Researchers at ETH Zurich and the Paul Scherrer Institute in Villigen, Switzerland, have produced a dense beam of cold atoms that they intend to use to test Einstein's theory of gravity.
This cold beam is composed of muonium; a second-generation antiparticle. The researchers want to examine its interaction with gravity to determine whether another force of nature exists that we have not yet discovered.
It should be noted that the matter we observe around us is made of protons, neutrons and electrons, which scientists call the first generation of particles. Two other generations of particles are heavier than the first-generation particles but are unstable and quickly decay into first-generation particles.
The muon, the heavier cousin of the electron, is from the second generation of particles. Researchers at the Paul Scherrer Institute have successfully produced muons and their antiparticles using a particle accelerator. When an antimuon combines with an electron, a neutral muonium atom is formed.
These scientists, together with researchers at ETH Zurich, want to measure how the muonium atom falls under the influence of gravity. From Galileo to Newton and then Einstein, physicists have confirmed the universality of free fall.
This equivalence principle states that all objects in a gravitational field fall with the same acceleration, regardless of their mass or internal structure.
According to ITNA and citing Interesting Engineering, Einstein used this principle as a foundational pillar of his famous theory of relativity, but the principle has only been tested on first-generation matter and antimatter particles. Measuring it on the muonium atom will be the first instance of testing this principle on a second-generation particle.
Muonium is well suited for this experiment because it is electrically neutral. By contrast, a charged particle can be affected by stray magnetic fields and obscure the weak effect of gravity.
According to the researchers, one of the main obstacles to performing such an experiment has been the rapid decay of muonium. These particles, in addition to decaying quickly in only 2.2 microseconds, in previous efforts moved at varying speeds, making the experiments difficult to perform.
Researchers at the Paul Scherrer Institute have used a quantum fluid to produce a steady stream of muonium atoms.
Jesse Zhang, a researcher at the Paul Scherrer Institute, says: "We used superfluid helium cooled nearly to absolute zero at minus 273 degrees Celsius. Superfluid helium is a quantum liquid in which helium atoms lose their individual identity and do not accept impurities within themselves."
It is reported that the antimuons produced in the Paul Scherrer Institute's particle accelerator are then fired into this quantum fluid, where they collide with free electrons and form muonium atoms.
A positive chemical potential formed in this reaction pushes the muonium atom to the surface, and the chemical potential is converted into kinetic energy and ejects it vertically out of the liquid.
The researchers now intend to build an interferometer that uses the wave properties of the muonium atom to create an interference pattern so that the effect of gravity on muonium can be detected.
It is said that building this device may take two to three years, but once ready it will answer an important long-unanswered question: does gravity act differently on exotic matter? If gravity truly acts differently, this would point to the existence of a fifth force of nature.
In conclusion, it should be said that at present the research group is focused on determining whether the equivalence principle also applies to second-generation particles.