Einstein right again: Antimatter falls “down” due to gravity like ordinary matter

CERN physicists have shown that antimatter falls downward due to gravity, just like regular matter, according to a new paper published in the journal Nature. It's not a particularly surprising result—it would have been huge news had antimatter been found to be repulsed by gravity and "fall" upward—but it does tell us a bit more about antimatter and brings physicists one step closer to resolving one of the most elusive mysteries surrounding the earliest moments of our universe.

As the name implies, antimatter is the exact opposite of ordinary matter, as it is made of antiparticles instead of ordinary particles. These antiparticles are identical in mass to their regular counterparts. But just like looking in a mirror reverses left and right, the electrical charges of antiparticles are reversed. So an anti-electron would have a positive instead of a negative charge while an antiproton would have a negative instead of a positive charge. When antimatter meets matter, both particles are annihilated and their combined masses are converted into pure energy. (It's what fuels the fictional USS Enterprise, as any Star Trek fan can tell you.)

As far as we know, antimatter doesn’t exist naturally in the known universe, although we can now create small amounts at places like CERN's Antimatter Factory. But scientists believe that 10 billionths of a second after the Big Bang, there was an abundance of antimatter. The nascent universe was incredibly hot and infinitely dense, so much so that energy and mass were virtually interchangeable. New particles and antiparticles were constantly being created and hurling themselves, kamikaze-like, at their nearest polar opposites, thereby annihilating both matter and antimatter back into energy in a great cosmic war of attrition.

Matter won. At some point in those first few fractions of a second, for reasons that continue to puzzle scientists, a small surplus of matter appeared. Even that tiny imbalance was sufficient to wipe out all of the antimatter in the universe in about one second. As the universe expanded, the temperature began dropping rapidly until it was too low to create new particle and antiparticle pairs. Only a small amount of “leftover” particles of matter remained; everything else had been annihilated, and their masses were emitted as radiation. Those bits and pieces make up the stars, planets, asteroids, and just about every other observable object in the universe.

Antihydrogen is electrically neutral and thus an ideal test particle because the electromagnetic force is so much stronger than the gravitational force. But it's a challenge to produce and trap antihydrogen particles; the only place that produces low-energy antiprotons is ALPHA, which does so by confining two plasmas of very cold particles (one using positrons, the other using antiprotons) within an electromagnetic trap. By bringing the two streams together, they can form antihydrogen. And by creating an incredibly strong magnetic field, it's possible to trap those antihydrogen particles.

Hangst and his fellow ALPHA team members had attempted an earlier gravitational measurement of antihydrogen back in 2013, but the results weren't sufficiently precise. So they built a new experimental apparatus: a tall cylindrical vacuum chamber with a magnetic trap in which one could vary the strength of the magnetic field. They gradually reduced the magnetic field until antihydrogen atoms started to escape and measured how many atoms escaped the trap by moving upward (antigravity) or falling downward. "Broadly speaking, we're making antimatter and we're doing a Leaning Tower of Pisa kind of experiment," said co-author Jonathan Wurtele, a plasma physicist at the University of California, Berkeley. "We're letting the antimatter go, and we're seeing if it goes up or down."

As the antihydrogen atoms escape, they touch the chamber walls and annihilate. Most of the annihilations occur

beneath the chamber, showing that gravity is pulling the antihydrogen down. Credit: CERN

The team repeated the same experiment many times, each time varying the magnetic field strength at the top and bottom of the apparatus. This helped rule out the possibility of errors in their measurements. The results: 80 percent of antihydrogan was annihilated beneath the trap, which is just how regular hydrogen atoms behave under the same conditions. So Einstein was right. Again. And that's sad news for physicists hoping for gravitational repulsion.

“There was a small but steady stream of papers from physicists at respectable institutions that made predictions about our universe and galaxy that relied on gravity being opposite for matter and antimatter,” co-author Joel Fajans of UC Berkeley told Symmetry magazine. “It seems like we foreclosed all of those theories.”

The next step is to upgrade the experiment and make it sufficiently sensitive to measure the rate at which antimatter particles fall downward. It should be identical to the rate at which matter falls, and that's where the smart money would bet. But if it isn't, that would be a tantalizing hint of exciting new physics. “Any discrepancy here would completely revolutionize physics,” said Fajans. “The great thing about doing this is that if we get a positive result, it’s a brave new world: the most exciting discovery in physics in the last 50 years. It’s certainly worth doing.”

Nature, 2023. DOI: 10.1038/s41586-023-06527-1  (About DOIs).