Half the Blink of an Eye
David Kaiser · Antimatter
Last week a team of physicists based at CERN announced that they had coaxed a handful of elusive antihydrogen atoms into existence: 38 of them, to be exact. Simply creating antimatter is no longer newsworthy; a competing team fabricated tens of thousands of antihydrogen atoms using a different method back in 2002. What’s new about the latest experiment – the result of five years’ work – is that the fragile atoms stuck around for as long as 172 milliseconds: nearly one-fifth of a second, about half as long as the blink of an eye. And when it comes to atoms of antimatter, that is an astonishingly long time.
When particles of ordinary matter meet their antimatter counterparts, they annihilate each other. This has made it difficult to study antiatoms: unless they can be trapped in isolation from ordinary matter, they will inevitably vanish in a puff of energy. For decades, physicists have used electric fields to trap electrically charged subatomic particles, such as positrons or antiprotons, and subject them to close scrutiny. Positrons, the antimatter counterpart of electrons – they have exactly the same mass, and an equal-but-opposite electric charge – are used routinely in PET (positron emission tomography) scans. Particle accelerators like the Tevatron at the Fermi National Accelerator Laboratory near Chicago, meanwhile, create their energetic fireworks by smashing trillions of negatively charged antiprotons into ordinary, positively charged protons every fraction of a second.
But an antihydrogen atom (an antiproton bound with a positron) is electrically neutral; the usual traps are worthless. Instead, the team at CERN designed a far more subtle containment device: a specially shaped magnetic field, powered by supercooled, superconducting magnets. The physicists sent ten million antiprotons and seven hundred million positrons into this trap, hoping that the particles would combine into neutral antihydrogen atoms. Three dozen pairs obliged – and stuck around for an unprecedented length of time.
Paul Dirac, somewhat reluctantly at first, introduced the idea of antimatter as a theoretical necessity in the early 1930s. Physicists began to find experimental evidence for positrons soon afterwards, catching glimpses of them in cosmic rays. Antiprotons were first detected in 1955, among the earliest discoveries at the Bevatron accelerator in Berkeley, California. The pride of Berkeley, the Bevatron was a symbol of the new postwar gigantism in particle physics. The Nobel committee acted with unusual dispatch, awarding the physics prize in 1959 to Owen Chamberlain and Emilio Segrè of the Berkeley lab for the antiproton discovery. (Thirteen years later, Oreste Piccioni sued Chamberlain and Segrè, claiming that they had taken his idea for the subtle magnetic field needed to identify the antiprotons while shutting him out of the experiment. The judge threw the case out, arguing that the statute of limitations had expired.)
It took a team of 42 physicists, from 16 institutions scattered across eight countries and four continents, to fabricate and trap the antiatoms in the recent experiment at CERN. They came together in the hope of shedding light on some of the most perplexing questions in modern physics. According to otherwise well-tested theories of particle physics and cosmology, the universe moments after the big bang should have consisted of equal parts antimatter and matter. Yet very quickly, the ratio dropped to one in a billion. Theoretical physicists have concocted dozens of schemes to account for the dramatic imbalance, but so far neither theory nor experiment has produced anything like a compelling explanation.
One way to try to account for the imbalance is to conduct high-precision tests of what is known as the ‘spectrum’ of antihydrogen atoms. Are a positron’s energy levels in an antihydrogen atom precisely the same as the corresponding energy levels of an electron in an ordinary hydrogen atom? The hydrogenic energy levels are among the most carefully measured physical quantities in all of science: they are known to 14 decimal places. The reigning theories of fundamental forces suggest that the energy levels of antihydrogen should be precisely the same, down to the 14th decimal place. Yet those same theories have as yet produced no convincing explanation for the stark imbalance between the amounts of matter and antimatter in the universe. Conducting sensitive measurements of the antihydrogen spectrum could isolate some new and unexpected behaviour of the most fundamental forces of nature, which in turn might offer the first serious clues to explain the matter-antimatter imbalance.
Before such hypersensitive measurements can be made, physicists must first trap hundreds (or more) of the lonesome antihydrogen atoms and make them sit still long enough for minute electromagnetic fields to tickle the positrons and measure the corresponding energy levels. One-fifth of a second isn’t enough time, unfortunately, but it’s a major step in the right direction.