University of Helsinki
Experiments with the new ALPHA-g apparatus at CERN offer support for Einsteins general relativity theory from 1915. Physicists dive into the mysterious interaction between gravity and antimatter. Their findings on these intriguing particles finally answer the question, whether antiparticles behave differently than matter particles in gravitational fields.
Antimatter has all the same properties as regular matter, but with opposite charge. You may have heard of it mostly in sci-fi literature and films, but nevertheless it is a real phenomenon. The ALPHA-g experiment focuses on producing and studying antiparticles, especially antihydrogen atoms. The “g” stands for gravity, as the experiment investigates how the antiparticles respond to gravity.
Einsteins theory about general relativity has taught us almost all we know about gravity, but one aspect he was not aware of, how do antiparticles respond to gravitational fields? About 15 years after Einsteins publication, the counterpart of the electron, the positron, was identified by Dirac. This new finding raised new questions and, may I say, even challenged Einsteins infamous theory of gravity. Will Einsteins theory hold for something that was discovered long after the publishment?
In the general theory of relativity, Einstein claims that regardless of a particle’s inner structure, all masses react identically to gravity. Antimatter particles have the same mass as their matter- counterparts and shouldn’t be an exception to this theory.
The research team at CERN made antihydrogen atoms by combining antiprotons and positrons. This resulted in antihydrogen atoms, with no charge, that could be shot up in the vertical ALPHA-g machine. The antihydrogen atoms are now trapped in the magnetic “atom trap.” Depending on the direction the atoms move in, the scientists could see how the anti-matter particles reacted to earth’s gravitational field.
The outcomes of the experiment were the same as those observed with regular hydrogen atoms, with around 80% of both moving downward after release. This reinforces Einsteins theory that anti matter and matter particles would react the same way to gravity. Regardless of how exotic anti particles may sound.
The results don’t only confirm the attractive gravitational force between Earth and the anti-particle, but also rule out the possibility that the gravitational force could be repulsive. Which has been a hot topic since the discovery of the first positron. That would mean that the anti-particle would be moving up instead of down like regular particles. With this experiment we can now confidently say that this is not the case. Unfortunately, it challenges some cosmological models relying on the repulsive matter-antimatter gravitation theory.
Anderson, E.K., Baker, C.J., Bertsche, W. et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023). https://doi.org/10.1038/s41586-023-06527-1