An artist's conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus. As the antihydrogen atoms escape, they touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down. Credit: Keyi "Onyx" Li/U.S. National Science Foundation
Key points:
- In first-of-its-kind experiments, scientists have shown that antimatter falls down due to gravity.
- The findings shed light on why antimatter is largely missing from the observable universe.
- Next, the team will study how antihydrogen interacts with electromagnetic radiation through spectroscopy.
In a unique laboratory experiment, researchers have now observed the downward path taken by individual atoms of antihydrogen, providing a definitive answer: antimatter falls down. In confirming antimatter and regular matter are gravitationally attracted, the finding rules out gravitational repulsion as the reason why antimatter is largely missing from the observable universe.
“Einstein's theory of general relativity says antimatter should behave exactly the same as matter,” said Jonathan Wurtele, University of California, Berkeley plasma physicist and project collaboration member. “Many indirect measurements indicate that gravity interacts with antimatter as expected, but until [this result] nobody had actually performed a direct observation that could rule out, for example, antihydrogen moving upwards as opposed to downwards in a gravitational field.”
For the experiment, undertaken by the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at CERN, the antihydrogen was contained within a tall cylindrical vacuum chamber with a variable magnetic trap, called ALPHA-g. The scientists reduced the strength of the trap's top and bottom magnetic fields until the antihydrogen atoms could escape and the relatively weak influence of gravity became apparent.
As each antihydrogen atom escaped the magnetic trap, it touched the chamber walls either above or below the trap and annihilated, which the scientists could detect and count.
The researchers repeated the experiment more than a dozen times, varying the magnetic field strength at the top and bottom of the trap to rule out possible errors. They observed that when the weakened magnetic fields were precisely balanced at the top and bottom, about 80% of the antihydrogen atoms annihilated beneath the trap—a result consistent with how a cloud of regular hydrogen would behave under the same conditions.
Thus, gravity was causing the antihydrogen to fall down.
The ALPHA collaboration researchers will continue to probe the nature of antihydrogen. In addition to refining their measurement of the effect of gravity, they are also studying how antihydrogen interacts with electromagnetic radiation through spectroscopy.
"If antihydrogen were somehow different from hydrogen, that would be a revolutionary thing because the physical laws, both in quantum mechanics and gravity, say the behavior should be the same," said Wurtele. "However, one doesn't know until one does the experiment."