since then, although to the outsider, down at the LHC, it is anything but that. A vast subterranean hall is festooned with cables and ducting, gantries and supports, and filled with complex shiny objects. One of these is a superconducting magnet and the cryogenic system needed to keep it slightly above absolute zero (−273.15 degrees Celsius). Its job is to curve the track of some particles released in the experiments while other detectors measure the energy of those that slam into them. Building a Higgs boson factory: China’s race to the frontier of physics Towering over all of this is a multistorey metal disc radiating yellow-brown trigger chambers, which tell the system when to take pictures, millions of them a second. It would be possible to believe that this is a time machine, and in a sense it is. The alternative to going back 13.8 billion years to look for now-vanished particles is to try to recreate them, to try to find flaws in the Standard Model of the universe, although it seems flawed enough already. It doesn’t account for gravity and it only describes the matter we can see. But the behaviour of the galaxies, spinning round without flying apart, indicates that detectable matter constitutes only five per cent of what there is. Other unseen matter or forces must be holding the galaxies together. Or perhaps our description of gravity still needs improvement. New study reveals time can flow either way in quantum physics But everyday Newtonian gravity is still important at CERN. A short drive across the site, which resembles a cross between a university campus and a factory complex, an unimposing, shed-like building has an incongruously exciting name: The Antimatter Factory. Inside, Michael Doser is working on AEgIS (antihydrogen experiment: gravity, interferometry, spectroscopy), one of several experiments using a much smaller accelerator. This involves working with antiparticles rather than particles, and decelerating rather than accelerating them. “We’re doing this,” he says, “to answer one of the most fundamental questions you can have about the universe – ‘Why is there something rather than nothing?’” Doser is also looking at the first moments of the Big Bang. “Any time energy is condensed and transformed into mass, you don’t produce a single particle. Because of symmetry you produce pairs of a particle and a mirror image of that particle – its antimatter partner,” he says. So the universe should consist of equal quantities of matter and antimatter, or the two should have got together and swapped back into energy, leaving nothing but light. So once again something massive is missing – the universe’s antimatter counterpart. “The absence of antimatter is in a way a huge embarrassment,” he says. “How can you lose a whole universe?” The only antimatter in the universe is manufactured, caught and kept here, and some of it is whizzing around in a vacuum, held in place by electric and magnetic fields, in a small ring below us – a sort of model village version of the LHC. There are purple magnets at the corners of the hexagonal device which are doing the bending, orange magnets between them focusing the beam of antiprotons and slowing it, and foil-covered exit tubes that deliver antiprotons to experiments. Since matter still exists and antimatter doesn’t, they cannot be exact mirror images of each other, but must have some differences, perhaps in reacting differently with the universe’s known forces. Doser’s experiment involves trying to drop some antimatter to see how it behaves under gravity. On Earth, any matter dropped will be moving at 9.81 metres per second one second later, but at the speeds even the supercooled and decelerated antimatter moves – hundreds of kilometres a second – any reaction to gravity is almost impossible to detect. Is there anything around here that’s actually simple? “No,” admits Doser ruefully. “Everything we’re trying to do goes well beyond what one knows how to do. Most likely we will learn [antimatter] behaves completely like we expect it to, which is like normal matter.” But down the rabbit hole that’s a good result. The Data Centre’s presentation room gives a view into a hall lined with row upon row of humming cabinets, much like those processing your Amazon purchase or Facebook post. And, as a bonus, an ordinary desktop computer on display is one of the first two ever to run the World Wide Web. Even though superfast computers select only the most interesting images to retain, CERN’s Data Centre is still receiving around 60 gigabytes a second from various experiments – data that must be safely stored, backed up and made available for analysis on demand. The cabinets house not only the CMOS flash drives now common at home, but also 10,000 conventional hard drives. “It’s warm, it’s loud and it’s uncomfortable,” warns data expert Dan van der Ster, and so it is. But the spaces between any two paired rows of inward-looking cabinets are cooled, and white panels stretch away into the distance like those of HAL in 2001: A Space Odyssey . The offline storage solution for all this data is the astonishingly lo-tech medium of tape, although now when an online request is received for archived material, one of dozens of robots will find the right cassette and insert it in a drive. So analysis of CERN’s data can continue during Covid-19 because scientists can access it from anywhere. The ordinary visitor can also discover CERN via visit.cern/discover-cern-online, and even take a virtual guided tour at visit.cern/virtual. But it may take a little more than five minutes to understand.