Ultracold neutrons don't disappear into the mirror world

For decades, a theory in physics has postulated the existence of a mirror world whose interaction with our own reality is extremely feeble. The particles in this mirror universe are also thought to be candidates for dark matter. The science-journalism coverage adds useful context, while the strongest evidential footing still comes from the underlying data, papers or institutional documentation.

This matters because cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. Editors have highlighted the following attributes while ensuring the content's credibility: Add as preferred source Physical Review Letters (2026). Researchers at the Paul Scherrer Institute PSI have now examined some 25 billion neutrons, thereby ruling out, with a very high degree of certainty, their disappearance into the.

The idea may sound like science fiction, but it has been put forward by researchers in theoretical physics. The results rule out, with very high probability, any transformation of neutrons into their mirror version.

Even though dark matter accounts for considerably more of the total mass of the universe than ordinary matter, its nature remains a complete mystery. This required, first, a very large number of ultracold neutrons, the PSI source is a world leader in producing these, and, second, magnetic field coils surrounding the container.

Discover the latest in science, tech, and space with over 100, 000 subscribers who rely on Phys. org for daily insights. Every five minutes, the team stored around 1.5 million neutrons in a large stainless-steel tank.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

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.

Because this item comes through Phys. org Physics as science journalism, it should be treated as contextual reporting rather than primary evidence. Good science reporting can identify why a result matters, connect it to the wider literature and make technical work readable, but the decisive evidence remains in the original paper, dataset, mission release or technical record. That distinction is especially important when a story is later repeated by aggregators, because repetition increases visibility, not evidential strength.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside.

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