If You Drop Antimatter, Which Way Does It Fall?

What are astronomy's most intriguing puzzles?
Feb. 11 2014 8:00 AM

Does Antimatter Fall Up?

A tantalizing possibility for unifying gravity and quantum mechanics.

CERN lab, Dec 2013.
The CERN lab, where scientists challenge gravity's authority.

Photo courtesy Anna Pantelia/CERN Photo Lab

Here's a pitch that didn't make it past the Chiquita marketing execs: Bananas are nature's antimatter dispensers. Those ubiquitous yellow fruits are packed with potassium, making them a quality addition to any breakfast (or burger, if you're Ron Swanson). But that potassium includes a relatively sizable serving of radioactive potassium-40, which sometimes spits out antimatter: a positron, the antimatter partner of the electron.

Almost as soon as it appears, the poor positron is annihilated. That's because antimatter is the rebel twin of regular matter: Each antiparticle has the same mass but opposing charge as its counterpart, plus a few other bizarro properties. When banana-fueled antimatter meets regular matter in your body, both vanish in a puff of energy. It's an incredibly powerful explosion on exceptionally tiny scales, so even through you do get a radiation dose, it's nothing to stress about.

In fact, antimatter is quietly being annihilated all around you. It pops up in naturally radioactive substances, is spawned in our upper atmosphere and during thunderstorms, and roils in vast amounts near the center of the Milky Way galaxy. If you've ever had a PET scan, you've been probed with the power of positrons.

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We know how to make beams of the stuff in particle physics labs and trap them in magnetic nets. Even so, antimatter's suicidal tendencies make it hard to handle, leaving us with some big unanswered questions. Perhaps the most basic query is also the most exciting: If you drop antimatter, which way does it fall?

Since the days of Galileo purportedly dropping balls off Italian towers, experiments have suggested that any two objects will fall down at the same rate, accounting for friction, regardless of their mass and composition. But no one has been able to test this directly for antimatter, which hints at the tantalizing possibility that it will do something unexpected.

"That would be the greatest revolution in physics in the past 20 to 30 years," says Joel Fajans, a physicist at the University of California–Berkeley. Bigger than the discovery of the Higgs boson? I ask him in disbelief. "Oh yeah, no question. There's a very low probability but an enormous reward if antimatter were to gravitate differently than we expect."

Particle physicists are gleeful anarchists. Many brilliant minds are hard at work trying to topple the current regime, known as the standard model, which describes almost all of the particles and forces at work in the universe.

The Higgs was the latest piece of the puzzle to be verified, and its existence at its expected mass is an excellent sign that the model works as predicted. The trouble is that we know something is amiss. The standard model doesn't always know what to do with gravity, and it is totally stumped by dark matter and dark energy. So sure, the people hunting the Higgs wanted to find it, but many of them were also subversively hoping that it would be a dark horse particle, something that didn’t look or behave at all as expected, one that would hammer cracks in the standard model's shining fortress.

So far, the Higgs has been annoyingly vanilla, which brings us back to antimatter and the dim but fervent hope that antiparticles won't follow the same rulebook. We know gravity is the weakest link in the standard model, and we've never actually seen what particles of antimatter do in response to gravity. Because antimatter is so weird in so many other ways, it's easy and exciting to speculate that antimatter could—just maybe—antigravitate. Antiparticles could be the ultimate rebels and fall up, or they may still fall down but be slightly faster than regular particles of the same mass. Take that, Galileo.

CERN lab, Dec 2013.
The CERN lab is pictured in December 2013.

Photo courtesy Anna Pantelia/CERN Photo Lab

Seeing either one happen could help us complete our understanding of gravity. Perhaps it could lead to a peace accord between Einstein's general relativity—the powerhouse theory of how gravity affects the very fabric of the universe—and its theoretical nemesis, quantum mechanics. It may also give us clues to a second antimatter mystery: Why is there more matter than antimatter in the universe?

When energy condenses into matter, it always produces a matched set of a particle and its antiparticle. That's a rule of physics, and we don't have much choice in the matter, says Michael Doser at CERN in Geneva. But that means something must have gone wrong during the Big Bang, because all the energy it generated should have congealed into equal amounts of particles and antiparticles.

Based on what we know about antimatter, all the particle pairs should have annihilated each other, leaving a universe filled with a stable soup of energy and no matter. Last I checked, I am not a being made of pure energy. So what gives? One theory holds that somehow the antiparticles peacefully annexed themselves before universewide annihilation. If so, and if they behave the same way under gravity that regular particles do, they should have grouped up into larger, detectable structures like their normal-matter cousins.

"If you look out into the universe, you'd expect half to be antimatter. But there's not a single antimatter galaxy out there as far as we can tell," says Doser. Curiouser and curiouser. Hence the drive to find out whether antimatter is secretly defying gravity.

Last April, Fajans and his colleagues reported results from the most sensitive test yet on gravity's influence on antihydrogen, an atom made of antimatter particles. Using the ALPHA experiment at CERN, they corralled antihydrogen with magnets, stopping it from running off and being annihilated willy-nilly. Then they simply turned the magnets off. The aim was to figure out when and where a controlled antihydrogen atom touches the wall and goes poof, so you can calculate whether it was being pulled by gravity—and whether it went up or down.

The first results are a solid start: They put some limits on how antihydrogen could be behaving. The team found that antihydrogen could not have been falling down more than 100 times faster than regular hydrogen, and it could not have been falling up more than 65 times faster than theory predicts. That's the best ALPHA can do for now. The experiment as it stands is not precise enough to directly see gravity's effect, in part because the antihydrogen made in labs is too darn hot.

"Particle physicists have one main tool in our box: machines to smash things together," Doser says. At CERN, a machine called the Antiproton Decelerator shoots protons at a copper target, and the energy is transformed into particles and antiparticles. The resulting antimatter is moving at nearly the speed of light. Because the antiparticles have electric charge, sending them through intense electric fields will slow them down. The particles are then sent to machines such as ALPHA, where they are assembled into antihydrogen atoms and placed in magnetic traps.

Even then, the antihydrogen is pretty warm and is racing around at a decent clip, so as soon as the magnetic net starts to dissolve, the atoms go flying off. ALPHA scientists haven't yet cooled their antiatoms enough to truly tease out the effects of the atoms' speed and gravity's pull.

Doser's team is working on another CERN machine, AEGIS, that will try to really chill out antimatter. The device should make antihydrogen later this year, with a goal of getting the atoms down to about 1 degree Kelvin above absolute zero. The trick to making such frigid antimatter is still being figured out, so Doser was shy about sharing the details. But if they can do it, he expects they will run their first gravity tests sometime in 2016.

Doser thinks the odds are slim that antimatter will fall differently, but no one is giving up until we're sure antimatter can't be used in the struggle to unify physics. "There are armies of people looking in every single corner trying to find the tiniest deviations in the standard model," he says. "You don't have revolutions without toppling statues. You have to show that everything you know is wrong, or tie everything together in a beautiful way."

Victoria Jaggard is the physical sciences news editor for New Scientist. Follow her on Twitter.