Does Antimatter Fall Up?

One of the biggest unknowns in physics is simply this: does antimatter fall up or down?

It's a serious question. If antimatter is repulsed by gravity, that could explain why we see so little of the stuff floating about space. If it's attracted just as matter is, but perhaps just a little more so, that could have implications for theories that attempt to unite quantum mechanics and general relativity. 

Physicists have speculated about the answer for decades, but there's been little data to feed those efforts. Finding the answer has proved to be an experimental difficulty. Antimatter is hard to wrangle: it annihilates as soon as it comes into contact with ordinary matter. Although electromagnetic fields can be used to steer charged antimatter particles quite easily, the forces involved can easily overwhelm any gravitational signal you might hope to see.

The best candidate that has emerged for studying gravity's effects is antihydrogen, an "antiatom" that contains an antiproton and a positron instead of a proton and an electron. Antihydrogen is electrically-neutral, long-lived (assuming you can trap it), and fairly heavy, which is good for gravity experiments. But it's tricky to work with. It must be synthesized from scratch from its antimatter components, and it must be made to move slowly enough for gravity to have a discernable effect before it annihilates.

Despite those challenges, physicists have started making inroads with the stuff. In a paper published today in Nature Communications, a team working on the ALPHA experiment at CERN is reporting the first direct measurement of antimatter’s reaction to gravity. But if you're looking for an answer to the up or down question, you'll have to wait a little longer.

ALPHA has been making headlines for several years already. In 2010, the experiment, which uses coils of wires to form a pickle-shaped magnetic trap, became the first to create and confine antihydrogen atoms. In 2011, the team showed they could use the experiment’s magnetic bottle to hold these antiatoms for more than 16 minutes, long enough for detailed study of the atom's properties.

ALPHA’s primary goal is to shine light on these atoms to see if their spectra differ from those of hydrogen atoms. But in late 2011, two physicists at the University of California, Berkeley, Joel Fajans and Jonathan Wurtele, got to talking about what else they might do with the data they’d collected. "Jonathan and I were just talking about whether we could see gravity in the experiment," Fajans says. "He was saying we could, and I was saying 'Are you kidding?'"

When Fajans started looking at the data, however, he realized ALPHA might be able to say something about gravity's effects. To detect individual antihydrogen atoms in the trap, ALPHA must let them go, by turning off the magnetic fields and letting the antiatoms escape. The atoms then annihilate on the walls of the trap, creating a spray of other particles that can be picked up by detectors. In principle, if antimatter falls up, more of these particles should come from the upper half of the trap. If it falls down, more will come from annihilations on the lower half.

The complication is the speed of the antihydrogen atoms. Most rattle around so fast in the magnetic trap that when the magnetic "walls" are lowered, they shoot out and annihilate far too fast for gravity to have a real effect on their trajectories. Still, when Fajans and his colleagues analyzed the data ALPHA had already collected on 434 trapped antihydrogen atoms, they found they could at the very least place some limits. Gravity likely pulls no more than 110 times harder on antimatter than it does on matter, assuming that antimatter falls in a gravitational field. If gravity instead repels antimatter, it probably does so with no more than 65 times as much force as it exerts on ordinary matter.

Despite these crude limits, Fajans says, “it was a really pleasant surprise that we could say anything at all about gravity.” He adds that this is the first time that physicists have been able to make any direct measurement of gravity’s effect on antimatter, performing a test akin to Galileo's fabled ball drop from the Tower of Pisa. Other experiments have attempted to infer gravity’s effects indirectly, he says, but “when people look at the assumptions behind these measurements you can always find something to quarrel with.”

The ALPHA team expects to improve the precision of their measurements, primarily by using lasers to cool the antihydrogen atoms. If this works, they'll be able to slow the speed of the atoms, which will mean gravity will be able to have a stronger effect. That could be enough to reveal whether antimatter falls up or down, Fajans says.

But ALPHA will have competition. Two experiments that are designed specifically for gravitational measurements are in the works at CERN. The first to come online will be the AEgIS (for Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy) experiment, which finished construction at the end of 2012 and will begin antihydrogen experiments in 2015. A second experiment, called GBAR, will begin a few years later, after an upgrade to CERN’s antiproton decelerator, which feeds all of these experiments.

Both AEgIS and GBAR will reveal gravity’s effect on antimatter with ballistic experiments, measuring how much a beam of horizontally-moving antihydrogen atoms moves up or down before annihilating. These experiments require new ways of creating antihydrogen and are "horrendously difficult", says Michael Doser, spokesperson for the AEgIS experiment. But if all goes well, they will be able to easily detect whether antimatter falls up or down. In fact, they'll be able to discern a difference between the gravitational acceleration of matter and antimatter of as little as 1%, nearly a factor of 10,000 times ALPHA's current sensitivity (ALPHA may ultimately be able to boost its own sensitivity by a factor of 100).

Doser says that if AEgIS and GBAR find a difference between the behavior of antimatter and matter, ALPHA could be a crucial independent cross-check. “All of us have our work cut out for us,” he says. “With three experiments chasing this up, the coming years look to be interesting.”



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