What units are used for time in the SI system?

The unit used for time in the SI system is the second.

The International System of Units (SI) is a globally accepted and widely used system of physical units. It is based on seven base units, one of which is the unit of time, the second. The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

This definition was adopted in 1967 by the 13th General Conference on Weights and Measures. Prior to this, the second was defined in terms of the Earth's rotation. However, it was found that the Earth's rotation is not constant, but varies slightly due to various factors such as tidal friction. Therefore, a more stable and precise definition was needed, which led to the adoption of the atomic definition.

The second is used as the base unit for many other units in the SI system. For example, the unit of frequency, the hertz, is defined as one cycle per second. The unit of speed, the metre per second, is defined as the distance travelled in one second. The unit of acceleration, the metre per second squared, is defined as the change in speed per second.

In addition to being a base unit in the SI system, the second is also a fundamental unit in many areas of physics. For example, in the study of motion, time is a key variable. In the study of electromagnetism, the frequency of electromagnetic waves is measured in hertz, which are inversely proportional to seconds. In quantum mechanics, the energy of a quantum state is related to its frequency by Planck's constant, again linking energy to time.

In conclusion, the second is a fundamental unit in the SI system and in the study of physics. Its definition based on atomic transitions provides a stable and precise standard for time measurement.