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.

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