Time is an ethereal concept. It is at once intriguing and frightening; think of the gravitational power of a black hole to bend spacetime or what some observers call the ability of humans to time travel. Popular movies and books aside, Einstein said the separation between past, present and future is only an illusion.
Yet, time is probably the most measured quantity on Earth, according to the National Institute of Standards and Technology. It tells us when to wake, go to work and school, and when to sleep. It helps organize our lives and coordinate our activities. Still, who hasn’t thought where has the time gone, or experienced the feeling of time dragging when doing something unpleasant?
In science, time is used to measure and better understand countless concepts and objects in our world. Without the ability to measure time, planes wouldn’t fly, the internet wouldn’t work … life would be chaotic.
Yet, we cannot measure time directly. We cannot see it, hear it, taste it, touch it or smell it. Instead, we measure time intervals, the lengths separating two events. This is what a clock actually measures; “time” is the accumulation of these events, NIST explained.
Humans first broke up time into day and night. Ancient Egyptians then divided day and night into 12 parts each, with daytime hours longer than nighttime hours during the summer, NIST said. Later, Ancient Greeks proposed splitting the day into 24 equal hours. However, people continued using hours of uneven lengths until the late medieval period in Europe, when monks developed mechanical clocks to establish fixed prayer and work periods.
Interestingly: “The concepts of the minute and the second date back to ancient times—but they originated in the realm of astronomy instead of timekeeping. Four thousand years ago, the Sumerians divided a circle into 360 degrees. About 1,000 years later, Babylonian astronomers, and then the Greeks, applied this concept to divide the sky into 360 degrees. Going further, they split each degree into 60 minutes, and each minute into 60 seconds. Centuries later, these units found their way into the increasingly precise mechanical clocks developed in medieval Europe,” NIST explained.
Today, atoms produce the most precise clocks. Atomic clocks enable scientists to split the second — the international unit of time — into extremely small parts, opening up new applications with each finer subdivision. Specially energized cesium atoms “tick” with a specific frequency.
“We count 9,192,631,770 of those ticks and call the elapsed time interval a second,” NIST said.
Today, commercially available cesium clocks keep time to within an amazing 1/3,000,000 of a second per year; new experimental clocks are a million times more precise still. The atomic clock is by far the most impressive timepiece that humans have developed but getting to this modern definition of the second took a long time.