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Antimatter definitely doesn't fall up, physicists confirm

If you drop a piece of antimatter, it will fall down to the ground just like regular matter, according to the first ever measurement of how these strange particles are affected by gravity. While this rules out suggestions that antimatter could fall up, along with the existence of repulsive matter and antigravity machines, there is still enough uncertainty in the measurement for there to be slight differences with regular matter and for new physics to be at play.

Quantum mechanics says that many particles should have an antimatter counterpart, identical in every way apart from an opposite electric charge. This flipped charge shouldn’t change how gravity affects the particle — all massive particles should move through space in the same way under gravity, according to Albert Einstein’s relativity. But it has been exceedingly difficult to test whether this is true because antimatter annihilates whenever it meets its opposite particle, making it hard to produce and store enough of it.

Now, Jeffrey Hangst at Aarhus University in Denmark and his colleagues have measured how gravity affects antihydrogen, which consists of an anti-electron, or positron, and an antiproton. While normal matter on Earth accelerates while falling at a rate of around 9.81 metres per second squared, also known as g, the team found that antimatter fell at a value between 0.46g and 1.04g – in other words, definitely downwards.

“Most people, when they think of antimatter, they think of the science fiction thing of ‘it’ll fall up’ — we can definitely rule that out,” says Hangst. “What we can’t rule out is there being some small difference between the accelerations [of matter and antimatter].”

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Hangst and his team built a series of vertically-stacked chambers to produce and store antihydrogen for their so-called ALPHA-g experiment at the CERN particle physics laboratory near Geneva, Switzerland. The chambers are fed with positrons from a radioactive source and antiprotons from a particle accelerator, both of which are slowed down and kept at temperatures just above absolute zero. The two types of antimatter particles are then combined in a single chamber, producing around 20 neutral antihydrogen atoms every 4 minutes that are held in place by powerful magnetic fields.

The researchers then slowly released the magnetic fields at the top and bottom of the chamber over 20 seconds and counted the atoms that came out in both directions. Because some of the atoms will randomly have enough energy to come out of the top of the trap, Hangst and his team were looking for statistical imbalances of more particles coming out at the bottom, towards Earth.

“From a technological point of view, it’s really outstanding,” says Tara Shears at the University of Liverpool, UK. Particle accelerators are typically concerned with making particles go as fast as possible, but to trap them at speeds slow enough to measure gravity’s effect is very difficult, she says.

While the team found that the antihydrogen falls towards Earth with enough precision to rule out the idea that antimatter repels, rather than attracts, more experiments currently under way, such as the AEgIS and GBAR experiments at CERN, will help us better understand if there are more subtle differences between matter and antimatter, says Shears.

Journal reference:

Nature DOI: 10.1038/s41586-023-06527-1

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