Antimatter may weigh more than matter of the same mass, say physicists at the University of California, Riverside, who are attempting to establish how it behaves in gravity.
The answer could explain why the universe seems to have no antimatter, and why it's expanding at an ever-increasing rate.
The researchers have taken the first step towards measuring the free fall of positronium – a bound state between an electron and a positron. The positron is the antimatter version of the electron, with the same mass, but a positive charge.
The team first separated the positron from the electron in positronium so that this unstable system would resist annihilation long enough for the physicists to measure the effect of gravity on it.
"Using lasers, we excited positronium to what is called a Rydberg state, which renders the atom very weakly bound, with the electron and positron being far away from each other," says David Cassidy, an assistant project scientist in the Department of Physics and Astronomy.
"This stops them from destroying each other for a while, which means you can do experiments with them."
At the Rydberg level, positronium's lifetime increases by a factor of 10 to 100.
"But that's not enough for what we're trying to do. In the near future we will use a technique that imparts a high angular momentum to Rydberg atoms," says Cassidy.
"This makes it more difficult for the atoms to decay, and they might live for up to 10 milliseconds – an increase by a factor of 10,000 – and offer themselves up for closer study."
The team's already made Rydberg positronium in large quantities in the lab, and now plans to excite it further to achieve lifetimes of a few milliseconds. They will then make a beam of these super-excited atoms to study its deflection due to gravity.
"We will look at the deflection of the beam as a function of flight time to see if gravity is bending it. If we find that antimatter and matter don't behave in the same way, it would be very shocking to the physics world," says Cassidy.
"Currently, there is an assumption that matter and antimatter are exactly the same – other than a few properties like charge. This assumption leads to the expectation that they should both have been created in equal amounts in the Big Bang. But we do not see much antimatter in the universe, so physicists are searching for differences between matter and antimatter to explain this."