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Blinking into Existence

by
Scope Correspondent

It is perhaps the raw and reckless potency of antimatter that so excites us. For Star Trek writers and hack fiction authors alike, the very name, anti-matter, seems less about what it is and more about what it does. Be careless enough to put it together with normal matter and it will cancel out, negate. Even the clinical term for that process is suggestive: it will “annihilate.” And it will do so spectacularly, in a flash of gamma rays. Kaboom.

There’s some small irony, then, that humanity’s first contact with antimatter came not at the instant of its destruction but at its creation.

This happened in the 1920s and 30s, when the world was made of far fewer basic building blocks than scientists know of today. Physicists now speak of muons, pions, neutrinos—an invisible menagerie, a whole rogues gallery of particles teeming at reality’s smallest scales. But back in 1928, only two small particles had been discovered.

There was the positively-charged proton, which was thought to make up the hard core at the heart of every atom of everything. Then there was the negatively-charged electron, flitting at arm’s length of the core, a tiny gnat when compared to the proton’s mass but an exact foil in terms of electric charge.

At the time, theoretical physicist Paul Dirac was trying to describe the behavior of electrons in the context of Einstein’s special theory of relativity. He succeeded, but he also stumbled on an interesting problem. Nothing in his equations said an electron had to have a negative charge. For that matter, nothing said a proton had to have a positive charge. If the math could be trusted, mirror versions of both particles should exist, opposite in charge but exactly alike in every other way. They would be anti-particles.

When Dirac described these ideas at conferences, other physicists were skeptical. What was this positively-charged particle doing in an equation describing electrons? Or was it a proton, the only particle known at the time to have a positive charge? If not, was it something new, something different? Yet where was the evidence for it? Dirac’s equations indicated these things should show up occasionally, but nobody had seen one.

Three years after Dirac’s idea that a positive electron should sometimes pop into being, Science News Letter reported that 27-year-old Carl Anderson had seen one in a lab at Caltech. Anderson was not the first to observe an anti-electron. But crucially, he was the first to link what he saw to Dirac’s idea. Anderson presented photographic evidence of an electron and its opposite, which the article dubbed a positron. Earlier sightings could now be understood in hindsight.

Anderson was able to take a picture of things so unfathomably tiny with a device called a cloud chamber. As particles streak through the chamber, moisture condenses around their paths like contrails behind the world’s smallest jet planes. These trails hang in the fog long enough for a picture to be snapped. The comings and goings of the subatomic world, usually untraceable, are clear like footprints in the snow.

Add a magnet off to the side of the cloud chamber, as Anderson and others did, and something interesting happens. Electrons, with their negative charge, are affected like tetherballs: they curve toward the magnet. But what if you see the path of a particle bending away from the magnet?

It could be a positively charged proton; only Anderson knew a proton was too big to make the trail he saw. It could be an electron coming up from the bottom of the chamber instead of through the top—only the particle seemed to move through the top half of the chamber first. Anderson was left with one choice: he’d seen a positron. Just as Dirac had predicted.

What Anderson saw was a positron blinking into existence, likely transmuted into being as a cosmic ray passed through the chamber.. As twentieth-century physics wore on, scientists were able to find or make antiprotons, antineutrons, and even anti-hydrogen, an antiproton paired with a positron.

So what about that little issue of annihilation: antimatter’s refusal to place nice with ordinary matter. A positron and an electron will seek each other out, each attracted by their mirror’s opposing charge. Drawing closer, they will initiate a spiraling dance that ends in one ten-millionth of a second as both converge into nothingness, leaving behind only the pure sum of their respective energies in a blinding flash of gamma rays.

It is in this, its death, that antimatter is most “useful.” The flash of antimatter meeting matter helps doctors pinpoint tumors in the human body during PET (Positron Emission Tomography) scans, and it allows particle physicists to simulate the white-hot energy that permeated the universe in the instant of the Big Bang.

But though the life of an antimatter particle is short, lasting only as long as it takes to stumble into matter, and though it will inevitably vanish in a blaze of glory, it is worth remembering Dirac’s remarkable insight and Anderson’s discovery. Antimatter must live before it dies.

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