Antimatter has always been full of surprises. The first was that it existed. The second was that it didn’t.
First things first. In the 1920s, British physicist Paul Dirac managed to marry quantum theory with Einstein’s special relativity to explain how tiny, fast-moving fundamental particles such as electrons work. But his austerely beautiful unifying equation, , had an unwanted consequence. For every matter particle like an electron, it predicted the existence of a second particle that was the same, but opposite in things like electric charge.
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Dirac initially brushed this under the carpet – out of “pure cowardice” he later said – but three years on, the antimatter version of the electron, the positron, was discovered in cosmic rays. Since then, as the standard model of particle physics was built on the foundation that Dirac and others laid, a very different problem has emerged. Antimatter shouldn’t just exist, it should be abundant: every time a matter particle is made, an antimatter particle should also be conjured from the void. “We should have a universe half full of antimatter,” says Michael Capell, an astroparticle physicist at the Massachusetts Institute of Technology. So where are these particles?
They can’t be near us because matter and antimatter mutually “annihilate” whenever they meet, and we would notice the flash of X-ray energy produced when they do. Various small-scale particle behaviours might allow there to be slightly more matter than antimatter, but none of these effects is nearly big enough to explain the size of the discrepancy we see.
Perhaps, then, the missing antimatter is elsewhere – in stars and galaxies made exclusively of the stuff, much as our sun and Milky Way are made solely of matter. Stars made of antimatter would give out the same light as ordinary stars, but also a wind of antiparticles, just as our sun gives out matter particles. When these antiparticles come into contact with ordinary matter outside their galaxy, they should produce X-rays that would again be visible across the universe.
One in a billion
We are yet to see anything of that ilk either, but the is performing a more direct test. This giant particle detector, lofted onto the International Space Station in 2011, can sort matter from antimatter in passing cosmic rays.
Positrons and antiprotons can be made relatively easily in today’s universe, for example when high-energy particles collide in the strong magnetic fields around dead stars. The real prize would be something bigger. Most helium was made in the first few minutes of the universe’s existence, so to find anti-helium could mean that the same process created the expected large quantity of antimatter. Stars are the only places where carbon and heavier nuclei can be made, so a single anti-carbon nucleus would confirm that there is an antimatter star somewhere in our universe.
It’s like looking for a needle in a haystack – you would expect one complex antiparticle for every billion or so matter particles AMS detects, says Capell, who works on the project. The experiment has just about collected enough events to start saying something meaningful, but it is a race against time. The hunt has to be conducted in space, because antiparticles annihilate on contact with our atmosphere, but space is harsh on technology. “AMS has been working like a champ but we can see that it is ageing,” says Capell. In 2014, one of its four cooling pumps stopped working – a worrying development for an experiment designed to last until 2024.
So fingers crossed for something soon to overturn the evidence of our eyes – that we live in an entirely matter-dominated universe.
This article appeared in print under the headline “What if… antimatter worlds exist?”
