With ‘Leaning Tower of Pisa-Type of Experiment’ Physicists Find Antimatter Affected by Gravity, Too

It was long assumed that antimatter, the opposite of the normal matter making up the tangible universe around us, wasn’t subject to the gravity that pulls normal matter downwards, but a group of physicists in Europe have proven for the first time that it is.

The discovery was made by scientists at the European Organization for Nuclear Research (CERN), who tested elusive atoms of antihydrogen to see if they fell downward, indicating they were pulled by gravity, or upward, as if pulled by a hypothetical antigravity that would only affect antimatter.

Antihydrogen is just like regular hydrogen except that it’s opposite. Hydrogen is made up of a proton and a neutron, and antihydogen is made up of an antiproton and an antineutron. However, matter and antimatter can’t coexist together: if the one touches the other, they’ll annihilate each other, so it can only exist in a special vacuum chamber created by scientists.

Jonathan Wurtele, a theoretical physicist at the University of California, Berkeley, compared the experiment to those performed in 17th century Italy by Galileo Galilei, who dropped different objects off the Leaning Tower of Pisa to see if gravity pulled some of them faster than others.

“Broadly speaking, we’re making antimatter and we’re doing a Leaning Tower of Pisa kind of experiment,” Wurtele quipped. “We’re letting the antimatter go, and we’re seeing if it goes up or down.”

Interestingly, physicists don’t quite know how normal gravity works, either. A hypothetical massless “graviton” particle is assumed to be the mechanism by which matter pulls on other matter, but it has never been observed.

The challenge was that of the four known elementary forces of nature, gravity is by far the weakest (the others are electromagnetism and the strong and weak nuclear forces), meaning the antimatter had to be protected from regular matter for a longer period of time before the effect could be observed.

The scientists managed to confine about 100 antihydrogen atoms in a 25-centimeter-long magnetic bottle that was open at both ends. They measured which direction the antimatter traveled by watching for the telltale flashes when they collided with atoms of regular matter and annihilated each other, finding far more flashes below the bottom of the bottle than above its top.

“Until you measure it, you just don’t know. That’s science,” said Jeffrey Hangst, a particle physicist at Aarhus University, Denmark, and spokesperson of the Antihydrogen Laser Physics Apparatus (Alpha) collaboration at CERN.

Hangst’s team was also the first, in 2010, to successfully capture antihydrogen for an extended period of time. Antimatter can be produced by certain types of nuclear reactions in nature, especially inside of stars, but it almost instantly disappears. In 2016, they were also able to measure how antiatoms absorb light.

“This is by far the most difficult thing that we’ve done,” Hangst said.

A paper describing their discovery was published on Wednesday in the journal Nature.