Topics: Applied Physics, Education, Research, Thermodynamics
Also note the Hyper Physics link on the Second Law of Thermodynamics, particularly “Time’s Arrow.”
“The two most powerful warriors are patience and time,” Leo Tolstoy, War, and Peace
The short answer
We can measure time intervals — the duration between two events — most accurately with atomic clocks. These clocks produce electromagnetic radiation, such as microwaves, with a precise frequency that causes atoms in the clock to jump from one energy level to another. Cesium atoms make such quantum jumps by absorbing microwaves with a frequency of 9,192,631,770 cycles per second, which then defines the international scientific unit for time, the second.
The answer to how we measure time may seem obvious. We do so with clocks. However, when we say we’re measuring time, we are speaking loosely. Time has no physical properties to measure. What we are really measuring is time intervals, the duration separating two events.
Throughout history, people have recorded the passage of time in many ways, such as using sunrise and sunset and the phases of the moon. Clocks evolved from sundials and water wheels to more accurate pendulums and quartz crystals. Nowadays when we need to know the current time, we look at our wristwatch or the digital clock on our computer or phone.
The digital clocks on our computers and phones get their time from atomic clocks, including the ones developed and operated by the National Institute of Standards and Technology (NIST).