Antimatter has intrigued and puzzled physicists for almost a century; The effect of gravity on antimatter has been a matter of debate. A new study may have settled the debate by ruling out that antihydrogen atoms, the antimatter counterparts of hydrogen, are affected by gravity in the same way as their matter counterparts, ruling out the existence of repulsive “antigravity.”

In the seventeenth century, Isaac Newton proposed the theory of gravity after watching an apple fall from a tree and asking why it fell straight down rather than sideways or up. Centuries later, Albert Einstein developed the theory of general relativity, which remains the most successful and proven description of gravity. However, antimatter was unknown to Einstein.

In 1928, British physicist Paul Dirac theorized that for every particle there was a corresponding antiparticle, and this predicted the existence of the positron or antielectron, which was first observed in 1932. Since then, there has been much speculation about the interaction between gravity and antimatter; some claimed that antimatter was repelled by gravity, others that it was attracted to it.

A new study by the Antihydrogen Laser Physics Instruments (ALPHA) collaboration at the European Organization for Nuclear Research (CERN) may have settled the debate by finding that antihydrogen atoms, the hydrogen counterpart of antimatter, fall to Earth just like their matter counterparts.

“In physics, you never know anything until you observe it,” said Jeffrey Hungst, author of the study. “This is the first direct experiment to actually observe the effect of gravity on the motion of antimatter. This is a milestone in the study of antimatter, which still surprises us due to its apparent absence in the universe.”

The ALPHA experiment concerns the creation, capture and study of antihydrogen atoms in a trapping device. Antihydrogen atoms are electrically neutral and stable antimatter particles, making them ideal for studying the gravitational behavior of antimatter. Antihydrogen is created by combining two components of antiparticles, antiprotons and positrons. An antiproton is a subatomic particle that has the same mass as a proton but a negative electrical charge.

The ALPHA team recently built a vertical probe called ALPHA-g; where “g” represents the local gravitational acceleration for matter, which is 32.2 ft/s.² (9.81 m/sec)²). ALPHA-g allows measuring the vertical positions where antihydrogen atoms meet the corresponding material (a process called annihilation) when the trap’s magnetic field is turned off, allowing the atoms to escape.

The researchers trapped groups of about 100 antihydrogen atoms, one group at a time. They then slowly released the atoms over 20 seconds and gradually reduced the current in the upper and lower trap magnets. Computer modeling predicted that 20% of the atoms would emerge from the top of the trap and 80% from the bottom; this difference is due to the downward force of gravity. Averaging the results of seven oscillation tests, the researchers found that the fractions of antiatoms emerging from the top and bottom were consistent with the simulations. So the antihydrogen atoms fell just like the hydrogen atoms under the influence of 1 g or normal gravity.

Using the ALPHA-g apparatus, researchers have effectively recreated Galileo’s famous gravity experiment. According to legend, an Italian scientist dropped iron balls of different weights from the top of the Leaning Tower of Pisa and they landed on the ground at the same time; This showed that gravity causes objects of different masses to fall with the same acceleration.

Although the researchers say their findings rule out the existence of repulsive “antigravity,” the current work marks only the beginning of detailed, direct investigations into the gravitational nature of antimatter.

“It took us 30 years to figure out how to make this anti-atom, how to hold it, and how to control it well enough to be able to drop it in a way that is sensitive to gravity,” Hangst said. “The next step is to measure the acceleration as accurately as possible. We want to test whether matter and antimatter really fall in the same way.”

The research was published in the journal NatureIn the video below prepared by CERN, Geoffrey Hungst explains how ALPHA-g works and the reasons and results of antimatter gravity experiments.