How are seconds defined in physics?

The definition of a second in physics is based on the time it takes cesium atoms to vibrate. Specifically, the second is 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. This definition emphasizes precision and consistency, which are crucial for scientific measurements.

Using cesium atoms provides a highly stable and accurate standard due to their predictable atomic vibrations, allowing for the precise measurement of time. This definition replaced earlier methods that relied on astronomical observations, such as the Earth's rotation or the movement of celestial bodies, which can vary due to many external factors like tectonic shifts or gravitational interactions. The atomic definition ensures a universal time standard that is not influenced by such irregularities.