The Second

1. The Second and the international system of units

2. Why an international system of units? • Uniformity: • to insure that you get the quantity you are told that you are getting. • To insure that when adjusting your watch or your computer time, you are synchronized with others • In order to provide that assurance to you, there is an international cooperation established by CGPM through BIPM defines the UNITS, basic and derived coordinates the realization of the UNITS

3. International system of units • Formal definitions of all SI base units are approved by the CGPM. The first such definition was approved in 1889 and the most recent in 1983. These definitions are modified from time to time as techniques of measurement evolve and allow more accurate realizations of the base units.

4. The Metre Convention • 1875: a political decision led 17 nations to sign the Metre Convention. It was the beginning of modern metrology. Each of the signatory nation was sacrificing some of its independence in order to maintain cohesion in the measurement systems around the world. • Diplomatic (political) decision based on scientific and business reasons.

5. Historical importance • The first task was to define the metre and this was started by French scientists before the Revolution. • This is the only mission started by the king that was agreed upon by the Revolution! • Metrology has survived to all wars since then!

6. The Metre Convention • The Convention of the Metre (Convention du Mètre) is a diplomatic treaty which gives authority to the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM), the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM) and the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM) to act in matters of world metrology, particularly concerning the demand for measurement standards of ever increasing accuracy, range and diversity, and the need to demonstrate equivalence between national measurement standards. The Convention was signed in Paris in 1875 by representatives of seventeen nations. As well as founding the BIPM and laying down the way in which the activities of the BIPM should be financed and managed, the Metre Convention established a permanent organizational structure for member governments to act in common accord on all matters relating to units of measurement. • The Convention, modified slightly in 1921, remains the basis of international agreement on units of measurement. There are now fifty-one Member States, including all the major industrialized countries.

7. Structure of international metrology DiplomaticTreaty Metre Convention1875 Associates CGPM Member states CIPM Consultative committees MRA National Metrology Institutes (NMIs) BIPM

8. CGPM • General Conference on Weights and Measures • meets every four years and consists of delegates from Member States.

9. CGPM… • The General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) is made up of representatives of the governments of the Member States and observers from the Associates of the CGPM. Each General Conference receives the report of the International Committee for Weights and Measures (CIPM) on work accomplished; it discusses and examines the arrangements required to ensure the propagation and improvement of the International System of Units (SI); it endorses the results of new fundamental metrological determinations and various scientific resolutions of international scope; and it decides all major issues concerning the organization and development of the BIPM, including the budget of the BIPM for the next four-year period.

10. CIPM • International Committee for Weights and Measures • consists of eighteen individuals elected by the CGPM • Its principal task is to promote world-wide uniformity in units of measurement and it does this by direct action or by submitting draft resolutions to the General Conference (CGPM). • It is charged with supervision of the BIPM and affairs of the Metre Convention • The CIPM meets annually at the BIPM

11. BIPM • International Bureau of Weights and Measures • International centre for metrology • Laboratories and offices at Sèvres (France) • Coordinates to some extent the activities of the National Metrology Laboratories through the Consultative Committees’work.

12. BIPM… • The International Bureau of Weights and Measures (BIPM) was set up by the Metre Convention and has its headquarters near Paris, France. It is financed jointly by the Member States of the Convention and operates under the exclusive supervision of the CIPM. Its mandate is to provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units (SI). This task takes many forms, from direct dissemination of units (as in the case of mass and time) to coordination through international comparisons of national measurement standards (as in length, electricity and ionizing radiation). • The BIPM has an international staff of over 70 and its status vis-à-vis the French government is similar to that of other intergovernmental organizations based in Paris. The budget for 2005 is over ten million euros.

13. Consultatives Committees • Ten Consultative Committees ( CCs ) normally chaired by a member of CIPM • to advise the CIPM • act on technical matters • take important role in CIPM MRA • comprise representatives of NMIs and other experts.

14. Consultative Committees • The CIPM has set up a number of Consultative Committees, which bring together the world's experts in their specified fields as advisers on scientific and technical matters. Among the tasks of these Committees are the detailed consideration of advances in physics that directly influence metrology, the preparation of Recommendations for discussion at the CIPM, the identification, planning and execution of key comparisons of national measurement standards, and the provision of advice to the CIPM on the scientific work in the laboratories of the BIPM. • The Committees meet at irregular intervals. The president of each Committee is designated by, and is normally a member of, the CIPM. The members of the Committees are metrology laboratories and specialized institutes agreed by the CIPM, which send delegates of their choice. In addition individual members are appointed by the CIPM, and there is also at least one representative of the BIPM.

15. Five of ten CCs • CCAUV: Consultative Committee for Acoustics, Ultrasound and Vibration • CCEM: Consultative Committee for Electricity and Magnetism • CCL: Consultative Committee for Length • CCM: Consultative Committee for Mass and Related Quantities • CCPR: Consultative Committee for Photometry and Radiometry

16. The other five CCs • CCQM: Consultative Committee for Amount of Substance - Metrology in Chemistry • CCRI: Consultative Committee for Ionizing Radiation • CCT: Consultative Committee for Thermometry • CCTF: Consultative Committee for Time and Frequency • CCU: Consultative Committee for Units

17. CIPM MRA At a meeting held in Paris on 14 October 1999, the directors of the national metrology institutes (NMIs) of thirty-eight Member States of the Metre Convention and representatives of two international organizations signed a Mutual Recognition Arrangement (CIPM MRA) for national measurement standards and for calibration and measurement certificates issued by NMIs.

18. What is a standard? Standards are documents that describe the important features of a product, service or system. There are thousand of standards in use around the world…(including moral standards) By applying standards, organizations can help to ensure that their products and services are consistent, compatible, safe and effective. Today, products are assembled from components made in different countries, and are then sold around the world, so standards are more important than ever. Expert committees representing manufacturers, users, regulators, consumers and other interested parties develop standards at both the national and international levels... Regulation! Competition Consistency Coordination Where are the measurements?

19. Where do you get true measurement information? Still at a standard organization: National Measurement Institutes

20. National Metrology Institutes • NMIs are the backbone of the International System of Units. • This where the standards are developed and maintained. • This is where certification of the measurement chain in a country can be done.

21. National System of Measurement Standards SI Units CGPM National Primary Standards NMI’s Traceability NMI’s Government Agencies Reference Standards Industry, Commerce, Community in general Working Standards

22. Why traceability? Interconnectivity of the modern world. • Interchangeability of components in a world-wide market. • Quality control. These are just a few reasons why the client or end-user needs links as good as possible to the SIreferences or national standards. Without traceability you may not be able to sell your product, however good it is!

23. What is traceability? Chain of measurements from end user device (quartz or client’s clock) to reference (atomic ensemble or National Laboratory’s clock). Well documented. Quantitative measurements. Uncertainty for each measurement. Key point in traceability : numerical evaluation of uncertainty There is quite a statistical difference between a 2s or a 4s deviation

24. Overestimatinguncertainty Leads to confusion: the client may think his $2000 Rb clock is not good enough. Client may then take a wrong decision and buy a $50000 Cs clock. The client is upset.

25. Underestimating uncertainties Leads to confusion: the clock may be unfit for the client’s job. May lead to unnecessary errors. The client is upset.

26. Where is the second? • In all this big picture of the SI units and metrology system, where is the SI second • What status among the other units? • But first, the second is the unit of time • What is time?

27. What is time? • “What then is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not . . . ”St. Augustine ~400 A.D. • “Time is that elusive entity whose inexorable passage allows us to experience everything else.Time is also that entity most copiously expended indeeper attempts at defining time.”I.M. “Joe” King ~2000 A.D. • “Time intervals (and average frequencies) can be measured more accurately than any other physical quantity. High levels of accuracy are widely available at low cost.” Any Metrologist ~2000 A.D.

28. The CC concerning the Second The Consultative Committee for the Definition of the Second (CCDS) was set up in 1956. Its name was changed to Consultative Committee for Time and Frequency (CCTF) by the CIPM in 1997. Present activities concern matters related to the definition and realization of the second, establishment and diffusion of International Atomic Time (TAI) and Coordinated Universal Time (UTC), and advice to the CIPM on matters related to time and time scales.

29. The SI Second • Why 1956? • Time was measured long before that by astronomers • 1956: atomic time was proven better than astronomical time. • Much better than anything else before. • Still we have to wait 1967 for the official definition of atomic time.

30. The Second The SI unit of time is the second (symbol s). The second was defined, by international agreement, in October, 1967, at the XIII General Conference of Weights and Measures. 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 cesium atom 133." The unit of frequency is defined as the hertz (symbol Hz). One hertz equals the repetitive occurrence of one "event" per second. 8-3

31. The SI second • 9192631770 cycles of the hyperfine transition of the ground state of Caesium atom at rest. • Needs a permanent record of events (clock) • Where do we realize it? • Proper time: National Metrology Institutes • Coordinated locally at the sea level reference by BIPM

32. Cs Hyperfine Energy Levels (F,mF) (4,4) . (4,0) . . (4,-3) (4,-4) (3,-3) . (3,0) . (3, 3) Cs mass= 133 amu Clock transition in the ground state (4,0)-(3,0) F=4 9.19263177 GHz H F=3 Energy levels converted in Hz

33. Where does the SI second fits with the rest of the SI units?

34. Units of measurements SI Base Units Electric Current ampere Luminous Intensity candela Amount of Substance mole Temperature kelvin Length meter Time second Mass kilogram m s kg A K cd mol S Coordinated Time international atomic time TAI K Celsius Temperature 0Celsius 0C cd sr Luminous Flux lumen lm SI Derived Units s-1 Frequency hertz Hz kg m s-2 Force newton N m-2cd sr Illuminance lux lx kg m-1s-2 Pressure pascal Pa S A Electric charge coulomb C kg m2s-2 Energy joule J kg m s-3 Power watt W kg m2s-3A-2 Resistance ohm  kg m2s-3 A-1 Electric Potential volt V Non-SI units recognized for use with SI day: 1 d = 86400 s hour: 1 h = 3600 s minute: 1 min = 60 s liter: 1 l = 10-3 m3 ton: 1 t = 103 kg degree: 10 = (/180) rad minute: 1’ = (/10800)rad second: 1” = (/648000)rad electronvolt: 1 eV  1.602177 x 10-19 J unified atomic mass unit: 1 u  1.660540 x 10-27 kg sr: the steradian is the supplementary SI unit of solid angle (dimensionless) rad: the radian is the supplementary SI unit of plane angle (dimensionless) s-1 Activity becquerel Bq kg m2s-2A-1 Magnetic Flux weber Wb kg-1 m2s4 A2 Capacitance farad F m2s-1 Absorbed Dose gray Gy kg m2s-2A-2 Inductance henry H kg-1 m2s3 A2 Conductance siemens S m2s-2 Dose Equivalent sievert Sv kg s-2 A-1 Conductance siemens S Electromagnetic measurement units Health related measurement units

35. Where mass is not needed SI Base Units Temperature kelvin Length meter Time second m s K S Coordinated Time international atomic time TAI K Celsius Temperature 0Celsius 0C SI Derived Units s-1 Frequency hertz Hz sr: the steradian is the supplementary SI unit of solid angle (dimensionless) Non-SI units recognized for use with SI day: 1 d = 86400 s hour: 1 h = 3600 s minute: 1 min = 60 s liter: 1 l = 10-3 m3 degree: 10 = (/180) rad minute: 1’ = (/10800)rad second: 1” = (/648000)rad rad: the radian is the supplementary SI unit of plane angle (dimensionless) s-1 Activity becquerel Bq m2s-1 Absorbed Dose gray Gy m2s-2 Dose Equivalent sievert Sv Health related measurement units

36. Where SI second is not needed SI Base Units Amount of Substance mole Temperature kelvin Mass kilogram kg K mol K Celsius Temperature 0Celsius 0C SI Derived Units sr: the steradian is the supplementary SI unit of solid angle (dimensionless) rad: the radian is the supplementary SI unit of plane angle (dimensionless) Non-SI units recognized for use with SI ton: 1 t = 103 kg degree: 10 = (/180) rad minute: 1’ = (/10800)rad second: 1” = (/648000)rad unified atomic mass unit: 1 u  1.660540 x 10-27 kg

37. The ultimate measurement! • With the possibility of replacing the kilogram by a Watt balance, mass would be ultimately defined by the second, like the meter and all the other units. • It leaves only the Kelvin and the amount of substance as independent units.

38. Where SI second is not needed SI Base Units Amount of Substance mole Temperature kelvin K mol K Celsius Temperature 0Celsius 0C SI Derived Units sr: the steradian is the supplementary SI unit of solid angle (dimensionless) rad: the radian is the supplementary SI unit of plane angle (dimensionless) Non-SI units recognized for use with SI ton: 1 t = 103 kg degree: 10 = (/180) rad minute: 1’ = (/10800)rad second: 1” = (/648000)rad unified atomic mass unit: 1 u  1.660540 x 10-27 kg

39. Progress in Timekeeping Time Period 4th millennium B.C. Up to 1280 A.D. ~1280 A.D. 14th century ~1345 15th century 16th century 1656 18th century 19th century ~1910 to 1920 1920 to 1934 1921 to present 1949 to present Clock/Milestone Day & night divided into 12 equal hours Sundials, water clocks (clepsydrae) Mechanical clock invented- assembly time for prayer was first regular use Invention of the escapement; clockmaking becomes a major industry Hour divided into minutes and seconds Clock time used to regulate people’s lives (work hours) Time’s impact on science becomes significant (Galileo times physical events, e.g., free-fall) First pendulum clock (Huygens) Temperature compensated pendulum clocks Electrically driven free-pendulum clocks Wrist watches become widely available Electrically driven tuning forks Quartz crystal clocks (and watches. Since ~1971) Atomic clocks Accuracy Per Day ~1 h ~30 to 60 min ~15 to 30 min ~2 min ~1 min ~100 s 1 to 10 s 10-2 to 10-1 s 10-3 to 10-2 s 10-5 to 10-1 s 10-10 to 10-4 s

40. Time as a base unit ?!? • All units can be reduced to a time measurement and yet there is no absolute time, as Einstein has taught us. • So how do we go around this conundrum of metrology? • Time is only defined as being the proper time; time in the clock’s own reference frame. • Then how can we have a common time reference? Answer: a common frame reference.

41. Reference frame on rotating earth V(r,q) is the gravitational potential at a distance r from the center of the earth at a latitude of q, WE is the sidereal angular rotation rate of the Earth, rE is the equatorial radius of the Earth. • This is equivalent to roughly 100 ns per day. • Nature makes it easy for us: sea level is the point where all clocks are running at the same rate when compared to each other.

42. Need for coordinated time • Because of relativity, time is dependant of the reference frame. • Proper time (local time) is different from coordinated time, at sea level. • Clocks are slowed down by the gravitational potential: • A clock will accelerate by about 1.09×10-16 per meter above sea level.

43. TAI and UTC • The proper time scale computed by BIPM is called TAI (Atomic Time International) • The sea level reference is UTC (Coordinated Universal Time) • Both have the same rate, UTC has the (in)famous leap seconds • Both are derived from the definition of the SI second

44. The place of time in metrology • The SI second is central to metrology. • It is a truly independent unit which is defined in term of physics without the need for another unit. • Just needs a counter. And imagination. • It is the basis for most other units. • Unlike mass, it does not depend on the history of an artifact. (Sèvres Grand K is measured once every 40 years or so.)

45. Conclusion • Time and frequency measurements are the ultimate measurements • Needed for all the other measurements. • Central to the international system of units.