Novel particle beam could challenge Einstein's theory of gravity

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Does gravity act equally on all particles in the universe, or are there differences between ordinary and exotic matter? Physics professor Anna Soter and her team at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen are investigating this question.

"We have taken an important step toward carrying out an exciting experiment on this topic," says Soter. "We want to measure the gravitational interaction of the muon." All matter surrounding us—including ourselves—consists of protons, neutrons and electrons. Physicists refer to this as the first generation of particles.

There are also two further generations comprising heavier particles. The muon, for example, is the heavier sibling of the electron and belongs to the second generation. At PSI, researchers artificially produce muons and their antiparticles in a large particle accelerator.

When a positively charged antimuon combines with a negatively charged electron, a neutral muonium atom is formed. The standard model of particle physics describes the generational structure of matter. "But we physicists do not yet understand why these additional generations exist in the first place," says Soter.

"And why are there three in total?" Another particularly fascinating question is whether the increasing masses of the second- and third-generation particles behave in exactly the same way in terms of gravity as the lighter particles of the first generation. In ordinary matter, all bodies fall at the same rate at a given point within a gravitational field. This universality of free fall was recognized as early as the time of Galileo Galilei and Isaac Newton and, as the equivalence principle between gravitational mass and inertial mass, is a cornerstone of Albert Einstein's theory of gravitation.

This puzzling principle, however, has so far only been demonstrated with ordinary matter or first-generation antimatter. By measuring the gravitational behavior of muonium, the researchers would, for the first time, investigate how a second-generation particle falls. "The exotic muonium is very well suited to this because it is a neutral atom," explains Soter.