Antimatter Gravity Confirmed: It Falls Down, Not Up

For decades, science fiction writers and theoretical physicists alike wondered exactly how antimatter reacts to gravity. Thanks to a breakthrough experiment at the European Organization for Nuclear Research (CERN), we finally have the answer. Antihydrogen falls down. This discovery resolves a massive cosmic mystery and confirms a core prediction of Albert Einstein.

The ALPHA-g Experiment at CERN

To test how antimatter responds to gravity, scientists at CERN built a highly specialized experiment called ALPHA-g. Antimatter is incredibly difficult to study because it annihilates the moment it comes into contact with regular matter. You cannot simply place it in a glass jar and watch it drop.

The ALPHA collaboration had to manufacture their own antimatter from scratch. They focused on creating antihydrogen, the antimatter counterpart to hydrogen. Regular hydrogen consists of one proton and one electron. Antihydrogen consists of one antiproton and one positron (an anti-electron).

To build these atoms, the CERN team took antiprotons from the facility’s Antiproton Decelerator and combined them with positrons gathered from a radioactive sodium-22 source. The scientists then trapped the resulting antihydrogen atoms inside a vertical, cylindrical vacuum chamber. To keep the antimatter from touching the walls of the chamber, they used powerful superconducting magnetic fields to hold the atoms suspended in the center.

Dropping the Antimatter

The actual dropping of the antimatter required extreme precision. The magnetic trap holding the antihydrogen was kept incredibly cold, at roughly 0.5 degrees Celsius above absolute zero. In September 2023, the research team published their experimental methodology and results in the journal Nature.

The researchers gathered about 100 antihydrogen atoms inside the vertical cylinder for each test run. To see how gravity affected the atoms, the team slowly reduced the strength of the magnetic fields at the top and bottom of the cylinder. This allowed the antihydrogen atoms to escape.

The physicists placed sensors at the ends of the cylinder to detect the flashes of energy that occur when antimatter escapes and annihilates against the regular matter of the chamber walls.

The results were clear. Approximately 80 percent of the antihydrogen atoms annihilated at the bottom of the cylinder. The remaining 20 percent exited through the top due to the random thermal motion of the atoms bouncing around inside the trap. This ⁸⁰⁄₂₀ split perfectly matches how a cloud of regular, ultra-cold hydrogen behaves under the exact same conditions. Gravity pulls it down.

Confirming Einstein's Predictions

Jeffrey Hangst, the spokesperson for the ALPHA collaboration, stated that this experiment was the first direct observation of gravitational effects on the motion of antimatter. The findings confirm a major component of Albert Einstein’s 1915 general theory of relativity.

Einstein’s theory relies on the weak equivalence principle. This principle states that all objects will fall at the exact same rate in a given gravitational field, regardless of what they are made of. A pound of lead and a pound of feathers fall at the same speed in a vacuum. Now, CERN has proved that a pound of regular matter and a pound of antimatter will also fall at the exact same speed.

If the ALPHA-g experiment had shown that antimatter falls up, scientists would have had to completely rewrite modern physics. Anti-gravity would have shattered our current understanding of how the universe operates.

The Lingering Cosmic Mystery

While the ALPHA-g results answer one massive question, they leave another cosmic mystery completely untouched. Physicists call this the problem of baryon asymmetry.

According to the standard model of particle physics, the Big Bang should have created equal amounts of regular matter and antimatter. Because matter and antimatter annihilate each other on contact, the early universe should have destroyed itself, leaving behind nothing but pure energy.

Obviously, that did not happen. We live in a universe dominated entirely by regular matter. Antimatter is incredibly rare, found only in cosmic ray collisions or high-tech laboratories like CERN.

Some physicists theorized that if antimatter experienced anti-gravity, the repulsive force might explain how matter and antimatter separated in the early universe, preventing total annihilation. Because the CERN experiment proves antimatter falls down just like regular matter, this separation theory is no longer valid. Scientists still do not know why regular matter won the battle of the early universe.

Future Plans for the ALPHA Collaboration

The current experiment proved the direction of the gravitational pull, but the ALPHA team is not finished. The next step is to measure the exact acceleration rate of the falling antihydrogen.

Earth’s gravity accelerates objects downward at a rate of 9.8 meters per second squared (often referred to as 1g). The CERN physicists want to know if antimatter falls at precisely 1g, or if it falls at 0.99g. Even a microscopic difference in the acceleration rate between matter and antimatter could hint at unknown physical forces.

To achieve this level of precision, the ALPHA team plans to upgrade their equipment using laser cooling techniques. By hitting the antihydrogen atoms with specifically tuned lasers, they can slow the atoms down even further. Colder, slower atoms will allow the researchers to measure the exact speed of the downward fall, potentially unlocking new secrets about the building blocks of our universe.

Frequently Asked Questions

What is antimatter? Antimatter is identical to regular matter but has the opposite electrical charge. For example, a regular electron has a negative charge, while an anti-electron (positron) has a positive charge. When antimatter and regular matter meet, they destroy each other and release energy.

Why does antimatter fall down? Antimatter falls down because it has mass. According to general relativity, gravity warps spacetime around any object with mass. Because antimatter has the exact same mass as its regular matter counterpart, it follows the same downward curve of spacetime created by Earth’s gravity.

Did physicists ever really think antimatter fell up? Yes. Because antimatter is so difficult to create and contain, scientists had never directly tested its gravitational properties before the ALPHA-g experiment. A small minority of physicists proposed that antimatter might exhibit negative mass or experience gravitational repulsion, which would cause it to fall up.

Where does CERN get its antimatter? CERN generates antimatter using a machine called the Antiproton Decelerator. They fire high-energy protons into a block of metal to create antiprotons. They then slow these antiprotons down and mix them with positrons to create neutral antihydrogen atoms for their experiments.