Does gravity affect every type of particle in the universe in the exact same way? It is one of the most fundamental questions in modern physics, and researchers at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen are now preparing an ambitious experiment that could finally test that assumption using one of nature’s most unusual and elusive particles.
"We have taken an important step towards carrying out an exciting experiment on this topic," says the professor of physics, outlining the team’s overarching mission: "We want to measure the gravitational interaction of the muon."
To understand the significance of the upcoming research, one must look at the building blocks of the physical world. The familiar matter that makes up people, planets, stars, and nearly everything around us consists of protons, neutrons, and electrons. In the standard lexicon of physics, these particles are classified as belonging to the first generation of matter. However, the universe also features two additional generations composed of heavier, more unstable particles. One of these is the muon, a heavier relative of the electron that belongs directly to the second generation of matter.
At PSI, researchers can generate muons and their antimatter counterparts, antimuons, by utilizing a large particle accelerator. When a positively charged antimuon captures and combines with a negatively charged electron, the pair forms a short-lived, neutral atom known as muonium. This exotic atom lies at the absolute heart of the team’s proposed experimental design.
Why Physicists Want to Test Muons
The Standard Model of particle physics successfully describes these different generations of fundamental particles, yet it leaves a profound gap in our understanding. It provides no explanation for why nature relies on multiple generations in the first place, nor does it tell us why there are precisely three generations in total.
"But we physicists do not yet understand why these additional generations exist at all in the first place," notes the researcher. "And why are there three in total?"
That enduring mystery raises another critical question that cuts to the core of gravitational physics. Do the heavier particles belonging to the second and third generations respond to gravity in precisely the same way as the much lighter particles found in the first generation?
Testing Einstein’s Equivalence Principle
For ordinary matter, classical physics dictates that objects at the same location in a gravitational field fall at the exact same rate, regardless of their mass or composition. Galileo Galilei and Isaac Newton recognized this fundamental universality of free fall centuries ago. Later, this observation became a cornerstone of Albert Einstein’s general theory of relativity through the equivalence principle, which fundamentally connects gravitational mass with inertial mass.
So far, however, experimental physicists have demonstrated this core principle only with ordinary matter or first-generation antimatter. Measuring how muonium behaves under the influence of gravity would provide the scientific community with its very first test involving a second-generation particle.
"The exotic muonium is very well suited to this because it is a neutral atom," explains the researcher. "After all, to make something fall, you need something neutral."
Electrical neutrality is an absolute requirement because gravity is extraordinarily weak when compared with electromagnetism. If researchers were to attempt this delicate experiment using a charged particle, stray electromagnetic fields in the laboratory environment would instantly overwhelm the subtle gravitational effect they are attempting to detect and measure.
Yet, working with muonium presents a major experimental obstacle. Muons are notoriously unstable, surviving for only about 2.2 microseconds before they spontaneously decay. Furthermore, earlier generation methods produced muonium atoms traveling at drastically different speeds and scattering in random directions, rendering them entirely unsuitable for the extreme precision required in modern gravity measurements.
Fortunately, researchers at PSI have recently engineered a breakthrough solution to bypass this severe limitation.
"We have managed to produce the muonium atoms in a ‘cold’ state, which is what makes the gravity experiment possible in the first place," the researcher states. "In this case, ‘cold’ means that the atoms propagate at similar speeds, almost parallel to one another."
Superfluid Helium Creates a Controlled Muonium Beam
The research team has detailed its innovative method for producing this specialized beam of muonium in the scientific journal Nature Physics.
"In order to achieve this, we used superfluid helium that had been cooled close to absolute zero at minus 273 degrees Celsius," explains Jesse Zhang, the lead author of the study. "Superfluid helium is what is known as a quantum fluid, in which the individual helium atoms lose their identity, and which does not tolerate any impurities within it."
The intricate process begins by directing a stream of antimuons from PSI’s high-power particle accelerator directly into a thin layer of superfluid helium. As the particles enter the fluid, they rapidly lose speed. When a slowed antimuon encounters a free electron within the cryogenic environment, the two particles bind together to form a muonium atom possessing a positive chemical potential.
That chemical potential effectively acts as a driving force, pushing the newly formed atom completely out of the liquid. The moment the atom reaches the surface, the chemical potential is converted entirely into kinetic energy, providing the muonium atom with a powerful vertical boost that sends it shooting upward.
"So we’re using the chemical potential as an atomic cannon," Zhang explains.
To succeed, the muonium atoms must pass cleanly through the quantum liquid without undergoing destructive collisions and must travel at a predictable, uniform speed. Because their lifespan is remarkably short, any significant delay would cause the atoms to decay before they could even escape the surface.
"For our experiments, we also rely on PSI’s particle accelerator, which generates the world’s most intense, continuous muon beams," the researcher adds. "Thanks to this high-quality source, a great many muonium atoms can be produced."
Watching Gravity Shift an Atomic Pattern
With the production method established, the research team is currently constructing a specialized scientific instrument known as an interferometer to measure precisely how Earth’s gravity influences the newly created muonium beam.
The device leverages the natural wave properties of atoms to produce an interference pattern. Earth’s gravitational pull is expected to cause an infinitesimally small shift in that pattern. By accurately measuring this displacement, the researchers will be able to determine, for the first time, how gravity acts upon a muon-based system.
"We hope to be able to test the method for the first time with the atomic beam this year, and if all goes well, the actual gravity experiment should follow in two or three years’ time," the researcher notes.
Beyond gravitational measurements, this novel atomic beam could also unlock significantly more precise laser spectroscopy experiments involving muonium. Such high-precision measurements could drastically improve scientists’ understanding of the muon’s mass and other fundamental physical constants, marking another long-term objective for the research group.
Could the Experiment Reveal a Fifth Force?
If future experiments show that muonium responds to gravity differently than ordinary matter, the physical implications would be profound and far-reaching.
"That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force," the researcher outlines.
Modern physics currently recognizes four fundamental interactions governing the universe: gravity, electromagnetism, the strong nuclear interaction, and the weak nuclear interaction. While physicists have frequently proposed the theoretical possibility of an additional fifth force, no such interaction has ever been experimentally confirmed.
Discovering a new force, however, is not the primary objective of the team’s current work. The immediate goal remains far more fundamental: determining whether one of Einstein’s most central assumptions truly holds true across different generations of matter.
"I am completely open-minded," the researcher concludes. "I simply want to measure, for the first time, whether the equivalence between gravitational and inertial mass also applies to the second generation of particles — this alone is quite an inspiring piece of work."
