Emptiness isn't always empty: it can hide energy, like supercooled water hides ice. Physicists call this false vacuum — a state that only needs a nudge to cascade into the true one.
A pendulum that almost nothing can knock off balance (not shaking, not heat, not magnetic fields) because it is hidden in the very heart of the atom. That is the kind of pendulum physicists used when they first switched on a nuclear clock. Previously, the best clocks, optical ones, relied on electron transitions.