# Testing Einstein's Gravity with Exotic Muonium Atoms ETH Zurich researchers prepare a groundbreaking experiment on second-generation particles. - **Source:** [Latest Science News -- ScienceDaily](https://brifing.si/source/0eef8637-6c5b-4e79-8f5b-6c9c6b36cf16) - **Published:** 2026-09-18 - **Topic:** [Data & AI](https://brifing.si/c/data-ai) - **Original article:** - **On Brifing:** https://brifing.si/s/a4e4b007-1e3d-4ddf-b8e9-7cb2b6733f58 ## Muons challenge gravity assumptions ETH Zurich and PSI researchers are preparing to test whether gravity affects second-generation particles like muons the same way it does ordinary matter. Muons are heavier relatives of electrons that decay in just 2.2 microseconds. The experiment would be the first to examine gravitational effects on particles beyond the first generation of matter, potentially challenging Einstein's equivalence principle. ## Why muonium is ideal Muonium - formed when an antimuon combines with an electron - provides a neutral test subject crucial for gravity measurements. Neutrality prevents electromagnetic interference from overwhelming the weak gravitational signal. The short-lived muons (2.2 μs lifespan) require extremely precise measurement techniques that previous methods couldn't achieve due to varying speeds and directions of muonium atoms. ## Breakthrough in cold muonium production Researchers developed a method to produce 'cold' muonium atoms using superfluid helium cooled near absolute zero (-273°C). This quantum fluid allows muonium formation with consistent speed and direction. The chemical potential of superfluid helium acts like an 'atomic cannon', ejecting muonium vertically at predictable velocities essential for gravity measurements. ## The interferometer approach The team is building an interferometer to detect gravity's effect on muonium by measuring wave pattern shifts. PSI's particle accelerator provides the world's most intense continuous muon beam, enabling sufficient muonium production. The experiment requires extreme precision to detect gravitational effects during the particles' brief 2.2 microsecond lifespan before decay. ## Testing Einstein's equivalence principle The experiment directly tests whether gravitational mass equals inertial mass for second-generation particles. While demonstrated for ordinary matter, this principle remains untested for heavier particles. A deviation could indicate new physics beyond the Standard Model, potentially suggesting an unknown fifth fundamental force in nature. ## Technical challenges overcome Key innovations include using superfluid helium to produce slow, parallel-moving muonium atoms and developing detection methods fast enough for the 2.2 μs window. The team's Nature Physics paper details their production method. These advances make the gravity measurement possible where previous attempts failed due to technical limitations. ## Broader scientific implications Beyond gravity tests, the cold muonium beam enables more precise laser spectroscopy to better measure the muon's mass and fundamental constants. The research could shed light on why nature has three particle generations - a mystery the Standard Model doesn't explain. Results may influence both particle physics and cosmology theories. ## Timeline for groundbreaking results Researchers plan to test the interferometer method this year, with full gravity experiments expected in 2-3 years. Supported by the NCCR Muoniverse, this work could fundamentally alter our understanding of gravity's universality. As lead researcher Soter states: 'I simply want to measure whether the equivalence principle applies to second-generation particles.' ## Sources covering this story - https://www.sciencedaily.com/releases/2026/09/260918024808.htm — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._