Einstein (unit)

2025-08-08T15:51:55Z

24.206.73.45: add link to frequency

{{Short description|Obsolete unit with conflicting definitions}}
The '''einstein''' (symbol '''E''') is an obsolete [[physical unit|unit]] with two conflicting definitions. It was originally defined as the [[energy]] in one [[Mole (unit)|mole]] of [[photon]]s ({{physconst|NA|round=3|unit=photons|ref=no}}).<ref>W. Albert Noyes and P. A. Leighton, The photochemistry of gases. p. 14. Rochester, New York. 1940. Viking</ref><ref>Albrecht Folsing, Albert Einstein: a biography. p. 299. New York. 1997. Viking</ref> Because energy is inversely proportional to [[wavelength]], the unit is [[frequency]] dependent. This unit is not part of the [[International System of Units]] (SI) and is redundant with the [[joule]]. If it were still in use, as of the [[2019 revision of the SI]], its value would be related to the frequency of the [[electromagnetic radiation]] by
: 1 einstein = 1 mol × {{math|''N''A}} {{math|''h''}} {{math|''f''}} = 1 mol × {{val|6.02214076|e=23|u=mol−1}} × {{val|6.62607015|e=-34|u=J.s}} × {{math|''f''}} = {{val|3.9903127128934321|e=-10|u=J.s}} × {{math|''f''}},
where {{math|''N''A}} is the [[Avogadro constant]], {{math|''h''}} is the [[Planck constant]], and {{math|''f''}} is the [[frequency]].

Sometime later, the unit was used differently in studies of photosynthesis to mean one mole of photons, rather than the energy in one mole of photons.<ref name=Incoll>Incoll, L. D., S. P. Long, and M. A. Ashmore. 1981. "SI units in publications in plant science". Commentaries in Plant Science. 2: pp. 83–96.</ref><ref>{{citation |author=K. J. McCree |year=1981 |title=Photosynthetically Active Radiation |publisher=Springer |quote=In photobiology, a mole (Avagadro's number, {{val|6.022|e=23}}) of photons is commonly referred to as an einstein (E) }}</ref><ref>{{citation |editor=Amos Richmond |date=2004 |title=Handbook of Microalgal Culture |publisher=Blackwell Science |isbn=0-632-05953-2 |quote=This unit is called an Einstein ({{nowrap|1=E = {{val|6.023|e=23|u=quanta}}}}). }}</ref><ref name=":0">{{citation |author=D.R. Hershey |title=The American Biology Teacher |date=1991 |publisher=JSTOR |quote=... uses the obsolete unit Einstein (E) instead of mol, where 1 E was defined as a mole of photons. }}</ref> As such, [[photosynthetically active radiation]] (PAR) was formerly often reported in microeinsteins per second per square meter (μE⋅m−2⋅s−1). This usage is also not part of the SI and when used this way it is redundant with the [[mole (unit)|mole]].

Since the unit does not have a standard definition and is not part of the SI system, it has long been considered obsolete.<ref name=Incoll /><ref name=":0" /> The same information about photosynthetically active radiation can be conveyed using the SI convention by stating something such as "a photon flux of 1500 μmol⋅m−2⋅s−1".

This unit was named after physicist [[Albert Einstein]].

== References ==
{{reflist}}

[[Category:Photochemistry]]
[[Category:Units of amount of substance]]
[[Category:Albert Einstein]]

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Einstein (unit)