Antimatter Responds to Gravity Just Like Regular Matter

Physicists have wondered for decades how antimatter behaves when you drop it. Thanks to a groundbreaking experiment at CERN, we finally have the answer. A recent study confirms that antihydrogen falls downward under the influence of gravity, definitively putting an end to theories that antimatter might experience anti-gravity.

The ALPHA-g Experiment at CERN

The European Organization for Nuclear Research (CERN) operates the largest particle physics laboratory in the world, located just outside Geneva, Switzerland. In September 2023, a research team known as the ALPHA collaboration published a historic paper in the journal Nature. They announced the results of their ALPHA-g experiment, which was designed specifically to observe the gravitational behavior of antimatter.

Studying antimatter is incredibly difficult because it annihilates the moment it touches regular matter. When a particle of antimatter meets its regular matter counterpart, they destroy each other in a flash of energy. Because our world is made entirely of regular matter, researchers cannot simply hold antimatter in a glass jar or drop it from a tower.

To run this test, the ALPHA team had to build their own antimatter atoms from scratch. They created antihydrogen by combining antiprotons (the antimatter equivalent of protons) with positrons (the antimatter equivalent of electrons). Antihydrogen is the simplest form of stable antimatter, making it the perfect candidate for testing gravity.

How Do You Drop Antimatter?

Once the scientists created the antihydrogen atoms, they had to figure out a way to drop them without letting them touch the walls of the container. To solve this, the ALPHA-g experiment used a sophisticated vertical magnetic trap.

Because antihydrogen atoms have a slight magnetic charge, strong magnetic fields can push them around. The researchers placed the newly created antihydrogen atoms inside a tall, vertical cylinder. They turned on powerful superconducting magnets at the top and the bottom of the cylinder to act as invisible lids, keeping the antimatter suspended in a vacuum.

To perform the drop test, the team slowly reduced the strength of the magnetic fields at the top and bottom of the vertical trap. As the magnetic containment weakened, the antihydrogen atoms escaped. The scientists then used specialized sensors to detect exactly where the atoms annihilated against the physical walls of the machine.

The data was very clear. Approximately 80 percent of the antihydrogen atoms escaped through the bottom of the cylinder, while only 20 percent exited through the top. This 80-to-20 ratio perfectly matches how a cloud of regular hydrogen gas behaves under the exact same thermal conditions. The atoms that exited the top did so because of random thermal bouncing, but the overwhelming majority were pulled down by Earth’s gravity.

Ruling Out Anti-Gravity

Before this experiment, a fringe idea in physics suggested that antimatter might fall upward. Some scientists proposed that antimatter might have a negative gravitational mass, meaning it would be repelled by the regular matter that makes up the Earth.

If antimatter fell upward, it would have rewritten our entire understanding of physics. It could have also provided a neat explanation for why the universe is expanding at an accelerating rate. However, the ALPHA-g results firmly close the door on the idea of gravitational repulsion. Antimatter falls down, just like apples, rocks, and everything else on our planet.

Einstein Was Right Again

This experiment provides another massive victory for Albert Einstein. In 1915, Einstein published his General Theory of Relativity, which remains our best scientific description of how gravity works.

A core component of General Relativity is the Weak Equivalence Principle. This principle states that all objects in a gravitational field will fall at exactly the same rate, regardless of what they are made of or how much mass they have. Until the CERN announcement in 2023, scientists had never directly tested the Weak Equivalence Principle on antimatter. The ALPHA-g experiment shows that Einstein’s century-old theory holds up flawlessly, even when applied to the rarest and most volatile substances in the universe.

The Unsolved Mystery of the Missing Antimatter

While the CERN experiment solved the gravity question, it leaves the biggest mystery in modern physics completely intact. According to the standard model of physics, the Big Bang should have created equal amounts of regular matter and antimatter.

Since matter and antimatter annihilate upon contact, the early universe should have destroyed itself, leaving nothing behind but pure light. Instead, we live in a universe filled with regular matter, and natural antimatter is practically non-existent.

Some researchers hoped that if gravity repelled antimatter, it might explain how massive clouds of matter and antimatter pushed each other apart before they could destroy each other. Because we now know that gravity pulls matter and antimatter together in the exact same way, scientists must look for other explanations for why we exist at all.

What Comes Next for Antimatter Research

Proving that antihydrogen falls downward is only the first step. The next phase of the ALPHA experiment will focus on extreme precision.

Right now, scientists know that antimatter falls down, but they do not know if it accelerates at the exact same speed as regular matter. Regular matter falls to Earth at an acceleration of 9.8 meters per second squared. To see if antimatter falls at that precise rate, the CERN team plans to use laser cooling technology. By hitting the antihydrogen atoms with specialized lasers, they can strip away almost all of their thermal energy. This will stop the atoms from bouncing around, allowing scientists to measure their exact gravitational acceleration down to the smallest fraction of a decimal point.

Frequently Asked Questions

What is antimatter? Antimatter is a material made of particles that have the exact same mass as regular matter but carry the opposite electrical charge. For example, a regular electron has a negative charge, but an antimatter electron (a positron) has a positive charge.

Why did scientists think antimatter might fall up? Because antimatter has the opposite electrical properties of regular matter, some physicists theorized it might also have opposite gravitational properties. This concept of “anti-gravity” has now been proven false.

How did CERN create antihydrogen? Scientists at CERN created antihydrogen by harvesting antiprotons from a particle decelerator and mixing them with positrons emitted by a radioactive sodium isotope. They combined these particles inside a highly controlled vacuum using strong magnetic fields.