When Bernhard Lauss and Geza Zsigmond set out to find mirror neutrons, they did not expect to discover them. In fact, they hoped not to. At the Paul Scherrer Institute in Switzerland, the two physicists and their team spent years examining roughly 25 billion individual neutrons — and their conclusion, published in the journal Physical Review Letters, suggests those mirror neutrons probably do not exist. Or at the very least, neutrons almost certainly do not vanish into a hidden mirror world and come back.
The idea of a mirror world sounds like science fiction, but serious physicists have proposed it for decades. In this theory, the universe contains a shadow realm filled with mirror electrons, mirror protons, and mirror neutrons. These mirror particles would barely interact with ordinary matter at all, communicating with our world only through gravity — which could explain dark matter, the mysterious substance that makes up more of the universe than everything we can actually see and touch. "Essentially, the two types of particle sense each other's presence through the force of gravity," Zsigmond explained.
The challenge is that mirror particles are almost impossibly shy. Because they interact with ordinary matter so rarely, catching one directly seems nearly out of reach. But there is one loophole in the theory: neutral particles like neutrons might occasionally oscillate into the mirror world and back again, like flipping a coin that sometimes lands on its edge. If this happened, scientists would see neutrons simply disappearing from their experiments.
That is exactly what the PSI team searched for. Working with partners from ETH Zurich in Switzerland and Jagiellonian University in Kraków, Poland, they used PSI's high-intensity proton accelerator to produce enormous quantities of neutrons, then cooled them down until they moved at a glacial pace. These ultracold neutrons were trapped inside a special stainless-steel container surrounded by precisely controlled magnetic coils. The team watched and waited, hunting for any neutron that might slip away.
After examining so many neutrons — 25 billion — the researchers found none that disappeared into a mirror world. This does not prove the mirror world does not exist, but it does rule out the possibility that neutrons transform into mirror neutrons with any significant frequency. There is still much about dark matter that remains unknown. But by narrowing down where mirror particles might hide, scientists move closer to solving one of the biggest mysteries in the universe.