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…
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