The University of Manchester

Scientists observes the influence of gravity on antimatter for the first time

Scientists have demonstrated the existence of gravity between antimatter and Earth, reaffirming Albert Einstein’s General Theory of Relativity.

It is thought that Isaac Newton’s historic work on gravity was inspired by watching an apple fall to Earth from a tree. But for decades, scientists have wondered what would happen to an “anti-apple” made of antimatter – would it fall in the same way if it existed?

Until now, the question has left scientists with an incomplete picture of the Universe’s gravitating content.

In a paper published today in Nature, the ALPHA collaboration at CERN’s Antimatter Factory, which includes academics from The University of Manchester, shows that within the precision of their experiment, atoms of antihydrogen – a form of antimatter – fall to Earth in the same way as regular matter.

Matter is anything that takes up space and has a mass and can be in the form of liquid, solid or gas. Things such as, air, water and rocks are all examples of matter. Antimatter is like the opposite of matter, made up of particles that have the opposite electrical charge. For example, matter has electrons, antimatter has positrons (antielectrons).

While matter is everywhere, its opposite is now incredibly hard to find, even though both were created in equal amounts in the infancy of our Universe.

The result pushes scientists one step closer to solving the mystery of antimatter.

Dr William Bertsche, Reader in the Accelerator Physics group at The University of Manchester and a Deputy Spokesperson for the collaboration, said: “Einstein’s General Theory of Relativity, which he introduced over a century ago, describes how gravity works. Until now, we weren’t entirely sure if this theory applied to antimatter. This experiment proves that it does, within the certainty levels of the results, and affirms one of the most celebrated scientific theories of all time.

“Understanding how gravity affects antimatter is crucial for both understanding mysteries surrounding both antimatter and gravity itself. The origin of the observed dominance of matter over antimatter in the universe remains an unsettled challenge to existing theories, which we aim to understand through careful observation of the behaviour of antimatter relative to matter. For its own part, gravity remains ununified with other theories, such as quantum mechanics, and therefore having a broader palette of observations will help further our understanding of it.”

ALPHA spokesperson Jeffrey Hangst added: “In physics, you don’t really know something until you observe it. This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the Universe.”

Following a proof-of-principle experiment with the original ALPHA set-up in 2013, the team trapped groups of about 100 antihydrogen atoms, one group at a time, and then slowly released the atoms over a period of 20 seconds.

Computer simulations of the ALPHA-g set-up indicate that this operation – for matter – would result in about 20% of the atoms exiting through the top of the trap and 80% through the bottom, a difference caused by the downward force of gravity. By averaging the results of seven release trials, the ALPHA team found that the fractions of anti-atoms exiting through the top and bottom are in agreement with the expectations from the simulations.

Jeffrey Hangst said: “It’s taken us 30 years to learn how to make this anti-atom, to hold on to it, and to control it well enough that we could actually drop it in a way that it would be sensitive to the force of gravity.

“The next step is to measure the acceleration as precisely as we can. We want to test whether matter and antimatter do indeed fall in the same way.  Laser-cooling of antihydrogen atoms, which we first demonstrated in ALPHA-2 and will implement in ALPHA-g when we return to it in 2024, is expected to have a significant impact on the precision.”

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