FIGURE 3. Atomic clocks.

a, The first caesium atomic clock, built by Jerrold Zacharias in 1953. Zacharias also proposed a caesium clock using a fountain of atoms to test Einstein's prediction that a clock would slow down in raised in a gravitational potential. He failed to construct an atomic fountain, and although he never published his proposal, his attempt was passed down to two generations of physicists by word of mouth. (Photo: MIT Museum.) b, A rubidium atomic fountain clock built by the BNM-LPTF (Bureau National de Métrologie — Laboratoire Primaire du Temps et des Fréquences) and the Ecole Nomale Supérieure, Paris. The atoms are trapped and cooled in the lower part of the apparatus before being launched upwards to a height of 1 metre. The estimated accuracy of this clock is 1 part in 10¹⁵, or 7 minutes over a period of time equal to the age of the Universe (14 billion years). (Photo: C. Solomon, ENS.)