If you think your Apple watch is the coolest thing in timekeeping, think again. Scientists at Germany's Physikalisch-Technische Bundesanstalt (PTB) have just published a paper in the journal Physical Review Letters on the most accurate timekeeping device in history. It beats out the previous champs, atomic clocks, by a large margin. In fact, the new single-ion clock is so accurate that it may force us to redefine what a "second" is.
To understand the single-ion clock, you have to understand its predecessor, the atomic clock. Scientists have spent a lot of time watching atoms. They've figured out that an atom's electrons do a lot of predictable and detectable things when they jump from energy level to energy level in an atom (remember those electron clouds you had to draw in school?). In an atomic clock, atoms are forced to sit relatively still and then blasted with energy from a known frequency (like lasers or microwaves). The electrons move as a result of this energy blast, and the clock counts those movements. The clock knows how much time has passed by combining two known things (the behaviors of the electrons and the properties of the energy blast) to figure out an unknown thing (how much time has passed).
Accuracy in timekeeping is measured by how precisely the length of each 'tick' matches the length of every other 'tick'. The atomic clock's form of timekeeping is so accurate that our current definition of what a 'second' is comes from the best atomic clocks currently around — cesium atomic clocks. Scientists measure clock accuracy by their level of systematic uncertainty, which is a type of error rate.