This graphic shows antihydrogen atoms falling and annihilating inside a magnetic trap, part of the ALPHA-g experiment at CERN to measure the effect of gravity on antimatter.
Credit: U.S. National Science Foundation
For those still holding out hope that antimatter levitates rather than falls in a gravitational field, like normal matter, the results of a new experiment are a dose of cold reality.
Physicists studying antihydrogen—an anti-proton paired with an antielectron, or positron—have conclusively shown that gravity pulls it downward and does not push it upward.
At least for antimatter, antigravity doesn't exist.
The experimental results have been reported in the Sept. 28 issue of the journal Nature by a team representing the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at the European Center for Nuclear Research (CERN) in Geneva, Switzerland.
The gravitational acceleration of antimatter that the team comes up with is close to that for normal matter on Earth: 1 g, or 9.8 meters per second per second (32 feet per second per second). More precisely, it was found to be within about 25% (one standard deviation) of normal gravity.
"It surely accelerates downwards, and it's within about one standard deviation of accelerating at the normal rate," said Joel Fajans, a UC Berkeley professor of physics who, with colleague Jonathan Wurtele, a theoretician, first proposed the experiment more than a decade ago. "The bottom line is that there's no free lunch, and we're not going to be able to levitate using antimatter."
The result will not surprise most physicists. Albert Einstein's theory of general relativity, though conceived before antimatter was discovered in 1932, treats all matter identically, implying that antimatter and matter respond the same to gravitational forces. All normal matter, such as protons, neutrons and electrons, have anti-particles that bear the opposite electrical charge and, when they encounter their normal matter counterpart, annihilate completely.
"The opposite result would have had big implications; it would be inconsistent with the weak equivalence principle of Einstein's general theory of relativity," said Wurtele, UC Berkeley professor of physics. "This experiment is the first time that a direct measurement of the force of gravity on neutral antimatter has been made. It's another step in developing the field of neutral antimatter science."
Fajans noted that no physical theory actually predicts that gravity should be repulsive for antimatter. Some physicists claim that, if it were, you could create a perpetual motion machine, which is theoretically impossible.
Nevertheless, the idea that antimatter and matter might be affected differently by gravity was enticing because it could potentially explain some cosmic conundrums. For example, it could have led to the spatial separation of matter and antimatter in the early universe, explaining why we see only a small amount of antimatter in the universe around us. Most theories predict that equal amounts of matter and antimatter should have been produced during the Big Bang that birthed the universe.
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