Scientists at the University of California, Berkeley are devising an experiment to test one of physics most widely-held principles: Can anything ignore gravity's pull?

Specifically, they will attempt to do a the equivalent of a "drop test" – think Galileo's fabled (literally,they didn't happen) experiments dropping objects off the Leaning Tower of Pisa – with antimatter to see which way it falls.

Joel Fajans, a physics professor at Berkeley and researcher on CERN's ALPHA (Antihydrogen Laser Physics Apparatus) experiment, says it's unlikely antimatter resists gravity and falls upward, but it's something worth looking into.

"I think it's a neat question which should be settled experimentally," he says. "The great measurement everybody dreams of making is, 'does antimatter by some miracle fall upward rather than downward?' We think in the next couple years we'll be able to test that."

Antimatter is matter that has the same mass as particles of standard matter, but has an opposite charge and spin. Earlier this month, results from a study aboard the International Space Station suggested that over the past 18 months, at least 400,000 positrons (the antimatter equivalent of electrons) had been found near the instrument.

Fajans describes the experiment in a paper published Tuesday in Nature Communications.

"There's been speculation that antimatter might fall differently than normal matter, which would be in violation of a deep principle of physics that says it shouldn't," Fajans says. "But this experiment is prompted by the fact that there are some mysteries out there that gravitational anomalies with antimatter might explain."

According to the Big Bang theory, thus far the most widely accepted model of the beginning of the universe, equal amounts of matter and antimatter were created. But scientists have had difficulty finding much antimatter, leading them to ask where it all went.

"If it turned out antimatter falls upward, just about everything would change about our understanding of the early universe. If antimatter repels matter and resists gravity, we could kind of casually say that most of the antimatter just segregated itself in some distant corner of the universe," he says.

If antimatter falls up, it would also disprove the theory that dark matter and dark energy are necessary to explain some of physicists biggest questions about the universe.

"One of the most important discoveries in theoretical physics over the last 20 years is the recognition that we need to have dark energy and dark matter in the universe, prompted by experimental observations that don't make any sense without them," Fajans says. "But we haven't had any direct measure of dark matter or dark energy yet, and it's getting to be embarrassing at this point."