Physicists are on the hunt for dark forces. These forces are not as ominous as they sound: “dark” simply refers to the fact that no one has observed them before. In this case, they would act between neutrons and electrons.
One path to investigating dark forces involves using lasers to make precision measurements of isotopes (atoms of an element possessing different numbers of neutrons). If there is a dark force working behind the scenes, it could affect an isotope’s energy levels—discrete regions around the atom’s nucleus where its electrons exist.
Now two teams have independently performed the most precise measurements of this type. Their findings, reported this month in Physical Review Letters, are mixed: One group, led by researchers at Aarhus University in Denmark, analyzed calcium isotopes and saw no deviation from predictions. But the other team, led by scientists at the Massachusetts Institute of Technology, used ytterbium isotopes and