Since 1964, in SI, the decimal submultiplier meaning 10⁻¹⁵, or one quadrillionth. Symbol, f.

The growth of nuclear physics created a need for such a submultiplier. It was first officially recognized by the physicists, when the general assembly of the International Union for Pure and Applied Physics approved a proposal of its Commission on Symbols, Units and Nomenclature in 1960. Use of the femto- with SI units was authorized by resolution 8 of the 12th CGPM in 1964. The prefix was suggested by the Danish word “femten,” fifteen.

One might wonder what such a small unit is good for. There are as many femtoseconds in 1 second as there are seconds in 30 million years. Yet by 1991, scientists working with lasers had succeeded in creating light pulses lasting only 6 femtoseconds.


Comptes Rendus des Séances de la Conférence Generale des Poids et Mesures, 94.

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