New Particle Beam to Test Einstein's Gravity with Exotic Matter

Researchers have developed a novel particle beam using exotic muonium atoms to test Einstein's theory of gravity for second-generation particles, a first for physics. This experiment could either confirm General Relativity's reach or reveal new physics, potentially a fifth fundamental force.

David Grossman

October 5, 2026 · 4 min read

In a major step toward probing the limits of modern physics, researchers at ETH Zurich and the Paul Scherrer Institute (PSI) have developed a novel particle beam that will allow for a groundbreaking test of Albert Einstein's theory of gravity. By creating an intense, cold beam of an exotic atom called muonium, scientists plan to investigate for the first time whether gravity's pull is the same for a rare, "second-generation" particle as it is for the ordinary matter that makes up our world. The experiment directly probes the universality of free fall, a foundational principle of General Relativity that has never been verified for this type of matter.

The principle, a cornerstone of physics first recognized in the time of Galileo, states that all objects in a gravitational field fall at the same rate, regardless of their composition. While this idea has been confirmed with extraordinary precision for everyday matter and even first-generation antimatter, its validity for other types of particles remains an open and tantalizing question. This new experiment, conducted at the PSI's particle accelerator in Villigen, Switzerland, aims to fill that crucial gap in our understanding of the universe.

Generations of Matter and Gravity's Reach

At the heart of the new experiment is the distinction between the fundamental particles that constitute the universe. The Standard Model of particle physics organizes these building blocks into three "generations" of increasing mass. All stable, ordinary matter—from stars and planets to people—is made from first-generation particles. The new experiment uses muonium, an exotic, short-lived atom containing an antimuon, which is a heavier, unstable cousin of the electron and a member of the second generation. According to Anna Soter, a lead researcher on the project from ETH Zurich and PSI, testing how muonium falls is a novel way to probe one of physics' most fundamental rules.

The gravitational interaction of second-generation particles has never been directly measured, representing a significant gap in the experimental verification of Einstein's equivalence principle. The table below outlines the distinction between these particle types and the unique role of the new experiment.

A comparison of particle generations and the scope of gravitational tests.
Particle Category	Description	Status of Gravity Test
First-Generation Particles	Includes electrons and the quarks that form protons and neutrons, making up all ordinary matter.	The universality of free fall has been extensively verified, confirming Einstein's equivalence principle for this type of matter.
Second-Generation Particles (e.g., Muonium)	Muonium is an exotic atom made of an electron and an antimuon. The antimuon is a second-generation particle, about 200 times more massive than an electron.	The gravitational interaction of second-generation particles has never been experimentally tested. This represents a significant gap in verifying the equivalence principle.
The New Experiment	Uses a newly developed "cold" and intense beam of muonium atoms to precisely measure their behavior in a gravitational field for the first time.	A result that deviates from General Relativity's prediction could imply the existence of new physics, such as a "fifth fundamental force."

Overcoming a Fleeting Existence

A significant hurdle for any experiment involving muonium is its extreme instability. The antimuon at its core has a lifetime of just 2.2 microseconds before it decays, leaving a vanishingly small window to perform any measurement. To overcome this, the research team developed a new method for producing the muonium atoms. The technique, described as an "atomic cannon," uses superfluid helium to generate a beam that is not only intense but also "cold."

In this context, "cold" means the muonium atoms are moving very slowly. This is crucial because it extends the observation time available before the atoms decay. The challenge is immense; calculations show that even over five of its lifetimes, a muonium atom at rest would fall a mere 600 picometers under gravity—a distance smaller than the atom itself. Measuring such a tiny displacement requires exquisite precision and, critically, enough time for the fall to be detectable. By creating a slow-moving beam, the researchers can observe the atoms for longer as they travel, making the measurement of their gravitational fall feasible. This technical achievement, detailed in a recent publication, clears the primary obstacle that has long prevented a direct gravitational measurement on a second-generation particle.