After decades of effort, physicists have probed the inner working of atoms of antihydrogen—the antimatter version of hydrogen—by measuring for the first time a particular wavelength of light that they absorb. The advance opens the way to precisely comparing hydrogen and antihydrogen and, oddly, testing the special theory of relativity—Albert Einstein’s 111-year-old theory of how space and time appear to observers moving relative to one another, which, among other things, says that nothing can move faster than light.
"It's a stunning result," says Alan Kostelecky, a theorist at Indiana University in Bloomington who was not involved in the work. For decades, experimenters have dreamed of measuring the spectrum of light absorbed by antihydrogen, Kostelecky says. "Here it is. They're doing it now."
Just as an atom of hydrogen consists of an electron bound to a proton, antihydrogen is an antielectron (or positron) bound to an antiproton. Of course, antihydrogen doesn't occur in nature. Because matter and antimatter particles annihilate each other, antihydrogen would vanish as soon as it touched matter. So physicists must make the stuff in the lab. Still, they expect the properties of antihydrogen to exactly mirror those of hydrogen.
Explaining exactly why special relativity requires antimatter to mirror matter involves a lot of math. But in a nutshell, if that mirror relationship were not exact, then the basic idea behind special relativity couldn’t be exactly right, Kostelecky says. Special relativity assumes that a single unified thing called spacetime splits differently into space and time for observers moving relative to each other. It posits that neither observer can say who is really moving and who is stationary. But, that can’t be exactly right if matter and antimatter don't mirror each other.
That's why physicists have been itching to measure the spectrum of antihydrogen. A hydrogen atom cannot absorb or emit light of any old wavelength. Instead, it can absorb or emit light only of certain distinct wavelengths, as the electron in it jumps from one quantized energy level to another—the fact that a century ago spurred the invention of quantum mechanics. If relativity is right, those wavelengths must be exactly the same for hydrogen and antihydrogen.
Now, Jeffrey Hangst of Aarhus University in Denmark, and 53 other physicists with an experiment called ALPHA-2 have measured the wavelength of light absorbed by antihydrogen as the positron in it jumps between two particular levels—the so-called 1s and 2s levels. Working at the European particle physics laboratory, CERN, in Meyrin, Switzerland, they measured that "spectral line" to a precision of a few parts in 10 billion, as they report online today in Nature. In hydrogen, that line has been measured 100,000 times more precisely. Still, the result marks the beginning of antihydrogen spectroscopy, Hangst says. "I've been working for more than 20 years to get to this point.
To make antihydrogen, physicists trapped about 1.6 million positrons and 90,000 antiprotons in opposite ends of a cylindrical trap using electric fields. They brought the positrons and antiprotons together to form about 25,000 uncharged antihydrogen atoms, which they immediately tried to snare with magnetic fields. They snagged about 14 atoms per trial. That’s about 10 times more antihydrogen per go than the team produced in 2012, when it first tickled antihydrogen atoms with radio waves.
Were they working with hydrogen, physicists could have excited the atoms with, say, electricity and analyzed the light they radiated. With so few antihydrogen atoms, they had to do something more subtle. They shined through the trap a laser tuned to excite the antimatter atoms. Once excited, an atom could “relax” back to the original state. Or it could absorb another photon and lose its positron or relax in a way that would flip the positron’s spin. The last two possibilities would change the atom so that the trap would no longer hold it.
As a result, Hangst and company could determine whether the atoms were absorbing the light by shining the laser on them for 10 minutes and then counting the atoms left in the trap. They merely turned off the trap and let the remaining anti-atoms float into a surrounding particle detector. “The antihydrogen atoms sort of blow up in your face,” Hangst says, “so you can count every one of them.” The researchers compared the count between instances when the laser was tuned to the positron transition, versus times when the laser was tuned away from it, or left off.
The rough first measurement already tests special relativity, Kostelecky says. In 1997, he developed a theoretical framework called the standard model extension (SME) that begins with the prevailing theory of particle physics and encompasses every possible violation of the concept behind special relativity. However, he says, because there are many types of particles, there is no single definitive test of the principle. For example, other physicists have compared the masses and lifetimes of particles called K mesons and their antiparticles. But those comparisons test different parts of the SME, Kostelecky says.
ALPHA-2 physicists can improve the measurement by carefully sweeping the laser’s wavelength through the 1s-2s transition, as they plan to do next year, Hangst says. They plan to measure other spectral lines and even test the pull of gravity on antihydrogen—to see if it is pulled down or pushed up.