A determination of the duration between two instants of time (epochs). Time intervals are measured with high precision with a digital display counter. An electronic oscillator generates pulses; the count begins with a start signal and ends with a second signal. Two atomic clocks can be compared in epoch to 1 picosecond (1 ps = 10-12 s) by electronic interpolation. See Atomic clock, Oscilloscope
Rapid motions can be studied at short intervals by means of a large variety of high-speed cameras, including stroboscopic, rotating film-drum, rotating mirror, streak, and image converter cameras. An electronic streak camera can separate two pulses 1 ps apart.
Ultrashort laser pulses are used to study rapid processes caused by the interaction of photons with an atom or molecule. Pulses as short as three wavelengths of 620-nm light, with &tgr; = 6 femtoseconds (1 fs = 10-15 s), have been formed. See Laser, Optical pulses
Radioactive decay is used to measure long time intervals, to about 5 × 109 years, concerning human history, the Earth, and the solar system.