Antimatter particles have the opposite properties of ordinary matter particles and are thought to exist mainly around black holes and in cosmic rays in the Universe.
Physicists zoom in on antimatter behaviour
For more than 50 years, scientists have debated whether gravity would attract or repel antimatter particles – whether they would fall down like conventional matter or ‘up’ due to a kind of antigravity. While the question remains unsolved for now, a team of scientists wrote in the journal Nature Communications that they had developed the beginnings of a test that should lead to a conclusive answer.
“This is the first word, not the last,” said Joel Fajans, a member of the research team at the European Organisation for Nuclear Research’s (CERN) Alpha experiment. “We’ve taken the first steps toward a direct experimental test of questions physicists and non-physicists have been wondering about for more than 50 years.”
First steps to answering 50 year-old question
Antimatter particles have opposite properties to ordinary matter particles, including their electric charge. A positively-charged positron, for example, is the antiparticle equivalent of the negatively-charged electron. When an opposing pair meets, particles and anti-particles annihilate each other in a flash of energy, which means that if an even balance had continued to persist after the Big Bang, the Universe would never have come into being.
But how this imbalance came about is a great riddle for particle physics. Scientists also scratch their heads over whether antimatter would respond in the same way as matter to gravity, or whether it would move in an different direction at a different speed. Some believe the fact the Universe is comprised almost entirely of matter could be explained if antimatter did indeed ‘fall’ up. But others assume it would behave the same as matter in reaction to gravity. No proof exists for either theory.
“Maybe we will be surprised”
“We certainly expect antimatter to fall down, but just maybe we will be surprised,” said Fajans, a University of California physics professor. “In the unlikely event that antimatter falls upwards, we’d have to fundamentally revise our view of physics and rethink how the universe works.”
The Alpha team reported the first direct measurements, though experimental and within a very wide range, of gravity’s effect on antimatter in free fall. The experiment works on manufacturing and trapping antihydrogen atoms – the antimatter counterparts of the simplest atom, hydrogen. The atoms are placed in a magnetic trap which is then switched off, allowing them to “fall out” and hit the trap walls in flashes of energy.
The researchers realised that by analysing where the atoms hit, and with what velocity, they could determine if gravity acted on antihydrogen differently than on hydrogen. If antimatter acted the same as matter, the rate of its gravitational mass to inertial mass would be the same: 1 – less than this and the antimatter would be falling upwards.
Gravitational mass is determined by a body’s response to gravity, while inertial mass measures to its resistance to acceleration. From its early measurements, the team was able to place outer limits on the ratio of plus 110 on the one side and minus 65 on the other – a margin that will be refined with more accurate tools and methods.
(Content courtesy of Mariette le Roux, Agençe France-Presse (courtesy of Cosmos Magazine : www.cosmosmagazine.com))