““A true standard requires us to find some way of defining what we mean by one unit of time which is the same for everyone no matter where or when they are observing. This desire for universality naturally moves us to seek some time standard that is determined by the constants of Nature alone. And this is indeed how modern absolute time standards are defined. They avoid the use of any characterisitic of the Earth or its gravitational field and focus instead upon the natural oscillation frequencies of certain atomic transitions between states of different energy. The time for one of these transitions to occur in an atom of caesium is determined by the velocity of light in a vacuum, the masses of the electron and proton, Planck's constant, and the charge on a single electron. All these quantities are taken to be constants of Nature. A time interval of one second is then defined to be a certain number of these oscillations. Despite the esoteric nature of this definition of time, it is a powerful one. It should allow us to communicate precisely what length of time we were talking about to the inhabitants of a distant galaxy.””
