The SI unit of frequency, equal to one cycle per second.The hertz (Hz) is the SI unit of frequency, defined as one cycle per second. It quantifies how often a periodic event repeats in a given time interval, from the vibration of atoms in a crystal to the oscillations of light waves. In physics, it is the standard measure for anything that cycles—sound waves, radio signals, or the ticking of atomic clocks—and its precision is crucial for experiments that probe the fundamental structure of matter.
The unit was named after Heinrich Hertz, who experimentally confirmed the existence of electromagnetic waves in the 1880s. It was formally adopted as the SI unit in 1960, replacing the earlier term 'cycles per second.'
One hertz means exactly one complete oscillation or cycle occurs every second. For higher frequencies, multiples like kilohertz (10^3 Hz), megahertz (10^6 Hz), and gigahertz (10^9 Hz) are used. In practice, frequency is measured by counting cycles over a known time, often using electronic counters or interferometric techniques that compare against a stable reference, such as a cesium atomic clock.
Hertz is used everywhere from tuning musical instruments and setting radio station frequencies to calibrating the ultra-precise optical clocks that underpin GPS and tests of fundamental physics, as in the cited research on helium-4 nuclei and nuclear clocks.
The frequency of the microwave radiation used in atomic clocks is about 9.2 billion hertz, and measuring it to 48 hertz precision—as in the helium-4 study—means detecting a change of just five parts in a billion.
