Summary

Chapter 18 is about minute measurements and the science of metrology. Kean writes about the International Prototype Kilogram, the world's official kilogram which is exact in its weight. Made out of platinum and iridium, the official kilogram is closely monitored and calibrated, and it is the standard by which all other kilograms are measured. Unlike the kilogram, the official meter was replaced with light instead of a physical rod in 1960. The definition of an exact second is difficult to determine precisely because the length of both calendar and astronomical years varies based on ocean tides. A more precise form of measurement is the atomic clock, which uses the movements of electrons to measure photons. Cesium is the key element in atomic clocks and the cesium standard is now the world's official measurement of time.

Kean then writes about the fine structure constant...