Nuclear clocks are a tick closer to reality: physicists measure energy of lowest nuclear excited state

Nuclear clocks are a tick closer to reality thanks to experiments that measured the energy of the lowest excited state of a thorium-229 nucleus to the highest precision ever. A clock based on transitions between such nuclear states would be much more accurate than existing atomic clocks and would therefore place tighter constraints on the Standard Model of particle physics.

Atomic clocks “tick” at frequencies set by the regular transitions of electrons within atoms or ions, as measured by a laser kept in resonance with these transitions. Today’s best atomic clocks are accurate to within one part in 1018, which means they would slow down by less than one second if left running for 13 billion years (the age of the universe). However, a clock that relied on nuclear transitions would be more accurate still, because the small size of an atomic nucleus relative to an atom’s electron shell means that the behavior of the former is less affected by external electromagnetic fields.