In Dan Brown's bestselling pulp novel Angels & Demons, the bad guys steal ¼ gram of antimatter, suspended by a magnetic field in a vacuum bottle, from the European particle physics lab, CERN, and try to blow up the Vatican. Now, physicists at the lab near Geneva, Switzerland, have indeed trapped a few dozen atoms of antihydrogen with a magnetic field. That's far too few to make a bomb, but comparisons of trapped hydrogen and antihydrogen atoms might someday blow a hole in Einstein's theory of special relativity.
"I don't think I can overemphasize how happy and relieved we were that this worked," says Jeffrey Hangst, a physicist at the University of Aarhus in Denmark and spokesperson for the 40 physicists working on trapping device called ALPHA. Trapping the antiatoms is likely the hardest step on the path to comparing hydrogen and antihydrogen, he says, as four groups at CERN are vying to do.
A hydrogen atom consists of a lightweight, negatively charged electron bound to a hefty, positively charged proton. So an antihydrogen atom consists of a positively charged antielectron, or "positron," bound to a negatively charged antiproton. Physicists have been able to produce and trap both types of antiparticles for decades. But it wasn't until 2002 that Hangst's team and a team led by Gerald Gabrielse of Harvard University produced the first antihydrogen atoms. They combined antiprotons generated by slamming a beam of protons into a metal target with positrons generated through the radioactive decay of the isotope sodium-22.
That's not as simple as it sounds. The antiprotons and positrons emerge as hot gas, in which the charged particles zip around randomly at high speed. So the charged particles must first be cooled and caught in the electrical fields produced by a device known as a Penning trap, a cylindrical arrangement of electrodes in a vacuum chamber. In fact, both teams captured the positrons and antiprotons in side-by-side clouds in a so-called nested Penning trap—essentially one Penning trap inside another—and then used a gentle electric field to slosh the antiprotons into the cloud of positrons, making antihydrogen atoms that quickly zipped away and disintegrated on contact with normal atoms.
Now, the ALPHA collaboration has gone a key step further, capturing a few of those antiatoms and holding them for a fraction of a second. That's difficult for two reasons. First off, uncharged atoms slip right through the electric fields of a Penning trap, so they must be held in a magnetic trap. Second, even after cooling, the positrons and antiprotons are still hot enough to make hot, fast-moving atoms that are hard to trap.
Nevertheless, the ALPHA team managed to catch a few of the things. The physicists again combined positrons and antiprotons in a nested Penning trap, beginning with a puff of 30,000 antiprotons cooled to 200˚ above absolute zero, or 200 kelvin, and 2 million positrons cooled to about 40 kelvin. But this time, they build the Penning trap inside a magnetic trap known as an Ioffe-Pritchard trap, as they report online today in Nature.
That trap was only "deep" enough to capture the slowest-moving antiatoms. To prove that it worked, after mixing the protons and positrons, Hangst and colleagues turned off the Penning trap and used electric fields to sweep any remaining charged particles out their device. They then turned off the magnetic trap and looked for any lingering antiatoms drifting into the material in the trap and annihilating to produce detectable particles. In 335 trials, the physicists saw a total of 38 trapped atoms—about one every 10 trials.
"I'm delighted that we see even a few atoms trapped," says Gabrielse, who pioneered studies of cold antiprotons and the development of the nested Penning trap. "I feel even a slight pride of parentage in that we developed some of the techniques they used." He says his team, which is working with a device called ATRAP, is taking a slightly different approach, concentrating on cooling as many as 3 million antiprotons to just a few kelvin to make it easier to capture larger numbers of antihydrogen atoms.
Ultimately, physicists want to compare hydrogen with antihydrogen. Thanks to quantum mechanics, an atom can absorb or emit light of only very specific wavelengths. In hydrogen, those wavelengths have been measured with a precision of a part in 1014. Scientists would like to know if antihydrogen's spectrum is the same. Physicists have already compared protons and antiprotons, but the comparisons of the atomic spectra would be far more precise, Gabrielse says.
The validity of Einstein's theory of special relativity hangs in the balance. A principle called charge-parity-time reversal (CPT) symmetry says that an atom and its antiatom must have the same spectrum. And if that's not true, then special relativity also cannot be exactly correct. That's because special relativity assumes a kind of symmetry, called Lorentz invariance, in the way observers moving at constant speed relative to each other will perceive quantities such as distance and time. And Lorentz invariance requires CPT symmetry to hold.
Particle physicists would be astounded if CPT symmetry did not hold, Hangst acknowledges. But other presumed symmetries have crumbled, he notes. For example, physicists once assumed all particle interactions were mirror symmetric, a symmetry known as parity. But in 1956, studies of radioactive decay proved that isn't so. "These symmetries are good until they're not," Hangst says. "One gets violated and somebody wins a Nobel Prize."