Article by: Juliana Bakhtsizina, on 30 September 2023 at 06:05 am PST

Albert Einstein's general theory of relativity, formulated in 1915, has stood the test of time as the most successful description of gravitation. Over the years, numerous experimental tests, from the 1919 solar eclipse to the recent detection of gravitational waves, have confirmed its predictions. However, as our understanding of the universe has evolved, with the emergence of concepts like dark matter and dark energy, it has become evident that there is much more to learn about the gravitational nature of the cosmos. Additionally, the theory's singularities and the absence of a quantum theory of gravity suggest that our current understanding of gravity is incomplete.

One intriguing question that has arisen in the realm of gravitational physics involves antimatter, a concept unknown to Einstein when he developed his theory. Dirac's theory of antimatter emerged in 1928, and the positron, the antimatter counterpart of the electron, was observed in 1932. Since then, scientists have speculated about how gravity interacts with antimatter. While the theoretical consensus suggests that laboratory masses, including antimatter, should be attracted by Earth's gravity, some authors have explored the cosmological implications if antimatter were to be repelled by matter.

The Weak Equivalence Principle (WEP) is a fundamental concept of Einstein's theory, asserting that all masses, regardless of their internal structure, respond identically to gravity. This principle has been tested for matter in Earth's orbit with remarkable precision. However, antimatter has presented a unique challenge for direct tests of the WEP due to the absence of stable, electrically neutral test particles. Electromagnetic forces acting on charged antiparticles make direct measurements within Earth's gravitational field exceedingly difficult. "Gravity is just so bloody weak, you really have to be careful," says Hangst, who is also a physicist at the University of Aarhus in Denmark. He and his collaborators reported the findings on 27 September in Nature.

Even more challenging is the interaction between antimatter and magnetic fields. Controlling stray magnetic fields to the necessary precision to observe the effects of gravity is extremely difficult. Experiments have demonstrated that confined, oscillating, charged antimatter particles behave as expected in a gravitational field but are limited in their potential for precise measurements.

Advancements in antimatter research, particularly the production and confinement of antihydrogen, have now made it possible to employ stable, neutral anti-atoms in dynamic experiments where gravity is expected to play a role. Early conceptual studies and a proof-of-principle experiment in 2013 hinted at the potential for such experiments.

In a groundbreaking experiment conducted within the ALPHA-g apparatus, researchers aimed to investigate the response of antihydrogen atoms to Earth's gravitational pull. The results were striking. The best-fit measurement yielded a value of (0.75 ± 0.13 (statistical + systematic) ± 0.16 (simulation)) g for the local acceleration of antimatter towards the Earth. This finding suggests that antihydrogen atoms are indeed attracted to the Earth by gravity, consistent with the predictions of Einstein's General Relativity.

Furthermore, the experiment virtually ruled out the existence of repulsive gravity, with a probability smaller than 10^-15. These results align with the predictions of General Relativity and do not support cosmological models reliant on repulsive matter-antimatter gravitation.

This groundbreaking experiment marks the beginning of direct inquiries into the gravitational nature of antimatter. While the initial measurements determined the sign and approximate magnitude of the acceleration, the next challenge is to measure the magnitude with even greater precision to provide a stringent test of the Weak Equivalence Principle.

Colder antihydrogen atoms are expected to enable more sensitive measurements. Laser cooling of trapped antihydrogen and adiabatic expansion cooling are promising developments in this direction. Alternative approaches by other collaborations at CERN are also underway.

The motion of antimatter in the Earth's gravitational field has long been a subject of speculation and indirect inference. Now, with this experiment, scientists have a solid experimental foundation to explore this phenomenon further. The quest to understand gravity's interaction with antimatter opens exciting new opportunities for fundamental physics and may eventually lead to a deeper understanding of the universe's gravitational dynamics.