Researchers at ETH Zurich and the Paul Scherrer Institute in Villigen, Switzerland, have produced a high-intensity beam of muonium atoms in a cold state, an experimental advance that could enable a direct test of whether gravity acts differently on second-generation matter. The peer-reviewed study in Nature Physics describes a new method for generating muonium in superfluid helium. The resulting atoms travel at similar speeds and almost parallel to one another, properties required for a future interferometer that would measure the tiny influence of gravity on muonium.

The study did not conduct that gravitational measurement or find evidence that gravity departs from its expected behavior for ordinary matter; it instead developed the beam source needed for the proposed test. The experiment is designed to examine the universality of free fall and the equivalence of gravitational and inertial mass, a cornerstone of Albert Einstein’s theory of gravitation. In ordinary matter, bodies fall at the same rate at a given point within a gravitational field, while the equivalence principle links gravitational mass with inertial mass.

According to the research team, this principle has so far been demonstrated only with ordinary matter or first-generation antimatter. Muonium is a neutral atom formed from a positively charged antimuon and a negatively charged electron, making it a second-generation probe. A gravity experiment with this exotic atom would test whether the equivalence continues to hold for particles outside the first generation.

Muonium has two characteristics that make it useful for this test: it is neutral, and it belongs to a heavier and less familiar generation of particles. A neutral object allows gravitational behavior to be studied without the weak gravitational signal being masked by stray electromagnetic fields, which can dominate the behavior of charged particles. At the same time, muonium is exceptionally difficult to produce and control.

Its constituent muon decays after only about 2.2 microseconds. Earlier sources generated muonium atoms that flew off in many directions at different speeds and were therefore unsuitable for the precision required in a gravity experiment. The researchers addressed this problem by creating a cold, intense and well-directed beam.

The antimuons were directed into a thin layer of superfluid helium that had been cooled close to absolute zero, at minus 273 degrees Celsius. Superfluid helium is a quantum fluid in which individual helium atoms lose their identity and in which impurities are not tolerated. Lead author Jesse Zhang said the researchers used the superfluid as an “atomic cannon,” exploiting the conversion of chemical potential into kinetic energy to propel the newly formed muonium atoms out of the liquid.

When an antimuon encounters a free electron in the superfluid helium, it forms a neutral muonium atom with positive chemical potential. The atom is forced out of the liquid, and as it reaches the surface, that chemical potential is converted into kinetic energy. The resulting boost sends the muonium vertically out of the helium.

Importantly, the atoms must move through the quantum liquid at a suitable speed without collisions, because excessive scattering would prevent them from reaching the surface within their very short lifetime. The high-intensity antimuon beam from PSI allows many muonium atoms to be generated for the planned experiments. Anna Soter, a professor of physics at ETH Zurich and member of the research team, said the cold state is what makes the proposed gravity experiment possible.

In this context, “cold” does not mean that the atoms are motionless; it means that they propagate at similar speeds and nearly parallel to one another. This improved control over direction and velocity is important because the planned instrument will use the wave properties of the atoms to create an interference pattern. A very small shift in that pattern caused by Earth’s gravitational pull would provide a measure of gravity’s effect on muonium.

The study establishes the upstream beam capability rather than reporting the result of that comparison. The next stage of the program is the development of a muonium interferometer. According to the researchers, they hoped to test the method for the first time with the atomic beam in the year of the announcement and, if that worked as expected, to carry out the actual gravity experiment two or three years later.

Those are planned milestones rather than completed measurements. If the interference pattern of muonium shifts differently from the pattern expected for ordinary matter, Soter said the result would be surprising and could, among other theoretical possibilities, point toward the existence of a fifth force. No fifth force has been proven, and the present work provides no evidence for one.

The team’s primary aim is to measure whether the equivalence between gravitational and inertial mass applies to a second-generation particle. Physics recognizes four fundamental interactions: gravity, electromagnetism, the strong interaction and the weak interaction. A fifth force has been proposed repeatedly, but it has never been established.

Soter emphasized that the researchers are open-minded about what the measurement may show while keeping the immediate objective focused on the first such test. The new beam is therefore an enabling experimental result, not a demonstration that Einstein’s theory has been challenged or overturned. Beyond the planned gravity measurement, the high-intensity muonium beam is expected to enable more precise laser-spectroscopy experiments.

These could allow muonium to be measured more accurately than in previous work and provide information about the muon’s mass and fundamental physical constants, which are stated goals of the research group. The same experimental source can thus serve two purposes: supplying a cold, directed beam for a gravitational comparison and providing a larger, better-controlled population of muonium for detailed spectroscopic study. The reported advance addresses a major technical obstacle while leaving the central scientific question unresolved.

A successful gravitational test will still require an interferometer capable of resolving a minuscule shift in an interference pattern while dealing with muonium’s short lifetime, beam quality and other sources of uncertainty. The study’s demonstrated result is the generation of a high-intensity, superthermal muonium beam in superfluid helium, with atoms emerging in the cold state needed for future work. Whether muonium falls exactly as expected for ordinary matter remains to be measured.

The beam nevertheless supplies the experimental foundation for testing gravitational universality in a second-generation particle, a comparison that had not previously been carried out with muonium.

Subject of Research: A cold muonium beam developed for future tests of gravitational universality and laser spectroscopy

Article Title: Novel particle beam could challenge Einstein’s Theory of Gravity

Article References: Novel particle beam could challenge Einstein’s Theory of Gravity. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: muonium, muon beam, gravity, gravitational universality, equivalence principle, interferometry, superfluid helium, laser spectroscopy, ETH Zurich, Paul Scherrer Institute, Novel, particle

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Scientists from the Paul Scherrer Institute (PSI), conducting a new series of experiments with millions of ultracold neutrons, have determined that the neutral particles do not switch between our universe and a theoretical “mirror universe” containing oppositely charged, identical counterparts of all elementary particles previously proposed by theoretical physicists.

Although the new experiments do not completely disprove the concept of a mirror universe, the experiments do cast doubt on one of the idea’s fundamental principles, making its existence less likely. The team behind the new study said their findings also aid future experiments by eliminating one potential area of inquiry.

Neutrons Are the Perfect Candidate to Test for a Mirror Universe

Although theoretical physicists sometimes propose radical concepts, one such idea proposes a mirror universe where every elemental particle has a “twin.” For example, the authors of the new study explain, there would be mirror protons, mirror electrons, and mirror neutrons. Still, they note, interaction between the particles in our universe and their mirror universe companions is likely to be minimal.

Geza Zsigmond, a researcher at the PSI Center for Neutron and Muon Sciences, said the theory suggests that the two mirror particles “sense each other’s presence” through gravity. Some physicists have even suggested that gravitational interaction with mirror-universe particles could explain dark matter. Unfortunately, experiments designed to hunt for the mirror universe have returned zero positive results.

Bernhard Lauss, leader of the research group for ultracold neutron physics at the PSI Center for Neutron and Muon Sciences, said, “There is no way of proving the existence of mirror particles using gravitational interaction alone.” Fortunately, the theory also offers a second testing possibility.

According to Zsigmond and colleagues, mirror universe particles would interact with our universe either through gravitation or through what they described as a “rare” oscillation of neutrons. This means that neutral particles like a neutron could occasionally disappear from our universe and reappear in the mirror universe. Even more surprisingly, the same theory says that these particles could magically return to our universe as well.

“Based on this hypothesis, neutral particles might oscillate to and fro between our normal matter world and the mirror world,” Lauss said.

The team said that this possibility, combined with new experiments at the Institut Laue-Langevin in France that “rekindled” speculation about the mirror universe, focused their new efforts on this “other” property of mirror universe particles.

Study Finds No Evidence for Neutral Particle Oscillations

After joining up with researchers from ETH Zurich and the Jagiellonian University in Kraków, the PSI team designed a series of experiments that should show if neutrons are zipping back and forth between universes. Zsigmond said that neutrons are ideal to test the concept for two reasons. First, they are neutral particles, and second, the researcher said they have a “strange property” the team hoped to exploit.

“Neutrons appear to have different lifetimes, depending on how this is measured,” Zsigmond explained.

Although several theories have been proposed to explain this discrepancy, the team acknowledged that the varying neutron lifetimes could be caused by some of these neutral particles disappearing from our universe and entering the mirror universe.

First, the team used the high-intensity proton accelerator at PSI to generate the millions of neutrons needed for their experiments. Next, the team slowed the neutrons down, creating what scientists call “ultracold” neutrons.

Next, the researchers trapped the ultracold neutrons in a specially designed non-magnetic stainless steel vacuum chamber. Because the theory suggests that the probability of neutrons jumping between universes is extremely sensitive to magnetic fields, the vacuum chamber was surrounded by magnetic field coils that the researchers could control with the required precision.

Throughout the experimentation phase, the researchers stored 1.5 million ultracold neutrons in the chamber every five minutes. After about 200 seconds, the team would empty the chamber and count how many neutrons remained. The team repeated this process over several months, resulting in the creation, storage, and measurement of approximately 25 billion neutrons.

Lauss said the researchers gradually varied the strength and direction of the magnetic field throughout the process to “scan all the relevant regions in which one might expect oscillations between neutrons and mirror neutrons to occur.” However, Lauss said, “we saw no evidence at all of such oscillations.”

Experiments “Set the Standard” For the Foreseeable Future

When discussing the significance of their findings, the PSI-led team said the lack of evidence of mirror universe oscillation by ultracold neutrons suggests a very high probability that such mirror universe neutral particle roundtrips “can be ruled out.” They also note that failing to find evidence of a mirror universe still resulted in benefits for particle physics by eliminating an area of inquiry.

“By restricting the scope for certain speculations, we are showing researchers in theoretical physics that they need to explore new avenues,” Zsigmond explained. “Physics needs that kind of impetus if it is to continue to evolve.”

Moving forward, the team said that “substantial improvements” to their experiments would require a much more elaborate setup. Until then, they said their experiment and its results are “the best of its kind in the world and will set the standard for the foreseeable future.”

The study “New High-Sensitivity Search for Neutron to Mirror-Neutron Oscillations at the PSI Ultracold Neutron Source” was published in Physical Review Letters.

Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org