Does gravity affect every type of particle in the universe in precisely the same way? For centuries, humanity has operated under the assumption that it does, grounded in the foundational principles laid out by Galileo Galilei, Isaac Newton, and later refined by Albert Einstein. Yet, a team of researchers at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen, Switzerland, is preparing to put this bedrock assumption to a rigorous, unprecedented test using one of nature’s more elusive and unusual 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 ambitions. "We want to measure the gravitational interaction of the muon."
To understand the significance of the upcoming experiment, one must look at the standard building blocks of the universe. The familiar matter that makes up human bodies, rocky planets, distant stars, and nearly everything visible in our daily environment consists of protons, neutrons, and electrons. In the language of modern physics, these particles are classified as belonging to the first generation of matter. However, the Standard Model of particle physics reveals that nature did not stop there. Two additional generations exist, populated by progressively heavier and more exotic particles.
One of these heavier relatives is the muon, a second-generation particle that shares many characteristics with the electron but possesses a significantly greater mass. At the Paul Scherrer Institute, researchers have the unique capability to generate muons and their antimatter counterparts, antimuons, utilizing a massive, high-powered particle accelerator. When a positively charged antimuon captures and binds with a negatively charged electron, the two particles form a short-lived, neutral atom known as muonium.
Why Physicists Want to Test Muons
While the Standard Model successfully categorizes these diverse generations of particles, it leaves one of the deepest mysteries of the subatomic world completely unexplained: why nature relies on multiple generations in the first place.
"But we physicists do not yet understand why these additional generations exist at all in the first place," notes the research team lead. "And why are there three in total?"
This lingering mystery opens the door to an equally profound question concerning fundamental forces. Do the heavier, more fleeting particles of the second and third generations respond to gravity in the exact same uniform way as the lighter, stable particles of the first generation?
For ordinary matter, objects situated at the same point in a gravitational field fall at precisely the same rate, regardless of their composition or mass. This universality of free fall was famously observed by Galileo and formalized by Newton before becoming a central pillar of Albert Einstein’s general theory of relativity through the equivalence principle. This principle establishes a fundamental link between an object’s gravitational mass and its inertial mass.
Up to this point, however, physicists have only been able to demonstrate this principle using ordinary matter or first-generation antimatter. Measuring how muonium behaves under the influence of gravity would provide the scientific community with its very first experimental test involving a second-generation particle.
"The exotic muonium is very well suited to this because it is a neutral atom," the researchers explain. "After all, to make something fall, you need something neutral."
Electrical neutrality is an absolute requirement for this type of experiment because gravity is astonishingly weak when compared to electromagnetism—the force that binds electrons to atomic nuclei. If scientists attempted to measure the gravitational pull on a charged particle, stray electromagnetic fields in the laboratory environment would easily overwhelm the minuscule gravitational effect they are attempting to isolate and detect.
Yet, working with muonium presents daunting technical hurdles. Muons are notoriously unstable, surviving for only about 2.2 microseconds before decaying into other particles. Furthermore, traditional methods for producing muonium resulted in atoms flying off in random directions and at wildly varying speeds, rendering them utterly useless for the extreme precision required in modern gravitational measurements.
Superfluid Helium Creates a Controlled Muonium Beam
Fortunately, a team of researchers at PSI has recently engineered a breakthrough technique to overcome this historical barrier. The new method allows them to produce muonium atoms in a controlled, low-energy state, paving the way for the upcoming gravitational tests.
"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 researchers note. In this context, "cold" does not refer to ambient temperature in the conventional sense; rather, it means that the generated atoms propagate at remarkably similar speeds, traveling almost completely parallel to one another in a coherent stream.
The team detailed their innovative methodology 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 complex production process begins by steering a stream of antimuons from PSI’s powerful particle accelerator directly into a thin layer of superfluid helium. As the high-energy antimuons plow into the liquid, they rapidly lose velocity. When a slowed antimuon happens to encounter a free electron drifting within the helium, the two particles bind together, forming a neutral muonium atom characterized by a positive chemical potential.
This chemical potential acts as a driving force, actively pushing the newly formed muonium atom out of the liquid medium. The moment the atom breaks through the surface of the superfluid, its chemical potential is instantly converted into kinetic energy. This explosive conversion provides the muonium atom with a sudden upward boost, propelling it vertically into space.
"So we’re using the chemical potential as an atomic cannon," Zhang explains.
For the experiment to succeed, the muonium atoms must traverse the quantum liquid completely free of unwanted collisions and at a precisely predictable velocity. Because their natural lifespan is so brutally short, even a fraction of a microsecond of unnecessary delay would mean the atoms would decay long before they could 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 researchers point out. Thanks to the exceptional quality and high intensity of this particle source, the laboratory can produce a sufficient volume of muonium atoms to make data collection feasible.
Watching Gravity Shift an Atomic Pattern
With the controlled atomic beam successfully developed, the research team is now constructing a highly sensitive scientific instrument known as an interferometer. This device will be used to observe and measure how the gravitational pull of the Earth subtly alters the trajectory of the muonium beam.
The interferometer harnesses the fundamental wave properties of atoms to generate a distinct interference pattern. The constant, downward tug of Earth’s gravity is expected to induce an infinitesimally small shift in this wave pattern. By accurately measuring this spatial displacement, the researchers will finally be able to calculate how gravity interacts with a second-generation particle.
"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 research team reports.
Beyond testing the limits of general relativity, this novel atomic beam opens the door to vastly more precise laser spectroscopy experiments involving muonium. Such high-precision measurements could significantly sharpen humanity’s understanding of the muon’s exact mass and refine fundamental physical constants, marking another major long-term objective for the laboratory.
Could the Experiment Reveal a Fifth Force?
If the upcoming experiments reveal that muonium responds to gravity differently than ordinary, first-generation matter, the ramifications for theoretical physics would be nothing short of revolutionary.
"That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force," the researchers suggest.
Modern physics currently recognizes four fundamental forces or interactions that govern the universe: gravity, electromagnetism, the strong nuclear interaction, and the weak nuclear interaction. Over the decades, theoretical physicists have repeatedly hypothesized the existence of an additional, undiscovered fifth force to explain various cosmic anomalies, though no such force has ever been empirically verified in a laboratory.
Uncovering a fifth force is not, however, the primary motivation behind the team’s current research agenda. The immediate goal remains far more fundamental: establishing whether one of Einstein’s most cherished principles holds true across entirely different generations of matter.
"I am completely open-minded," the researchers state. "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."
This groundbreaking research is supported by the National Centre of Competence in Research Muoniverse.
