Nestled in the heart of CERN’s antimatter factory, surrounded by intensely powerful magnetic fields and within a vacuum sparser than interstellar space, is some of the most sensitive material on Earth. Inside a filing-cabinet-sized box, which weighs a few hundred kilograms less than a Ford Focus, are a handful of antiprotons that have sat for weeks in extraordinary stillness. Most other particles produced in this building might expect to be probed and prodded, but these antiprotons have just one job: to sit tight and wait for their ride.

These hundred or so antimatter particles will soon be transported on the back of a truck around a 4-kilometre loop of road around the CERN campus, which will be the first demonstration of a future antimatter delivery service that will one day see antimatter transported to laboratories around Europe.

I have come to CERN’s campus, near Geneva, Switzerland, to see the experiment, called the Symmetry Tests in Experiments with Portable antiprotons (STEP), in its final preparations before the big day, as project leader Christian Smorra shows me around the facility.“It’s groundbreaking for antimatter science,” he says. “The idea of transporting antiprotons existed, in principle, since the time when this facility started, and now it’s the first time that it has become possible to actually do that.”

We have known since the 1920s that many particles have a near-identical counterpart, save for an opposite charge, called antimatter. But it took nearly a half-century for scientists to be able to produce and store the simplest antimatter – an antiproton – in significant quantities due to its propensity to annihilate and vanish when encountering its matter counterpart, the abundant proton.

The first experiments to confine antiprotons were carried out at CERN in the 1980s, where they were produced by smashing together protons into metal targets. Today, CERN’s Antimatter Decelerator hall, known as the antimatter factory, is the only place in the world that can produce millions of antiprotons on demand and store them for further study. It is home to seven different antimatter experiments, including the Baryon Antibaryon Symmetry Experiment (BASE), of which STEP is a part of.

All these experiments are testing antimatter’s fundamental properties to extreme precision to see how it might deviate from regular matter. Any differences could shed light on why we appear to live in a matter-dominated universe, with a near-total absence of antimatter.

But to really probe down to the extraordinary precision required, it is necessary to filter out noisy radiation that might interfere with measurements, which is a problem for the antimatter factory. When antiprotons enter the hall, they are travelling at nearly the speed of light and must be slowed using powerful magnetic fields, which are impossible to fully block out.

In 2018, Smorra and his team realised that they would need to move antimatter away from the factory to somewhere quieter – and hatched an escape plan. “We had seen the impact of the magnetic field fluctuations, so it was clear that we would eventually need to continue our precision measurements [elsewhere],” says Smorra.

This wasn’t an easy task. Containing antimatter typically requires powerful magnetic fields produced by superconducting magnets, which need to be kept at near-absolute zero, requiring huge amounts of power. Smorra and his team designed STEP to use just a 30-litre tank of liquid helium to keep the magnets cool, so the electronics can instead run on a simple diesel generator. For the upcoming test run, though, it will use only battery power.

The magnet also has to be engineered to cope with the stop-start accelerations that occur while driving, as well as a bespoke vacuum system to ensure that the absence of problematic regular matter can be maintained while the antiprotons are loaded into and unloaded from the trap.

In 2024, Smorra and his team demonstrated that STEP works for regular protons by driving their contraption around the CERN campus on a truck. Now, Smorra and his team are about to try the real thing.

The preparations so far have been relatively straightforward. About a week before I arrived, around 100 antiprotons were slowed down and entered into the complex system of vacuums and electromagnetic fields that will hold them.

Since then, they have been sitting there idly at the centre of a tangle of wires and liquid helium pipes. Smorra and his team can check on their antimatter’s vital signs using a small oscilloscope screen affixed to the machine, where the characteristic frequency that antiprotons vibrate at takes the form of a two humps. They have affectionately pinned two googly eyes above each peak.

In the early hours of Tuesday morning, a crane will lift the entire 850-kilogram trap onto the back of a truck, driven by someone who will have had specialist training to drive CERN’s sensitive equipment around, making sure they don’t accelerate or stop too suddenly.

The truck will then take a 4-kilometre loop around the CERN campus, arriving back at the antimatter factory where it started.

If their test is successful, the eventual goal for Smorra and his team will be to drive their antimatter capsule on roads beyond CERN, delivering it to laboratories across Europe. One such facility is currently under construction at Heinrich Heine University Düsseldorf in Germany, where the antimatter will be studied in the absence of almost any external magnetic fields. However, this goal could take several years, as CERN will largely shut down in July to upgrade the Large Hadron Collider to operate at higher powers. That upgrade won’t finish until late 2028.

But once the antimatter delivery service is up and running, you could be driving down a Swiss or German motorway and find yourself next to a truck full of antimatter. It will look just like a normal truck, but its contents will be anything but normal. This might sound like a concerning proposition, given antimatter’s tendency to annihilate when it meets regular matter, but people shouldn’t be fearful, says Smorra.

“There’s nothing dangerous about the transport of antimatter, because the amount that we are transporting is so small,” says Smorra. “If you transport 1000 antiprotons and it gets lost, you won’t even notice it.”