attohertz

Attohertz is the SI unit of frequency equal to 10^-18 hertz. The symbol for the unit is aHz. Because hertz measure cycles per second, an attohertz corresponds to an extremely slow oscillation: 1 aHz equals 10^-18 cycles per second, and conversely 1 Hz equals 10^18 aHz.

In practical terms, the corresponding period is enormous. A frequency of 1 aHz has a period of 10^18 seconds, about 3.17×10^10 years. This places attohertz in a domain far beyond ordinary laboratory oscillations and even well beyond the timescales typically considered in most physical measurements.

The prefix atto- denotes 10^-18, and attohertz sits at the lower end of the frequency scale alongside other SI prefixes such as femto-, pico-, nano-, micro-, and milli- for smaller frequencies. Attoseconds, with the same prefix, are a time unit (10^-18 seconds), illustrating the consistent use of prefixes across time and frequency.

Attohertz is primarily a theoretical or niche unit. It is used to discuss extremely long-term or cosmological phenomena, or to describe hypothetical limitations in metrology and fundamental-constant investigations. In current experimental practice, measurable frequencies lie well above the attohertz range; real-world frequency measurements typically cover kilohertz to gigahertz and, in some specialized areas, nanohertz.