C2 Report

C2 Report

Report to the 2002 General Assembly for 1999-2002
Berlin, Germany
October 7-12, 2002


As the somewhat cumbersome title of C2 implies, the subject area of many of the particular physicists who are measuring atomic masses and fundamental constants also lies, of course, within the parts of physics covered by other IUPAP Commissions, and C2 takes care to avoid duplication of their coverage. However, measuring quantities with the highest possible accuracy is as much a specialty as working in any other part of physics, and brings its own particular needs for the physicists concerned. Their measurements provide important extra-disciplinary tests of particular aspects of theory at the highest accuracy. The greatly improved accuracies of today means that projects are likely to be long, and expensive, and if the work is sustained in the longer term it does not lead to many publications. It is becoming increasingly difficult to attract and retain physicists into the part of physics represented by C2, particularly to work on the measurement of atomic masses and fundamental physical constants in University research laboratories.

It is difficult for physicists in the subject area of C2 to meet together and discuss common problems. The Commission therefore was pleased to sponsor the biannual Conferences on Precision Measurement and Fundamental Constants (CPEM) held in Sydney, June 2000, and the 2002 CPEM conference held in Ottawa. Both of these conferences led to good publicity for IUPAP. Conferences relating to precision measurements of atomic masses have been more difficult to sponsor, as this is also a comparatively small research area that does not attract large numbers of physicists to conferences. Recently, there has been an increase in activity in this field and the emphasis has been on ultra-precise (up to 10-10) measurements among stable nuclides and less precise (up to 10-8) among unstable nuclides. The Commission sponsored the 3rd International Conference on Exotic Nuclei and Atomic Masses (ENAM 2001) held in Hameenlinna, Finland, July 2000. The next meeting in this series will be in 2004 (organized by the Physics Division of the Oak Ridge National Laboratory, USA) and an application for sponsorship is in preparation. Opportunities to meet together as a Commission are rare and those C2 members who attended sponsored CPEM conferences met together to discuss C2 affairs. A similar meeting took place amongst those involved in Atomic Mass Measurements at the Atomic Physics at Accelerators: Mass Spectrometry meeting held in Cargese, September 2000.

The Commission also sponsors the IUPAP SUN-AMCO Medal that is awarded to those physicists who have made outstanding contributions to the area of physics within the remit of the Commission. Its name commemorates the present Commission being derived from the merging of the former IUPAP: SUN and AMCO Commissions. The Commission was pleased to announce awards of Medals to Dr C. A. Hamilton and Professor T. W. Hänsch.

There have usually been several members of the C2 who are also to be members of the CODATA Task Group on Fundamental Constants. the Task Group has produced a new set of Recommended Values for the Fundamental Physical Constants[i] (see: http://physics.nist.gov/cuu/Constants , further combinations of the constants using the full variance co-variance uncertainty matrix may also be evaluated on this site. These values co-ordinate the measurements that were essentially available in firmly published form to December 31st 1998. The new values replace the 1986 values. They represent a tenfold improvement in accuracy in many cases. The 1998 values have been universally accepted and are being rapidly introduced throughout the world. It is likely that there will be more frequent, interim, reviews than hitherto. These will have to be propagated with care if scientists are to be clear as to which particular review values colleagues are using. Atomic masses are evaluated elsewhere[ii]. One or more of the evaluators are usually represented on C2 see http://isotopes.lbl.gov/toimass.html, and http://csnwww.in2p3.fr/AMDC/web/amdcw_en.html.

C2 has produced its own Guide on symbols, units, etc[iii] to guide physicists, who may well invent several specialized unit systems before coffee, an idea of the units, etc., that they are likely to be expected (by pedants, examiners, and Journal Editors) to be using. There has been considerable discussion within the Commission concerning how this should be revised and an initial attempt has been discussed and junked. A shorter draft C2 Web entry giving the recommended authoritative sources has been produced, and awaits modification and endorsement by the future members of C2.

As a reincarnation of one of the first IUPAP Commissions, the SUN Commission, and as the current title of the Commission suggests, there is a strong level of representational interaction with outside bodies and, of course, their views may prevail over the wishes of physicists. These needs can be argued against, but physicists cannot necessarily veto them and physicists may prefer to go their own way. Certain nominated Members of IUPAP Commission C2 have represented IUPAP interests on several outside committees, including IUPAC-IDCNS, IEC-TC25, ISO TC12, CCU and the JCGM, that is the Joint Working Committee on Guides on Metrology (including vocabulary (the VIM) and uncertainties (the GUM).

There has been some discussion [iv] of the problems associated with the introduction of leap seconds into disseminated Time Scales and their impact on the Global Satellite Navigation Systems. Thus Glonas uses Universal Co-ordinated Time (UTC), and the GPS uses the Atomic Time (AT). It is not clear at present whether these user problems are best solved by educating the user, by improving the associated software, or by making a fundamental change to the present system. The various timescales were introduced as a compromise because there was a variety of user needs that were judged impossible to meet by a single timescale. Users are accustomed to changing their local timescales by one or more hours and to the fact that their ‘noon’ differs from that of their local sundial. It may be that it will be possible to abandon the UTC at some future date.

Although attempts to universalize the use of the comma for the exclusive representation of the decimal point have abated, this may be temporary. There has been considerable effort by some of the members of the Consultative Committee for Units to introduce the neper, but not the bel, into the SI as a dimensionless derived unit. Discussions are still rumbling in the background, with little evidence of real demand in the open literature for the present situation to change. Many physicists find the bel a very useful unit, but not necessarily within the SI. This topic is likely to resurface at the next meeting of the CCU. Dimensionality and the method of dimensions plays an important part in physics, and certainly becomes more difficult to teach if there are concepts such as dimensionless units to confuse the unthinking student.

There have been essentially three modifications to the SI derived units in the last few years[v]

(i) the SI class of supplementary units has ceased and the radian and steradian are now included as dimensionless derived SI units whose use is optional;

(ii) second the katal (mol/second) has been added as a special name for catalytic activity, and

(iii) the neper has been provisionally moved into the class of dimensionless SI derived units. The equally useful bel (and decibel) remains a dimensionless unit approved for use alongside the SI – although there is a large and vociferous user demand to treat the bel in the same way as the neper.

One can anticipate demand from other scientific disciplines for the addition of further dimensionless derived units to the SI in the future. Physicists may prefer to stay with the earlier SI or CGS systems.

Reporting any measurement requires an agreed vocabulary and an indication of how well the measurement has been performed. In an attempt to get universal interdisciplinary agreement on this the very successful VIM [vi] and GUM Guides on Vocabulary and Uncertainties[vii] respectively were produced a few years ago, with IUPAP participation, under the aegis of the ISO. The JCGM was subsequently split off organizationally into a separate entity, and work on the second editions is in progress.

Although elementary particles such as quarks or the Higgs boson can be named well ahead of their discovery, different traditions apply to the process of naming the new elements. In the latter area of physics/chemistry, attribution of discovery is an important part of the naming process. As in any difficult research area further work sometimes brings unexpected surprises. The Chairman of C2 has served as a nominated IUPAP member of the joint IUPAC-IUPAP Joint Working Group concerning the discovery of the heavy elements. The Joint Working Group has recently published a report on the discovery of element 110[viii] and is in process of examining the latest overall case for the discovery of elements 111 and 112, etc. and a report is nearing completion.


[1] P. B. Mohr and B. N. Taylor, Rev. Mod. Phys. 72, 351 (2000)

[1] G. Audi, and A. H. Wapstra, 1995, Nucl. Phys. A 595, 409-480

[1] Symbols, units, nomenclature and fundamental constants in physics, prepared by E. R. Cohen and P. Giacomo, Document IUPAP-25 (SUMAMCO 87-1), Physica 146A, 1-68 (1987)

[1] The leap second: its history and possible future R. A. Nelson; D. D. McCarthy; S. Malys; J. Levine; B. Guinot; H. F. Fliegel; R. L. Beard; T. R. Bartholomew, 38, 509 529 (2001)

[1] Les Système International d’Unités (SI), 7th edition (BIPM, Sèvres, 1998); Supplement 2000, addenda and corrigenda to the 7th edition (1998) (BIPM, Sèvres, 2000)

[1] International Vocabulary of Basic and General Terms in Metrology (VIM) ( published by the ISO, 1993)

[1] Guide to the Expression of Uncertainty in Measurement (published by the ISO, 1993)

[1] On the discovery of the elements (110-112) P. J. Karol, H. Nakahara, B. W. Petley and E. Vogt, Pure and Applied Chemistry 73, (no. 6), 959-968 (2001).