A breakthrough in chronometry decades in the making could redefine the limits of how we keep time. Using atoms of thorium-229, physicists have built functional clocks based not on the oscillations of electrons , but on the back-and-forth energy shifts of atomic nuclei themselves. What makes this achievement even more exciting is that it has been achieved twice, by two independent teams of scientists, in Europe and China. Both teams have detailed their momentous work in preprints on arXiv. "The