Ytterbium

Ytterbium closes the 4f shell, and that filled, spherically symmetric core gives it a narrow optical transition at 578 nm that atomic physicists have turned into the best clock ever built. Ytterbium lattice clocks at NIST and elsewhere reach fractional uncertainties near 10⁻¹⁸ — a drift of less than a second over the 13.8-billion-year age of the universe — and they are precise enough to detect the gravitational time dilation from raising the apparatus a centimetre. They are among the leading candidates to redefine the SI second away from caesium. More prosaically, ytterbium-doped fibre lasers at 1030 nm now dominate industrial metal cutting, and ytterbium's electrical resistance rises sharply under pressure, which makes it a useful stress gauge in shock experiments.