Measurement of Time intervals

A clock measures a time interval. Historically many kinds of clocks have been used -- electric oscillators, electronic oscillators, solar clocks, quartz-crystal clocks -- but today's atomic standard rests on the periodic vibration of a Cesium atom (Figure 1.8): the second is defined via 9,192,631,770 periods of a specific Cesium-133 radiation (Table 1.2), giving accuracy to about three-millionths of a second per year. Other techniques used for time measurement include the decay of elementary particles and radioactive dating.

Table 1.7 lists the order of magnitude of time intervals physics deals with, spanning from $10^{-24}$ s (the lifespan of the most unstable particles) to $10^{17}$ s (the age of the universe) -- twenty-one orders of magnitude on either side of the human-scale 'one second'. …