Does gravity affect every type of particle in exactly the same way? Researchers at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen are preparing an experiment that could test that fundamental assumption using one of nature's more unusual particles. "We have taken an important step towards carrying out an exciting experiment on this topic," says the professor of physics: "We want to measure the gravitational interaction of the muon." The familiar matter that makes up people, planets, and nearly everything around us consists of protons, neutrons, and electrons.
Physicists classify these particles as belonging to the first generation of matter. Two additional generations also exist, made up of heavier particles. One of them is the muon, a heavier relative of the electron that belongs to the second generation.
Researchers at PSI can create muons and their antiparticles using a large particle accelerator. When a positively charged antimuon combines with a negatively charged electron, the pair forms a neutral atom known as muonium. The Standard Model of particle physics describes these different generations of particles, but it does not explain why nature has multiple generations in the first place.
"But we physicists do not yet understand why these additional generations exist at all in the first place," says Soter. "And why are there three in total?" That mystery raises another important question. Do the heavier particles of the second and third generations respond to gravity in exactly the same way as the lighter particles found in the first generation?
Testing Einstein's Equivalence Principle For ordinary matter, objects at the same location in a gravitational field fall at the same rate. Galileo Galilei and Isaac Newton recognized this universality of free fall centuries ago. It later became central to Albert Einstein's theory of gravity through the equivalence principle, which connects gravitational mass with inertial mass.
So far, however, researchers have demonstrated this principle only with ordinary matter or first-generation antimatter. Measuring how muonium behaves under gravity would provide the first test involving a second-generation particle. "The exotic muonium is very well suited to this because it is a neutral atom," explains Soter: "After all, to make something fall, you need something neutral." Neutrality is essential because gravity is extremely weak compared with electromagnetism.
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