Scientists at ETH Zurich and the Paul Scherrer Institute are preparing to conduct a novel experiment examining gravity’s effects on muons, exotic particles that belong to the second generation of matter. The investigation aims to test a foundational principle of Einstein’s theory of gravity by measuring whether these heavier particles respond to gravitational fields identically to ordinary matter composed of protons, neutrons, and electrons.
The equivalence principle, central to Einstein’s gravitational theory, states that all objects at the same location in a gravitational field fall at the same rate. While this principle has been confirmed for ordinary matter and first-generation antimatter, it has never been tested with particles from higher generations. Muons, heavier relatives of electrons, present an ideal candidate for such testing. When a positively charged antimuon combines with a negatively charged electron, they form a neutral atom called muonium. Neutrality is crucial because gravity is extremely weak compared with electromagnetism, and charged particles would be overwhelmed by stray electromagnetic fields that could interfere with measurements.
A major challenge in conducting this experiment involves muons’ extremely brief lifespan of approximately 2.2 microseconds before they decay. Researchers at PSI have overcome this obstacle by developing a method to produce muonium atoms in a controlled state. The technique uses superfluid helium cooled near absolute zero, which slows incoming antimuons. When antimuons encounter electrons within the superfluid, muonium atoms form and are ejected upward by chemical potential acting as what researchers describe as an atomic cannon.
The team plans to use an interferometer device to measure minute shifts in atomic interference patterns caused by Earth’s gravity. Initial testing of the method with the atomic beam is targeted for later this year, with the full gravity experiment anticipated to follow within two to three years. If successful, the research could provide insights into fundamental questions about particle physics, including why nature contains multiple generations of particles. Should muonium respond to gravity differently from ordinary matter, the findings could suggest the existence of a previously unknown fundamental force beyond the four currently recognized interactions.
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