Testing the Universality of Free Fall with Exotic Muonium
For ordinary matter, objects at the exact same location in a gravitational field fall at identical rates. Galileo Galilei and Isaac Newton established this universality of free fall centuries ago, which later formed the foundation of Albert Einstein’s theory of gravity via the equivalence principle connecting gravitational mass with inertial mass. According to physics professor Soter, testing whether this principle applies to leptons like muons requires a neutral atom. Soter explains that the exotic muonium atom suits this purpose well because it is neutral, and neutrality is required to make something fall in a controlled manner.
However, muonium presents a severe experimental obstacle. Muons survive for roughly 2.2 microseconds before decaying. Furthermore, older production methods generated muonium atoms traveling at varied speeds and in multiple directions, rendering them useless for precise gravity measurements. To solve this, PSI researchers engineered a method to produce cold muonium atoms propagating at similar speeds and almost parallel to one another, as detailed in Nature Physics.
Did you know?
Muons decay in just 2.2 microseconds, requiring researchers to measure their gravitational behavior before they vanish.
Superfluid Helium Acts as an Atomic Cannon
To cool and direct the beam, the research team used superfluid helium cooled close to absolute zero at minus 273 degrees Celsius, according to lead study author Jesse Zhang. Zhang explains that superfluid helium is a quantum fluid where individual helium atoms lose their identity and impurities are completely expelled. This chemical potential drives newly formed muonium atoms out of the liquid. Upon reaching the surface, the chemical potential converts into kinetic energy, giving the muonium atom a vertical boost.
The team refers to this mechanism as an atomic cannon. The muonium atoms pass through the quantum liquid without collisions at a predictable speed. Because their lifespan is so brief, any delay would cause them to decay before reaching the surface. With this method secured, the team is currently building an interferometer to measure how Earth’s gravity affects the muonium beam.
| Parameter | Traditional Methods | PSI Superfluid Helium Method |
|---|---|---|
| Atom State | Hot, multi-directional speeds | Cold, parallel propagation |
| Primary Obstacle | Decay before measurement (2.2 µs limit) | Fast ejection via chemical potential |
| Measurement Tool | Incompatible with high-precision setups | Interferometer for wave patterns |
Could the Muon Experiment Reveal a Fifth Force?
The interferometer uses the wave properties of atoms to generate an interference pattern. Earth’s gravitational pull should introduce an extremely small shift in that pattern, allowing researchers to calculate how gravity acts on the muon. According to Soter, the team hopes to test the method with the atomic beam, with the actual gravity experiment scheduled in two or three years. This beam could additionally enable precise laser spectroscopy experiments to refine measurements of the muon’s mass and fundamental physical constants.
Soter notes that such a result would be surprising and could point to the existence of a fifth force alongside gravity, electromagnetism, the strong interaction, and the weak interaction. While scientists have proposed additional fundamental forces repeatedly over decades, none have been experimentally confirmed. This research operates under the support of the National Centre of Competence in Research Muoniverse.
Pro Tip for Physics Enthusiasts
Frequently Asked Questions
What is muonium?
Muonium is a short-lived atom. It serves as a valuable model for studying leptonic interactions because it acts as a neutral atom.
Why do muons decay so quickly?
What role does superfluid helium play in the experiment?
Superfluid helium, cooled to near absolute zero at minus 273 degrees Celsius, expels impurities and uses its chemical potential to eject newly formed muonium atoms upward at predictable speeds.
What is the equivalence principle?
Rooted in Galileo and Newton’s work on the universality of free fall, Albert Einstein’s equivalence principle states that gravitational mass and inertial mass are equivalent, meaning objects fall at the same rate regardless of their composition.
What are your thoughts on the search for a fifth fundamental force? Share your perspective in the comments below, explore our related physics coverage, and subscribe to our newsletter for weekly updates on cutting-edge research.