It’s official: Antimatter falls down, not up.
In a first-of-its-kind experiment, scientists dropped antihydrogen atoms and watched them fall, showing that gravity attracts antimatter toward Earth, rather than repelling it.
The study confirms a pillar of Einstein’s general theory of relativity known as the weak equivalence principle. According to that principle, gravity pulls on every object in the same way, no matter what it’s made of. “This concept is at the heart of our comprehension of gravitation,” says physicist Ruggero Caravita, who was not involved with the new work.
Antimatter is the mirror image of matter, carrying the opposite electric charge but the same mass. An electron’s antiparticle, 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 entertain the idea that antimatter could fall up instead of down, says Jeffrey Hangst of Aarhus University in Denmark. But scientists had never been able to directly test it before. “Antimatter is kind of mysterious … so we want to actually confirm that behavior,” says Hangst, who is the spokesperson for the Antihydrogen Laser Physics Apparatus, or ALPHA, collaboration, which reported the new result.
Not only did the antimatter fall as expected, but it also dropped with roughly the same acceleration as normal matter, the team found.
The results, described in the Sept. 28 Nature, showcase scientists’ growing control over antimatter, and antihydrogen in particular. Antimatter is a wily substance that can be difficult to work with. If it touches anything made of matter — the walls of a storage container or molecules of air — it quickly annihilates. It has taken decades of work to measure any effect of gravity on antimatter at all, Hangst says.
In the experiment, performed at the European laboratory CERN near Geneva, scientists trapped antihydrogen atoms with strong magnetic fields. Those 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 how many atoms went up versus down. If gravity treats antimatter and matter equally, most atoms should fall down, with a few flying upward due to the initial jostling motions of the atoms. That’s just what the researchers found.
“It’s a very nice, very neat and very simple concept,” says theoretical physicist Yunhua Ding of Ohio Wesleyan University in Delaware, Ohio, who was not involved with the study.
To further confirm that the antihydrogen behaved as expected, the researchers altered the magnetic fields to push atoms upward, canceling out gravity’s effect. In that test, roughly equal numbers of atoms went up and down. Further varying the magnetic fields likewise matched expectations.
Previous experiments already hinted that gravity treats matter and antimatter the same. In 2022, the BASE experiment, also at CERN, reported that oscillations of confined antiprotons indirectly confirmed that matter and antimatter feel the same tug of gravity (SN: 1/5/22). But ALPHA’s experiment is the first to directly observe antimatter particles falling.
The idea that different types of objects fall with the same acceleration far predates Einstein. Legend states that in the 16th century, Galileo dropped different objects off the Leaning Tower of Pisa to demonstrate this effect. Scientists have since tested it in a variety of situations, even with objects in orbit around Earth (SN: 9/14/22). But they’d never done the test with antimatter until now.
Although physicists didn’t expect the antimatter to fall up, some researchers have proposed that antimatter may fall with a slightly different acceleration than normal matter. “If we find even the tiniest difference, this would be an indication that something new is happening,” says Caravita, of the National Institute for Nuclear Physics in Trento, Italy, who is the spokesperson of the AEgIS collaboration at CERN. AEgIS is one of a cadre of experiments there also working toward measuring gravity’s effect on antimatter.
The current experiment isn’t precise enough to suss out those subtle differences. But new techniques, such as cooling antihydrogen atoms with lasers, could make future tests more precise (SN: 4/5/21). That could help scientists see, when it comes to matter and antimatter, whether gravity is truly agnostic.