MOTION MOUNTAIN

328 a units, measurements and constants
Vol. I, page 98
Ref. 294
‘The candela is the luminous intensity, in a given direction, of a source that emits
monochromatic radiation of frequency540⋅10 12 hertz and has a radiant intensity in that
direction of (1/683) watt per steradian.’ (1979)*
We note that both time and length units are defined as certain properties of a standard
example of motion, namely light. In other words, also the Conférence Générale des Poids
et Mesures makes the point that the observation of motion is aprerequisite for thedefinition
and construction of time and space. Motion is the fundament of every observation
andofallmeasurement.Bytheway,theuseoflightinthedefinitionshadbeenproposed
alreadyin1827byJacquesBabinet.**
Fromthesebasicunits,allotherunitsaredefinedbymultiplicationanddivision.Thus,
all SI units have the following properties:
SI units form a system withstate-of-the-artprecision: all units are defined with a precisionthatishigherthantheprecisionofcommonlyusedmeasurements.Moreover,
the
precision of the definitions is regularly being improved.The present relative uncertainty
of the definition of the second is around10 −14 , for the metre about10 −10 , for the kilogramabout10
−9 ,fortheampere10 −7 ,forthemolelessthan10 −6 ,forthekelvin10 −6 and
for the candela10 −3 .
SI units form an absolute system: all units are defined in such a way that they can
be reproduced in every suitably equipped laboratory, independently, and with high precision.Thisavoids
as much as possible any misuse by the standard-settingorganization.
(The kilogram, still defined with the help of an artefact, is the last exception to this requirement;extensive
research is under way to eliminate this artefact from the definition
– an international race that will take a few more years.There are two approaches: counting
particles, or fixingħ. The former can be achieved in crystals, e.g., crystals made of
pure silicon, the latter using any formula whereħappears, such as the formula for the de
Broglie wavelength or that of the Josephson effect.)
SI units form apractical system: the base units are quantities of everyday magnitude.
Frequently used units have standard names and abbreviations.The complete list includes
the seven base units just given, the supplementary units, the derived units and the admittedunits.
The supplementary SI units are two: the unit for (plane) angle, defined as the ratio
of arc length to radius, is the radian (rad). For solid angle, defined as the ratio of the
subtendedareatothesquareoftheradius,theunitisthesteradian(sr).
The derived units with special names, in their official English spelling, i.e., without
capitallettersandaccents,are:
* The respective symbols are s, m, kg, A, K, mol and cd. The international prototype of the kilogram is
a platinum–iridium cylinder kept at the BIPM in Sèvres, in France. For more details on the levels of the
caesium atom, consult a book on atomic physics. The Celsius scale of temperatureθis defined as:θ/°C=
T/K−273.15;note the small difference with the number appearing in the definition of the kelvin. SI also
states: ‘When the mole is used, the elementary entities must be specified and may be atoms, molecules,
ions, electrons, other particles, or specified groups of such particles.’ In the definition of the mole, it is
understoodthatthecarbon12atomsareunbound,atrestandintheirgroundstate.Inthedefinitionofthe
candela, the frequency of the light corresponds to555.5 nm, i.e., green colour, around the wavelength to
whichtheeyeismostsensitive.
**JacquesBabinet(b.1794Lusignan,d.1874Paris)wasaphysicistwhopublishedimportantworkinoptics.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net