The weight of antihydrogen atoms (a positively charged positron or anti-electron orbiting a negatively charged antiproton) balances the weight of hydrogen atoms (a negatively charged electron orbiting a positively charged proton).
The weight of antihydrogen atoms (a positively charged positron or anti-electron orbiting a negatively charged antiproton) balances the weight of hydrogen atoms (a negatively charged electron orbiting a positively charged proton).
Image courtesy of Chukman So, U.C. Berkeley

The Science

Scientists have indirect evidence that antimatter falls the same way as matter. This would mean that a normal ball dropped on Earth would fall down, and so would a ball made of antimatter. However, they have never before conducted a direct experiment establishing this prediction. Such an experiment would involve scientists essentially dropping antimatter atoms and comparing their acceleration to that of normal matter atoms. A new experiment conducted at CERN establishes that antihydrogen atoms fall with the same acceleration as hydrogen atoms. Since antihydrogen falls downward, this means all antimatter likely falls downward.

The Impact

The experiment tackled a fundamental question: does antimatter fall up or down? The researchers discovered that it falls downward. This result demonstrates the importance of combining ideas and methods from plasma, particle, and atomic physics. This marks the start of a new era in the study of how gravity affects neutral antimatter. This groundbreaking achievement also sets the stage for future experiments. Using the same equipment, the researchers plan to refine their measurements to a precision of 1%. A follow-up experiment called an atomic fountain could achieve even greater accuracy.

Summary

Einstein's theory of general relativity, created in 1915, constitutes science’s best explanation of gravity. It predicted new physics, such as light bending in a gravitational field, which was verified in the 1919 solar eclipse observations, as well as gravity waves, which were detected only recently. However, much is unknown about the composition of the universe, including questions such as the preponderance of matter over antimatter, the natures of dark energy and dark matter, and the coupling of gravity to quantum mechanics.

Most scientists think antimatter should be attracted to Earth like regular matter, but some have proposed that it might be repelled instead. This proposal contradicts general relativity, which predicts that everything should respond to gravity the same way, no matter what it is made of. In this study, scientists used the ALPHA-g device to synthesize, trap, and release antihydrogen atoms from a magnetic balance conceptually similar to a classic pan balance. They found that antihydrogen atoms are attracted to Earth. This means that the idea of antigravity is not correct, at least for antihydrogen. This work is a proof-of-concept for higher-precision measurements of antihydrogen in the Earth’s gravitational field.

Contact

Joel Fajans
University of California, Berkeley
joel@physics.berkeley.edu

Jonathan Wurtele
University of California, Berkeley
wurtele@berkeley.edu

Funding

This research was supported in part by the Department of Energy Office of Science, Fusion Energy Science program, and the National Science Foundation in the United States; the National Council for Scientific and Technological Development, the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro, and Rede Nacional de Física de Altas Energias in Brazil; the Natural Sciences and Engineering Research Council and TRIUMF in Canada; the Carlsberg Foundation of Denmark; the Science and Technology Facilities Council , Engineering & Physical Sciences Research Council, the Royal Society, and Leverhulme Trust in the United Kingdom; the Israel Science Foundation; and the Swedish Research Council.

Publications

Anderson, E.K., et al., Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023). [DOI: 10.1038/s41586-023-06527-1]

Hamilton, P., et al., Antimatter Interferometry for Gravity Measurements, Physical Review Letters 112, 121102 (2014). [DOI: 10.1103/PhysRevLett.112.121102]

Amole, C., et al., Description and first application of a new technique to measure the gravitational mass of antihydrogen. Nature Communications 4, 1785 (2013). [DOI: 10.1038/ncomms2787]

Zhmoginov, A.I., et al., Nonlinear dynamics of anti-hydrogen in magnetostatic traps: implications for gravitational measurements. Classical and Quantum Gravity. 30, 205014, (2013) [DOI: 10.1088/0264-9381/30/20/205014]