Researchers at the Swiss institutes of ETH Zurich and Paul Scherrer Institute (PSI) in Villigen have produced an intense beam of cold atoms that they plan to use to test Einstein’s theory of gravity. This cold beam consists of muonium, a second-generation antiparticle, and the researchers want to test its interaction with gravity to know if another force of nature also exists that we haven’t spotted yet.
The matter we see around us is made up of protons, neutrons, and electrons, which scientists call the first generation of particles. The other two generations of particles are heavier than the first-generation particles but also unstable and rapidly decay into first-generation particles.
A muon is the heavier cousin of an electron from the second generation of particles. Researchers at PSI have successfully generated muons and their antiparticles using a particle accelerator. When such an antimuon combines with an electron, a neutral muonium atom is formed. Together with researchers at ETH Zurich, the scientists want to measure how the muonium atom falls under gravity.
Gravity on second-generation particles
From the times of Galileo, through Newton and then Einstein, physicists have confirmed the universality of free fall. This equivalence principle states that all bodies inside a gravitational field fall with the same acceleration, irrespective of their mass or internal structure.
Einstein used this principle as a foundational pillar for this famous theory of relativity, but this has only been tested on matter and antimatter particles of the first generation. Measuring this on a muonium atom would be the first instance of testing it on a second-generation particle.
Muonium is well suited for this experiment because it is electrically neutral. In comparison, a charged particle can be impacted by stray magnetic fields and mask the effect of the weak force of gravity.
How muonium beam helps
A major hurdle in performing such an experiment has been the quick decay of muonium. In addition to their rapid decay in just 2.2 microseconds, muonium produced in previous attempts also traveled at different speeds, making it difficult to conduct experiments with them.
PSI researchers used a quantum fluid to generate a steady flow of muonium atoms. “We used superfluid helium that was cooled to near absolute zero at minus 273 degrees Celsius,” explained Jesse Zhang, researcher at PSI in a press release. “Superfluid helium is a so-called quantum liquid in which the individual helium atoms lose their identity and do not like impurities inside.”
Antimouns generated in the PSI’s particle accelerator were then shot into the quantum liquid, where they meet free electrons and form the muonium atom. A positive chemical potential formed during this reaction kicks the muonium atom to the surface, where the chemical potential is converted to kinetic energy, shooting it vertically out of the liquid.
The researchers now plan to build an interferometer that will use the wave properties of the muonium atom to create an interference pattern capable of detecting the impact of gravity on the muonium. The device might take two to three years to build, but when ready, it will answer an important question that has long been pending: does gravity act differently on exotic matter?
If gravity does indeed act differently, it would point to the presence of a fifth force of nature. For now, the team is focused on determining if the equivalence principle applies to the second generation of particles.
The research findings were published in the journal Nature Physics.