Physicists from the ALPHA Collaboration at CERN’s Antimatter Factory have demonstrated the existence of gravity between antimatter and Earth, reaffirming Albert Einstein’s general theory of relativity.
Anderson et al. show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Image credit: Keyi ‘Onyx’ Li / U.S. National Science Foundation.
Our bodies, the Earth, and most everything else physicists know about in the Universe are overwhelmingly made of regular matter consisting of protons, neutrons, and electrons, like atoms of oxygen, carbon, iron and the other elements of the periodic table.
Antimatter, on the other hand, is regular matter’s twin, though with some opposite properties.
For example, antiprotons have a negative charge while protons have a positive charge. Antielectrons (also known as positrons) are positive while electrons are negative.
“However, perhaps most challenging for experimenters, as soon as antimatter touches matter, it blows up,” said Dr. Joel Fajans, a plasma physicist at the University of California, Berkeley and a memeber of the ALPHA Collaboration.
“The combined mass of matter and antimatter is transformed entirely into energy in a reaction so powerful that scientists call it an annihilation.”
“For a given mass, such annihilations are the densest form of energy release that we know of.”
“But, the amount of antimatter used in the ALPHA experiment is so small that the energy created by antimatter/matter annihilations is perceptible only to sensitive detectors.”
“Still, we have to manipulate the antimatter very carefully or we will lose it.”
“Einstein’s theory of general relativity says antimatter should behave exactly the same as matter,” said Dr. Jonathan Wurtele, a plasma physicist at the University of California, Berkeley and a memeber of the ALPHA Collaboration.
“Many indirect measurements indicate that gravity interacts with antimatter as expected, but until the result today, nobody had actually performed a direct observation that could rule out, for example, antihydrogen moving upwards as opposed to downwards in a gravitational field.”
In their experiments, the ALPHA physicists 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.
“Understanding how gravity affects antimatter is crucial for both understanding mysteries surrounding both antimatter and gravity itself,” said Dr. William Bertsche, a physicist at the University of Manchester and a deputy spokesperson for the ALPHA Collaboration.
“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 behavior 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.”
“In physics, you don’t really know something until you observe it,” said ALPHA spokesperson Dr. Jeffrey Hangst.
“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.”
“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.”
The results were published in the journal Nature.
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E.K. Anderson et al. 2023. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716-722; doi: 10.1038/s41586-023-06527-1
