Precise time keeping is of the utmost importance in science and technology and has been since the beginning of recorded time. Yet no one really knows what time is. If you look up the word “time” in a good dictionary you will see one of the longest entries, with dozens of definitions.
St. Augustine, in 400 AD, said “What, then, is time? If no one asks me, I know; if I wish to explain to him who asks, I know not.” Time is what a clock measures. The first clocks were natural phenomena, like the sun rising and setting and the motions of the stars in the heavens. Sundials were developed and then water clocks and sand-filled hour glasses. Later, pendulum clocks became the ultimate in precision. Physicists needed accurate clocks to measure natural phenomena and accurate time keeping was crucial to navigation. In the eighteenth century we came to understand light as one example of electromagnetic waves with very short wavelengths and very high frequency. Frequency is just the inverse of time or how many times something happens in a given time interval. Thus, the measurement of time and frequency came to be crucial to the most cutting-edge physics.
In the twentieth century, Einstein’s theory of relativity defined time as part of a four-dimensional space-time. Furthermore, time was relative, that is, measured time was different for observers moving at different speeds. Communicating with our satellites depends on taking relativistic time into account, and our GPS systems would not work properly without doing so. To meet the timekeeping needs of modern technology, the most accurate clocks are atomic clocks and crystal oscillators. Atomic clocks, which depend on the frequencies of energy transitions in atoms, are the most accurate measuring device for long periods of time. Crystal oscillators are most accurate for times less than 1 second, which turns out to also be crucial. The very best clocks today, like in satellites, include both an atomic clock and a crystal oscillator working together.