Antimatter is essentially a mirror image of regular matter. An antiproton has the exact same mass as a proton, but a negative charge instead of a positive one. A positron (an antielectron) has the same mass as an electron, but a positive charge. Because Einstein’s theory of General Relativity dictates that gravity acts on mass and energy regardless of a particle's composition, standard physics models strongly suggested that antimatter should fall downward just like regular matter. However, experimental proof is the gold standard of science, and checking this required overcoming immense engineering hurdles.
The core problem is that antimatter annihilates the moment it touches regular matter, releasing a flash of energy. Researchers cannot simply place an antiparticle on a scale or drop it in a standard vacuum chamber, because it will immediately collide with the walls or stray gas molecules and vanish.
To solve this, researchers at the CERN laboratory in Switzerland built a monumental experiment called ALPHA-g. They created antihydrogen—the antimatter equivalent of hydrogen, made of one antiproton and one positron—and suspended these neutral atoms inside a tall, vertical vacuum cylinder using incredibly precise magnetic fields. Once the antimatter atoms were securely trapped, the scientists slowly weakened the magnetic fields at the top and bottom of the cylinder to see which way the atoms would escape under the influence of Earth's gravity.
If antimatter experienced "anti-gravity," the atoms would have drifted upward. Instead, the results, which were finally published in late 2023, were definitive: antihydrogen fell downward.
The experiment confirmed that gravity treats antimatter exactly the same way it treats regular matter. The gravitational acceleration of antimatter matches the standard downward pull of Earth's gravity, closing the door on the idea that antimatter repels regular matter. Gravity, it turns out, does not care whether a particle is matter or antimatter; mass is simply mass, and it all falls down.