Question

What is a second in physics?

Answer 1

In physics, a Second is the base unit of time in the International System of Units (SI). It is defined based on a precise physical phenomenon to ensure accuracy and consistency. 

Current Definition:

Since 1967, the second has been defined as:

The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom at 0 K (absolute zero).

This definition relies on the extremely stable vibrations of cesium atoms in atomic clocks, which are used as the standard for measuring time.

Why Cesium-133?

Cesium-133 was chosen because its atomic transitions are highly regular and predictable, making it an excellent reference for timekeeping. Atomic clocks based on cesium are accurate to within a few billionths of a second per day.

Historical Context:

• Before 1967, the second was defined based on the Earth's rotation and its orbit around the Sun (1/86,400 of a mean solar day).
• However, the Earth's rotation is not perfectly constant, so the cesium-based definition was adopted for greater precision.

In summary, the second is a fundamental unit of time in physics, defined by the behavior of cesium atoms, and it underpins all modern timekeeping systems.