[LEAPSECS] Nit-pick: SI second
Magnus Danielson
magnus at rubidium.dyndns.org
Wed Feb 9 18:24:48 EST 2011
On 07/02/11 20:10, Tom Van Baak wrote:
>> It is also why TAI's rate was adjusted in the 1990's to compensate for
>> the red-shifted data that had been collected at NIST in Boulder, since
>> it sits at about 5400' (1700m) above sea level (as well as other
>> facilities not at sea level).
>>
>> Warner
>
> Are you sure you aren't confusing red shift (which has been
> a known factor since the 60's) with blackbody shift (which
> only arose in the 90's as clocks hit the 1e-15 level)?
These are distinct in the face of SI. The nit-picker could find all the
material needed in
http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
The pieces being:
"1.5 SI units in the framework of general relativity
The definitions of the base units of the SI were adopted in a context
that takes no account of relativistic effects. When such account is
taken, it is clear that the definitions apply only in a small spatial
domain sharing the motion of the standards that realize them. These
units are known as proper units; they are realized from local
experiments in which the relativistic effects that need to be taken into
account are those of special relativity. The constants of physics are
local quantities with their values expressed in proper units.
Physical realizations of the definition of a unit are usually compared
locally. For frequency standards, however, it is possible to make such
comparisons at a distance by means of electromagnetic signals. To
interpret the results the theory of general relativity is required since
it predicts, among other things, a relative frequency shift
between standards of about 1 part in 10^16 per metre of altitude
difference at the surface of the Earth. Effects of this magnitude cannot
be neglected when comparing the best frequency standards."
"Considering that a very precise definition of the unit of time is
indispensable for science and technology, the 13th CGPM (1967/68,
Resolution 1; CR, 103 and Metrologia, 1968, 4, 43) replaced the
definition of the second by the following:
The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the
ground state of the caesium 133 atom.
It follows that the hyperfine splitting in the ground state of the
caesium 133 atom is exactly 9 192 631 770 hertz, ν(hfs Cs) = 9 192 631
770 Hz.
At its 1997 meeting the CIPM affirmed that:
This definition refers to a caesium atom at rest at a temperature of 0 K.
This note was intended to make it clear that the definition of the SI
second is based on a caesium atom unperturbed by black body radiation,
that is, in an environment whose thermodynamic temperature is 0 K. The
frequencies of all primary frequency standards should therefore be
corrected for the shift due to ambient radiation, as stated at the
meeting of the Consultative Committee for Time and Frequency in 1999."
" SI unit of time (second) (CR, 103 and Metrologia, 1968, 4, 43)
Resolution 1
The 13th Conférence Générale des Poids et Mesures (CGPM), considering
• that the definition of the second adopted by the Comité International
des Poids et Mesures (CIPM) in 1956 (Resolution 1) and ratified by
Resolution 9 of the 11th CGPM (1960), later upheld by Resolution 5 of
the 12th CGPM (1964), is inadequate for the present needs of metrology,
• that at its meeting of 1964 the CIPM, empowered by Resolution 5 of the
12th CGPM (1964), recommended, in order to fulfil these requirements, a
caesium atomic frequency standard for temporary use,
• that this frequency standard has now been sufficiently tested and
found sufficiently accurate to provide a definition of the second
fulfilling present requirements,
• that the time has now come to replace the definition now in force of
the unit of time of the Système International d’Unités by an atomic
definition based on that standard, decides
1. The SI unit of time is the second defined as follows:
“The second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the
ground state of the caesium 133 atom”;
2. Resolution 1 adopted by the CIPM at its meeting of 1956 and
Resolution 9 of the 11th CGPM are now abrogated.
At its 1997 meeting, the CIPM affirmed that this definition refers to a
caesium atom at rest at a thermodynamic temperature of 0 K."
" Definition of TAI (PV, 38, 110-111 and Metrologia, 1971, 7, 43)
Recommendation S 2
International Atomic Time (TAI) is the time reference coordinate
established by the Bureau International de l'Heure on the basis of the
readings of atomic clocks operating in various establishments in
accordance with the definition of the second, the unit of time of the
International System of Units.
In 1980, the definition of TAI was completed as follows (declaration of
the CCDS, BIPM Com. Cons. Déf. Seconde, 1980, 9, S 15 and Metrologia,
1981, 17, 70):
TAI is a coordinate time scale defined in a geocentric reference frame
with the SI second as realized on the rotating geoid as the scale unit."
Cheers,
Magnus
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