In a paper published today in Nature, the ALPHA collaboration at CERN’s Antimatter Factory shows that, within the precision of their experiment, atoms of antihydrogen, a positron orbiting an antiproton, fall to Earth in the same way as their matter equivalents.
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“In physics, you don't really know something until you observe it,” says ALPHA spokesperson Jeffrey Hangst.
“This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the Universe.”
Gravity is the attractive force between any two objects with mass.
It is by far the weakest of the four fundamental forces of nature. Antihydrogen atoms are electrically neutral and stable particles of antimatter. These properties make them ideal systems in which to study the gravitational behaviour of antimatter.
The ALPHA collaboration creates antihydrogen atoms by taking negatively charged antiprotons, produced and slowed down in the Antimatter Factory’s AD and ELENA machines, and binding them with positively charged positrons accumulated from a sodium-22 source. It then confines the neutral – but slightly magnetic – antimatter atoms in a magnetic trap, which prevents them from coming into contact with matter and annihilating.
Until now, the team has concentrated on spectroscopic studies in the ALPHA-2 device, shining laser light or microwaves onto the antihydrogen atoms to measure their internal structure.
But the ALPHA team has also built a vertical apparatus called ALPHA-g, which received its first antiprotons in 2018 and was commissioned in 2021.
The ‘g’ denotes the local acceleration of gravity, which, for matter, is about 9.81 metres per second squared.
This apparatus makes it possible to measure the vertical positions at which the antihydrogen atoms annihilate with matter once the trap’s magnetic field is switched off, allowing the atoms to escape.
The full study involved repeating the experiment several times for different values of an additional “bias” magnetic field, which could either enhance or counteract the force of gravity. By analysing the data from this “bias scan”, the team found that, within the precision of the current experiment (about 20% of g), the acceleration of an antihydrogen atom is consistent with the familiar, attractive gravitational force between matter and the Earth. ■