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When the truck pulls away from the building at Cern, the European particle physics laboratory near Geneva, all eyes will be on its precious cargo, a one-tonne device containing some of the most exotic material on Earth.
The 20-minute test run around the campus, pencilled in for later this month, will mark the world’s first attempt to transport antimatter, a substance so delicate that when it meets normal matter, both are consumed in a burst of pure energy.
To reach this moment has taken years. But if the test goes well – meaning the truck returns with the antimatter intact – it will pave the way for Cern to transport the material to other laboratories. In those facilities, researchers will perform precision measurements in the hope of learning why our universe is built from matter and not these bizarre mirror particles.
“A core question we want to understand is where did matter come from. And then, if you know about antimatter, it’s natural to ask, why is that not here? The process is not understood and we are hunting for clues as to why it happened,” says Dr Christian Smorra, a physicist on the Baryon Antibaryon Symmetry Experiment (Base) at Cern.
Antimatter, a name that implies an almost ideological opposition to the bedrock of our existence, is warmly embraced in science fiction. In Star Trek, it powers the Enterprise’s warp drive and photon torpedoes. In Dan Brown’s Angels and Demons, a canister containing a quarter of a gram of antimatter is stolen from Cern in a plot to blow up the Vatican.
The reality is reassuringly mundane. Antimatter emitters are readily available at supermarkets in the form of bananas, which emit antiparticles through the radioactive decay of potassium. Sadly, they have limited value for understanding the universe. The device on Cern’s truck will carry about 1,000 antimatter particles, weighing about a billionth of a trillionth of a gram. Should the containment fail, and the antimatter make contact with normal matter, the resulting pulse of energy would be so feeble, the load doesn’t even warrant a radioactive label.
Antimatter was first predicted in 1928 when the physicist Paul Dirac married quantum theory, which describes the behaviour of subatomic particles, with special relativity, Einstein’s theory of space and time. The work earned Dirac a Nobel prize and described a universe in which every particle has a corresponding antiparticle, identical but oppositely charged.
The first antimatter was detected four years later. Carl Anderson at the California Institute of Technology spotted what turned out to be an antielectron, or positron, tearing through an instrument that captured particle showers unleashed by cosmic rays. He too won a Nobel prize for his work.
Scientists have since confirmed the full panoply of antiparticles. Antimatter versions of electrons, protons and neutrons can assemble into anti-atoms and anti-molecules. In a different universe, anti-planets might be warmed by anti-suns in shimmering anti-galaxies.
“If we were all made of antimatter and lived in a universe made entirely of antimatter we wouldn’t notice any difference,” says Dr Jack Devlin, a Royal Society university research fellow at Imperial College London. “What’s strange is that somehow the laws of physics allow the existence of this stuff that seems to behave in the same way as normal matter.”
According to modern models of the universe, equal amounts of matter and antimatter were created in the big bang. But what happened next? When matter and antimatter meet, the particles convert directly into energy. So, why is the cosmos not a sprawling expanse of energy?
“We seem to have ended up in a universe which is completely overwhelmed with regular matter and has almost no antimatter in it at all, and that is the heart of the mystery,” says Devlin.
Subtle differences in matter and antimatter, which are already emerging, are expected to explain how matter came to dominate, but uncovering them calls for extremely precise comparisons of the particles’ properties. It also requires a reliable supply of the material.
Enter Cern’s appropriately named Antimatter Factory. Researchers at the facility smash high energy protons, the nuclei of hydrogen atoms, into a dense metal target, creating showers of secondary particles. Among them are antiprotons, which are steered into a decelerator, slowed down, and ultimately captured in an antimatter trap.
But while the factory can produce antimatter, it’s not the best place for precision measurements. The decelerator that slows the antiprotons to about one tenth the speed of light uses powerful fields that make it impossible to perform sensitive measurements nearby. Other laboratories could measure the antimatter with 100 times more precision, researchers say.
With a view to conducting such experiments, Smorra and his colleague, Stefan Ulmer, are building a device to receive antiprotons at Heinrich Heine University in Düsseldorf. To survive the trip from Cern, the antimatter would need to be contained for more than 10 hours: two for loading and unloading the trap and the rest for the 500-mile drive.
The trap itself is a feat of engineering. It must hold antimatter in such a way that it never comes into contact with normal matter. To do this, the chamber is held under ultra-high vacuum, comparable to the emptiness of interstellar space. It is cooled to -269C, causing any stray gas to freeze on the chamber walls. Strong magnetic and electric fields are then used to hold the antiprotons in the centre of the cryogenic chamber.
The fields are strong enough to hold the antimatter in place should the truck hit bumps or brake sharply in transit. Perhaps the greatest threat to the material is getting stuck in traffic and the power supply failing. For the test run at Cern, the trap will be powered by batteries that last about four hours. Longer trips will need a dedicated generator on board.
“If we ever want to do experiments with antiprotons somewhere else, we need to get this on the road and that’s what we’re trying to do,” Smorra says. “First of all we have to show we can move the antimatter and this is the big milestone for us.”
