Laser excites antimatter atoms for first time

For the first time, physicists probed an atom of antimatter with laser light – and found no detectable difference between the “excitement” of a hydrogen atom and its antimatter twin, antihydrogen. 

The work, published today in Nature by an international team, is the culmination of hundreds of millions of dollars and two decades of research at the European Organisation for Nuclear Research (CERN).

Antimatter is the stuff of Bizarro World – families of particles identical to those of matter, but with opposite charge.

This surreal nature has made antimatter a favourite for sci-fi authors for decades. Not only was it warp drive fuel on the Starship Enterprise, antimatter was also the explosive that threatened Vatican City in Dan Brown’s novel Angels and Demons.

The reason antimatter makes for good munition or fuel is because it annihilates with matter on contact, producing pure energy in the most efficient reaction known to physics.

But that same property makes antimatter devilishly difficult to hold and study in the lab. Contact with any stray molecule of air, or a brush against the walls of the container, kills the experiment.

And one of the great mysteries of physics is why antimatter did not destroy the universe at the beginning of time.

The Standard Model of particle physics says the Big Bang should have produced equal amounts of matter and antimatter – a mixture that would have gone up in a giant puff of gamma rays.

Instead, more matter than antimatter was produced and we don’t know why. Finding a difference between matter and antimatter (aside from their charge) would be a crucial clue to figuring out what happened to the missing antimatter.

“Our current theory about how the universe is screwed together says that matter and antimatter should behave in the same way,” says Jeffrey Hangst, a physicist at Aarhus University in Denmark and Antihydrogen Laser Physics Apparatus (ALPHA) project leader at CERN.

“What we’re doing is looking for small differences in things we know well to see if new physics turns up.”

For decades, physicists were confined to studying antimatter particles in isolation, such as the positrons (the antimatter version of the electron) created when cosmic rays strike the upper atmosphere.

But over the past 20 years, physicists at CERN have perfected the art of creating and storing actual atoms of antimatter, enabling them to study it at a whole new level.

Specifically, CERN physicists can make antihydrogen. Being a positron orbiting an antiproton, it’s the antimatter counterpart to hydrogen, which is an electron around a proton.

In 2011, CERN’s ALPHA team managed to trap antihydrogen atoms for 1,000 seconds, the current world record. In 2014, they measured antihydrogen’s electric charge and found it to be zero, just like hydrogen. But these were just warm-ups.

Now, the same team have performed the measurement for which their apparatus was built – probing the structure of antihydrogen with laser light.

In the experiment, the team used magnetic fields to trap about a dozen antihydrogen atoms in a small cylinder the size and shape of a relay runner’s baton.

They then bathed the antihydrogen atoms with light of a frequency of 243 nanometres (the kind of ultraviolet light against which the ozone layer protects us).

When this light hits a regular hydrogen atom, its electron is kicked up from its lowest orbital to the next spherical one up. In this “excited” state, the electron whizzes around a bit further from the proton than before.

Physicists have nailed down the energy needed to do the kicking in a hydrogen atom to just a few parts in 10¹⁵. This is a measurement so exact it’s like defining the distance from the Earth to the sun to within a millimetre.

The question posed by Ahmadi, Hangst and the team was whether the same frequency excited the positron in an antihydrogen atom. And their measurement shows that it does, to a precision of two parts in ten billion. 

This means, as far as they can tell, the laws of physics (such as the forces binding the particles together and those governing the interaction of light) are identical for antihydrogen and hydrogen.

But the story does not end there. “This is just the first step,” Hangst says.

“We have football field resolution and we need blade of grass resolution. There’s a lot of work to do to rule out any difference between hydrogen and antihydrogen.”