The idea may sound like science fiction, but it has actually been put forward by renowned researchers in theoretical physics. A so-called mirror world might exist alongside our ordinary reality, in which every type of elementary particle has a corresponding mirror particle. There would then be mirror electrons, as well as mirror protons and mirror neutrons. However, according to this theory, normal particles are unlikely to interact much with mirror particles. “Essentially, the two types of particle sense each other’s presence through the force of gravity,” says Geza Zsigmond, a researcher at the PSI Center for Neutron and Muon Sciences.
Despite decades of searching, these mysterious mirror particles have never been detected. Although the results of recent experiments at the Institut Laue-Langevin in France rekindled speculation about mirror neutrons, a new experiment by PSI researchers has now shed further light on the question, achieving an unsurpassed level of precision. The results rule out, with very high probability, any transformation of neutrons into their mirror version.
The fact that mirror particles scarcely interact with ordinary matter at all is precisely what makes the search for them so difficult. They can only interact with the matter we know in two ways: either through gravitation or through the rare oscillation of neutral particles. This is also why mirror particles may be candidates for what is known as dark matter. Even though dark matter accounts for considerably more of the total mass of the Universe than ordinary matter, its nature remains a complete mystery. The weakness of the interaction between mirror particles and our own world is precisely the reason why they could account for dark matter.
Searching for rare oscillations
“There is no way of proving the existence of mirror particles using gravitational interaction alone,” says Bernhard Lauss, leader of the research group for ultracold neutron physics at the PSI Center for Neutron and Muon Sciences. “Because of this, we focused on the other property of mirror particles, namely that, based on this hypothesis, neutral particles might oscillate to and fro between our normal matter world and the mirror world.” This means that a neutral particle, such as the neutron, could – on very rare occasions – turn into a mirror particle and simply vanish from our world. After a while, it might then reappear out of nowhere.
“Neutrons are ideal for this experiment because, firstly, they are electrically neutral and, secondly, they possess a strange property,” adds Zsigmond. “Neutrons appear to have different lifetimes, depending on how this is measured.” Part of this discrepancy could be due to the fact that some neutrons “disappear” into the mirror world while they are being measured.
To get to the bottom of this peculiar behaviour, the PSI researchers joined forces with researchers from ETH Zurich and the Jagiellonian University in Kraków and devised a sophisticated experiment. They significantly slowed down neutrons – which are produced in large quantities at the high-intensity proton accelerator at PSI – thereby creating what are known as ultracold neutrons. They then trapped these in a special non-magnetic stainless-steel vacuum container. This required, firstly, a very large number of ultracold neutrons – the PSI source is a world leader in producing these – and secondly, the magnetic field coils surrounding the container had to be controlled with the utmost precision. This is because the probability of neutrons crossing into the mirror world and back into our world again is extremely sensitive to the surrounding magnetic fields.
No indication of a mirror world – despite world-best results
Every 5 minutes, the team stored around 1.5 million neutrons in a large stainless-steel tank. Each time, the researchers emptied it again after about 200 seconds and determined how many neutrons remained. They repeated the process over a period of several months, by which time they had measured around 25 billion neutrons in total.
“Over time, we gradually varied the strength and direction of the magnetic field so as to scan all the relevant regions in which one might expect oscillations between neutrons and mirror neutrons to occur,” says Lauss. “But we saw no evidence at all of such oscillations.” These findings mean that there is a very high probability that previous speculations about their transformation into mirror particles can be ruled out.
The PSI experiment is the best of its kind in the world and will set the standard for the foreseeable future. Further substantial improvements would require an extremely elaborate set-up. “We are also proud that many young people were able to participate in this experiment,” notes Lauss. Two doctoral theses were completed in collaboration with ETH Zurich as part of the research, and in addition, many students were involved in the experiment.
Although the study found no evidence of a spectacular mirror world, its findings are nonetheless important for particle physics. “By restricting the scope for certain speculations, we are showing researchers in theoretical physics that they need to explore new avenues,” says Zsigmond. “Physics needs that kind of impetus if it is to continue to evolve.”
Text: Dirk Eidemüller
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of future technologies, energy and climate, health innovation and fundamentals of nature. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2300 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 450 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).
Physical Review Letters
Experimental study
Not applicable
New High-Sensitivity Search for Neutron to Mirror-Neutron Oscillations at the PSI Ultracold Neutron Source
28-Jul-2026