CERN experiment proves gravity pulls antimatter the way Einstein predicted

In sci-fi, antimatter is the opposite of all the matter around us. Does that mean it should fall up? Not at all, scientists at Europe's largest sub-atomic physics lab have proved for the first time.

The result was predicted by Einstein's Theory of General Relativity more than 100 years ago, though getting experimental evidence has proved stubbornly elusive until now.

To test the idea, researchers had to power up CERN's ominous-sounding Antihydrogen Laser Physics Apparatus (ALPHA), constructed in 2018.

Antimatter is the opposite of regular matter, which makes up almost everything scientists observe. While the protons which help make up the atomic nucleus are positively charged, antiprotons have a negative charge. Similarly, antielectrons – also known as positrons – are positive where electrons are negative, but crucially have the same mass and magnitude of charge as electrons. Positrons and antiprotons can combine to help create antihydrogen, the antimatter twin of the universe's simplest and most abundant element.

Antihydrogen is tricky to study. Whenever it comes into contact with regular matter, it disappears in a puff of energy.

The research team led by Jeffrey Hangst, professor of experimental physics at Denmark's Aarhus University, suspended antihydrogen between two magnetic fields inside a tall vacuum chamber. By reducing the strength of the field at the top and the bottom of the trap, the antimatter was allowed to move above or below the trap and collide with the apparatus wall, releasing energy which experimenters could detect.

While the magnetic fields were accurately balanced, about 80 percent of the antihydrogen atoms fell below the trap, which the researchers concluded was down to gravity.

University of California, Berkeley plasma physicist and ALPHA collaboration member Jonathan Wurtele said in a statement: "Einstein's theory of general relativity says antimatter should behave exactly the same as matter. Many indirect measurements indicate that gravity interacts with antimatter as expected, but until the result today, nobody had actually performed a direct observation that could rule out, for example, antihydrogen moving upwards as opposed to downwards in a gravitational field."

The result does leave a problem, though. The laws of physics predict that antimatter and matter should exist in equal quantities, but scientists' observations show otherwise. One explanation put forward was that antimatter could have been repelled by gravitational forces in the early stages of the universe if it reacted to gravity in the opposite way to conventional matter. That proposal has been ruled out by the ALPHA experiment.

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"The results are thus far in conformity with the predictions of General Relativity. Our results do not support cosmological models relying on repulsive matter–antimatter gravitation," the researchers said in a paper published in Nature today.

Nonetheless, the ALPHA study could help tests of the weak equivalence principle and help scientists understand the gravitational nature of antimatter, they said. ®