Direct Measurements of Antimatter Gravity Reveal Surprising Results

Wednesday - October 4 2023, 18:25 UTC - 1 year ago

A new study published by a team in Nature has revealed that antimatter behaves the same way to gravity as matter does, providing us with the first direct validation of the weak equivalence principle for antimatter particles. This experiment brings us one step closer to understanding why there is a matter-antimatter asymmetry in the universe.

A substance called antimatter is at the heart of one of the greatest mysteries of the universe. We know that every particle has an antimatter companion that is virtually identical to itself, but with the opposite charge. When a particle and its antiparticle meet, they annihilate each other—disappearing in a burst of light.

Our current understanding of physics predicts that equal quantities of matter and antimatter should have been created during the formation of the universe. But this doesn’t seem to have happened as it would have resulted in all particles annihilating right away.

Instead, there’s plenty of matter around us, yet very little antimatter—even deep in space. This enigma has led to a grand search to find flaws in the theory or otherwise explain the missing antimatter.

One such approach has focused on gravity. Perhaps antimatter behaves differently under gravity, being pulled in the opposite direction to matter? If so, we might simply be in a part of the universe from which it is impossible to observe the antimatter.

A new study, published by my team in Nature, reveals how antimatter actually behaves under the influence of gravity.

Other approaches to the question of why we observe more matter than antimatter span numerous sub-fields in physics. These range from astrophysics—aiming to observe and predict the behavior of antimatter in the cosmos with experiments—to high-energy particle physics, investigating the processes and fundamental particles that form antimatter and govern their lifetime.

While slight differences have been observed in the lifetime of some antimatter particles compared to their matter counterparts, these results are still far from a sufficient explanation of the asymmetry.

The physical properties of antihydrogen—an atom composed of an antimatter electron (the positron) bound to an antimatter proton (antiproton)—are expected to be exactly the same as those of hydrogen. In addition to possessing the same chemical properties as hydrogen, such as color and energy, we also expect that antihydrogen should behave the same in a gravitational field.

The so-called "weak equivalence principle" in the theory of general relativity states that the motion of bodies in a gravitational field is independent of their composition. This essentially says that what something is made of doesn’t affect how gravity influences its movements.

This prediction has been tested to extremely high accuracy for gravitational forces with a variety of matter particles, but never directly on the motion of antimatter.

Even with matter particles, gravity stands apart from other physical theories, in that is has yet to be unified with the theories that describe antimatter. Any observed difference with antimatter gravitation may help shed light on both issues.

To date, there have been no direct measurements on the gravitational motion of antimatter. It is quite challenging to study because gravity is the weakest force.

That means it is difficult to distinguish the effects of gravity from other external influences. It has only been with recent advances in techniques to produce stable (long-lived), neutral, and isolated antimatter—recently achieved at CERN's Antiproton Decelerator—that an experiment of this kind has become feasible.

The CERN team has observed that antihydrogen and hydrogen atoms experience gravity similarly in agreement with the weak equivalence principle. Antimatter and matter can be distinguished under a unique form of gravitation measurements using gravimeters, allowing for precise comparisons. These new results are the first direct validation of this principle to be tested on antimatter particles.

In a first-of-its-kind experiment, CERN brings us one step closer to understanding why we observe more matter than antimatter in the universe. It is hoped that with further experiments we may soon be able to unravel this cosmic mystery.