Antimatter falls like matter, confirming Einstein's theory of gravity

It's official: antimatter is falling, not rising.

In a first-of-its-kind experiment, scientists dropped antihydrogen atoms and watched them fall, demonstrating that gravity pulls antimatter toward Earth, rather than pushing it away.

The study confirms a pillar of Einstein's general theory of relativity known as the weak equivalence principle. According to this principle, gravity pulls on every object the same way, regardless of its composition. “This concept is central to our understanding of gravitation,” says physicist Ruggero Caravita, who was not involved in the new work.

Antimatter is the mirror image of matter, carrying the opposite electrical charge but the same mass. The antiparticle of an electron, for example, is a positively charged particle called a positron. A proton's alter ego is a negatively charged antiproton, and so on.

Most physicists didn't seriously consider the idea that antimatter could fall upward instead of downward, says Jeffrey Hangst of Aarhus University in Denmark. But scientists had never been able to test it directly before. “Antimatter is a bit mysterious…so we really want to confirm this behavior,” says Hangst, a spokesperson for the Antihydrogen Laser Physics Apparatus, or ALPHA, collaboration, which reported the new result.

Not only did the antimatter fall as predicted, but it also fell with roughly the same acceleration as normal matter, the team found.

The results, described in the September 28 Nature, highlight scientists' growing control over antimatter, and antihydrogen in particular. Antimatter is a tricky substance that can be difficult to work with. If it hits an object made of matter – the walls of a storage container or air molecules – it quickly annihilates. It took decades of work to measure the effect of gravity on antimatter, Hangst says.

In the experiment carried out at Europe's CERN laboratory near Geneva, scientists trapped antihydrogen atoms with powerful magnetic fields. These antihydrogen atoms were made by mixing antiprotons created at CERN with positrons from a radioactive source.

The researchers then released the antihydrogen from its magnetic cage, counting the number of atoms rising and falling. If gravity treats antimatter and matter the same, most atoms should fall, with a few flying upward due to the atoms' initial jostling movements. That's exactly what the researchers found.

“It's a very interesting concept, very neat and very simple,” says theoretical physicist Yunhua Ding of Ohio Wesleyan University in Delaware, Ohio, who was not involved in the study.

To further confirm that the antihydrogen behaved as expected, the researchers changed the magnetic fields to push the atoms upward, negating the effect of gravity. In this test, approximately equal numbers of atoms moved up and down. Additional variation in magnetic fields also matched expectations.

Previous experiments already suggested that gravity treats matter and antimatter in the same way. In 2022, the BASE experiment, also at CERN, reported that oscillations of confined antiprotons indirectly confirmed that matter and antimatter felt the same gravitational pull (SN: 05/01/22). But the ALPHA experiment is the first to directly observe the fall of antimatter particles.

The idea that different types of objects fall with the same acceleration long predates Einstein. Legend has it that in the 16th century, Galileo dropped various objects from the Leaning Tower of Pisa to demonstrate this effect. Since then, scientists have tested it in various situations, even with objects orbiting Earth (SN: 09/14/22). But until now, they had never done a test with antimatter.

Although physicists did not expect antimatter to fall, some researchers have proposed that antimatter may fall with a slightly different acceleration than normal matter. “If we find even the slightest difference, it would indicate that something new is happening,” says Caravita, of the National Institute of Nuclear Physics in Trento, Italy, and spokesperson for the AEgIS collaboration at CERN. AEgIS is part of a set of experiments that also aim to measure the effect of gravity on antimatter.

Current experience is not precise enough to assess these subtle differences. But new techniques, like cooling antihydrogen atoms with lasers, could make future tests more precise (SN: 05/04/21). This could help scientists see, when it comes to matter and antimatter, whether gravity is truly agnostic.

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