Secrets of antimatter

The Big Bang created equal parts matter and antimatter. So what happened to all the antimatter? The answer may finally be within reach, CATHAL O’CONNELL reports.

OUT OF THE BIG BANG’S unimaginable light, two armies emerged and engaged in a frenzied attack. On one side were the particles of matter – electrons and protons. On the other side, antimatter – identical to the matter particles, except with opposite charge. Electrons clashed with their positron counterparts and destroyed one another. Protons duelled antiprotons, to the same violent end. It appeared the two sides were headed toward mutually assured destruction.

Then, a millionth of a second later, the fighting ceased.

In the aftermath lay a wasteland of photons and empty space. But when the dust settled, one side remained standing. For every billion pairs of matter and antimatter particles, a single particle of matter emerged unscathed from the melee. Today, billions of years later, everything we see – galaxies, stars, all of the atoms and molecules that make your body and mine – are descended from those surviving particles.

There’s just one problem: Everything we know about physics says we shouldn’t exist. Matter and antimatter are always created in equal parts, so the two armies should have wiped each other out, leaving the Universe empty, dark and lifeless. That it’s not means there’s something seriously wrong – or embarrassingly incomplete – about our best theory of how the Universe is screwed together.

The triumph of matter is one of the greatest mysteries in all of science. The big question that’s stumped physicists for decades is: what gave us the edge?

Now, using sophisticated new instruments that are as large as their targets are small, scientists are closing in on an unlikely solution. As it turns out, the secret may lie in a quiet corner of particle physics where a newly discovered quirk in the personality of neutrinos is challenging scientists to rethink the origin of the Universe.

TODAY ANTIMATTER is a well accepted ingredient in the recipe book of particle physics. We know that every matter particle has a twin antimatter particle that’s the same mass, but with opposite charge. We even use positrons in medical imaging ( see box ‘Everyday Antimatter’). But until 89 years ago, we had no inkling of antimatter.

It was the winter of 1927, and a young British physicist named Paul Dirac was in the com-mon room at the University of Cambridge, staring at the fire. As taciturn as Dirac was (his colleagues would later define a unit of a “dirac” as one word per hour) he was even more absorbed than usual. He was thinking about a thorny problem.

Physics was in the throes of a revolution. The new theory of quantum mechanics – which recast the subatomic world in terms of particles that could hop between energy levels – was beginning to explain previously mystifying effects. Just two years earlier, Erwin Schrödinger had placed the fledgling theory on a firm footing with his equation describing how electrons orbit an atomic nucleus. This equation allowed physicists to explain the structure of atoms with unprecedented detail. Yet some of those details, such as an electron’s magnetic readings, were off.

The problem was speed. Electrons whizz around the nucleus at about 2,000 kilometres per second and Schrödinger hadn’t accounted for how matter that is approaching the realms of light speed might warp the laws of physics. To address that question he needed Albert Einstein’s theory of special relativity. But nobody had been able to meld quantum mechanics and special relativity into one.

As Dirac stared at that fire, he had a sudden burst of insight – a piece of mathematical wizardry conjured from the flames that allowed him to skirt around the impasse that had stumped so many other physicists. He quickly wrote down a handful of mathematical symbols that combined the ideas of Schrödinger and Einstein in one equation. This new equation spat out a measure of the electron’s magnetism that was bang on. It also corrected some bugs in how Schrödinger’s equation dealt with the structure of the hydrogen atom. Most compelling of all, it provided a meaningful rationale for why electrons seemed to spin like a top – so-called “quantum spin”.

EVERYTHING WE KNOW ABOUT PHYSICS SAYS WE SHOULDN’T EXIST.