People often use the word "instant" to describe how fleeting time is. Converting the ancient definition, an instant is approximately 0.018 seconds. There are about 12,000 "finger-flicks", 240,000 "moments", and 4.8 million "instants" in a day and night. Such definitions of time that surpass human perception and ability are the simple, primitive, and romantic imaginings of the ancients about "time observation". Meanwhile, they also inspired rational thinking and scientific exploration of time standards among humans.

Historically, the time unit "second" was once defined as 1/86,400 of the time it takes for the Earth to complete one rotation on its axis (a solar day). With the development of science and technology, scientists discovered that the Earth's rotation is not stable and that solar days are not exactly the same throughout the year. So in 1956, based on the law of the Earth's revolution around the Sun, the "second" in the International System of Units was revised to 1/31,556,925.9747 of the time it takes for the Earth to complete one revolution around the Sun. This definition was approved at the 11th General Conference on Weights and Measures in 1960. However, the time measurement errors caused by the instability of the Earth's rotation were not overcome by the Earth's revolution, and at the same time, there were also problems with the difficulty of reproducing the Earth's revolution for measurement purposes.

To observe time more accurately, it is inseparable from the development of atomic physics and quantum mechanics.

Atoms are the basic units that make up matter, and their structure can be divided into atomic nuclei and extranuclear electrons. The extranuclear electrons move around the atomic nuclei and follow the laws of quantum mechanics, distributing themselves in different electron orbits. Imagine that there are many different "steps" inside an atom, and electrons can jump between these steps. Such jumps are called quantum energy level transitions. When a transition occurs, an electron jumps from one step to another, just like the pendulum in the macroscopic world, emitting a stable "tick-tock" sound and radiating electromagnetic waves with extremely stable frequencies.

Different types of atoms have unique oscillation frequencies. When atoms are under electromagnetic radiation of this frequency, the electrons orbiting inside the atoms will undergo quantum energy level transitions. Atomic clocks designed based on this quantum phenomenon can define time by measuring the electromagnetic waves emitted during energy level transitions.

At the end of the 1940s, the National Bureau of Standards of the United States successfully developed the ammonia molecular clock. In 1955, the National Physical Laboratory of the United Kingdom developed the world's first cesium atomic clock, marking that mankind officially entered the era of atomic clocks. Compared with traditional time measurement methods based on astronomical phenomena, atomic clocks will not change due to changes in geographical location or time, demonstrating better consistency and stability.

In 1967, the 13th General Conference on Weights and Measures adopted atomic time to define the "second", that is, 9,192,631,770 times the period of the transition between the two hyperfine energy levels of the ground state of the cesium-133 isotope under the condition of no external interference. Up to now, the definition of the "second" is still maintained by the more advanced cesium atomic fountain clock.