Galileo put gravitational theory to the test by dropping balls from the Leaning Tower of Pisa. Four hundred years on, scientists have performed a higher tech version of the experiment to demonstrate for the first time that antimatter also falls downwards.
The study, by scientists at Cern, showed conclusively that gravity pulls antihydrogen downwards and that, at least for antimatter, antigravity does not exist.
“Broadly speaking, we’re making antimatter and we’re doing a Leaning Tower of Pisa kind of experiment,” said Prof Jonathan Wurtele, a theoretical physicist at the University of California, Berkeley. “We’re letting the antimatter go, and we’re seeing if it goes up or down.”
Antimatter is a mirror version of ordinary matter, with some basic properties like electrical charge reversed. Antiprotons have the same mass, but a negative charge, while antielectrons (also called positrons) are positively charged.
When matter and antimatter meet, they annihilate each other and produce energy, meaning that in a matter-dominated world like our own, antimatter only fleetingly comes into existence. But most theories predict that equal amounts of matter and antimatter should have been produced during the big bang, and the mystery of what happened to all the antimatter is a central question in fundamental physics.
For some, the concept of antigravity has been an enticing potential explanation to this conundrum. This could have led to the spatial separation of matter and antimatter in the early universe, meaning that we only see a small amount of antimatter in the local universe. There are other theoretical reasons that make this idea unlikely, but as the question had never been put to the test, it had remained a fringe possibility.
“Until you measure it, you just don’t know. That’s science,” said Jeffrey Hangst, a particle physicist at Aarhus University, Denmark, and spokesperson of the Antihydrogen Laser Physics Apparatus (Alpha) collaboration at Cern.
A direct measurement of antimatter falling is hugely challenging because the gravitational force is so weak compared with the other three known forces of nature and scientists have struggled to preserve antimatter for long enough to carry out experiments. The latest study, published in Nature, used antihydrogen atoms cooled to half a degree above absolute zero (-273.15C). About 100 of the antimatter atoms were confined in a 25cm-long magnetic bottle with an opening at the top and bottom. Meticulous measurements showed that the atoms were more likely to escape, and meet their annihilation, at the bottom of the bottle due to gravitational forces.
The gravitational acceleration was found to be within 25% of normal gravity, meaning that it could be identical to the gravitational force experienced by ordinary matter – or at least, similar.
“It has taken us 30 years to learn how to make this anti-atom, to hold on to it, and to control it well enough that we could actually drop it in a way that it would be sensitive to the force of gravity,” said Hangst. “The next step is to measure the acceleration as precisely as we can.”
