In the grandfather clock, the pendulum swings back and forth, or oscillates, under the force of gravity. One second is accumulated by adding together a fixed number of periods of oscillations. Credit: AGCuesta/ShutterstockĪll clocks, including grandfather clocks, atomic clocks, and the clocks in your phone, use a periodic oscillation to define time. In a grandfather clock, the pendulum swings back and forth, or oscillates, under the force of gravity. First developed in the late 1600s by Christiaan Huygens, pendulum clocks lost only 15 seconds per day. Prior to the Industrial Revolution, the best timekeepers were pendulum clocks, the mechanical basis for grandfather clocks. You may have learned to count “one, 1,000” or “two, Mississippi” in school, but the way NIST defines the second with atoms is much more complex. Here, I hope to explain how we define time with atoms, how we use timing in modern society, and whether better timing and a future redefinition of the second might affect our lives and modern technology. Now, if you could wait long enough, the best atomic clocks would lose only one second every 15 billion years! Since then, every decade has seen better than a tenfold improvement in timekeeping performance. The first atomic clocks were developed during the 1940s and became commercially available in the 1960s. Previously, the standard of mass, the kilogram, was based on a human-made cylinder of platinum and iridium, whose mass was defined to be one kilogram. This did away with the last “artifacts” in the SI. That’s why, after 1967, the second’s definition changed to one based on the exquisitely stable energy levels in atoms hence, the term “atomic clock.”Īs an aside, the most recent redefinition of the International System (SI) of units in 2019, more commonly referred to as the metric system, made it such that all units are defined by the unchanging laws of physics and the constants in nature. This presents a challenge for accurate timekeeping. But Earth’s rotation can slow down and speed up based on the gravitational influence of the moon and the movement of the outer shell of the Earth. Prior to 1967, the official time reference for the second was the stable but slow rotation of the Earth relative to the cosmos. These redefinitions may have happened in your lifetime and you probably didn’t notice. Since then, the second has undergone multiple redefinitions. In 1960, the second was defined as an international standard unit of measurement for time. The standardization of time has also become essential for the stability in power grids, data synchronization over the internet, global travel, and telecommunications. As the world has modernized and our pace of life has increased, our relationship to time has changed.
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