2016 / mitteilungen S 2016 Special Issue Experiments for the New SI PTB-Mitteilungen Special Issue No. 2 2nd Edition 07 / 2018 Braunschweig and Berlin Braunschweig Physikalisch-Technische Bundesanstalt Physikalisch-Technische Physikalisch-Technische Bundesanstalt Braunschweig and Berlin National Metrology Institute Experiments for the New SI Cover picture: On the way towards a new kilogram definition. In the sphere interferometer, the diameter of the silicon sphere lying in the center is measured extremely accurately. Source: PTB Special Journal for the Economy and Science Official Information Bulletin of the Physikalisch-Technische Bundesanstalt Braunschweig and Berlin Volume 126 (2016), No. 2 Contents Experiments for the new SI ■ Foreword 3 Jens Simon ■ Fundamental constants are to define the units A Paradigm Change in the International System of Units (SI) 5 Rainer Scharf, Thomas Middelmann ■ Frequency of an atomic transition How Does an Atomic Clock Tick? Realization of the Second from 1955 to the Present Day 17 Andreas Bauch, Stefan Weyers, Ekkehard Peik ■ Speed of light Interferometry – How Do I Coax a Length out of Light? 35 René Schödel ■ Elementary charge Counting Electrons to Measure Current 53 Hansjörg Scherer, Uwe Siegner ■ Planck’s constant & Avogadro constant Counting Atoms for Mass and Amount of Substance 63 Peter Becker, Horst Bettin Electro-Mechanical Balance – the Watt Balance 79 Michael Gläser ■ Boltzmann constant How Much Energy Does Temperature Contain? 89 Joachim Fischer, Bernd Fellmuth, Christof Gaiser ■ Photometric spectral luminous efficacy A Measure for Visible Light – Development and Importance in the International System of Units 99 Armin Sperling, Stefan Kück 2 EXPERIMENTS FOR THE NEW SI Foreword Jens Simon* * Dr. Dr. Jens Simon, Today, we can already say with certainty that the “metrologically holy” piece of metal from the safe PTB Press and Infor- autumn of 2018 will earn an entry in the history of the International Bureau of Weights and mation Office; e-mail: books of science. And perhaps it will be more than Measures in Sèvres, France). [email protected] just the history of science that takes note of this The notion of defining the units in such a event, but the history of civilization itself. universal way is not a new one, but was born in For it is in the autumn of 2018 that an under- the closing decades of the 19th century. As early taking will be signed and sealed that is the sum as 1870, James Clerk Maxwell was concentrating total of years and even decades of work at national more on atomic quantities to provide a definition metrology institutes in the very highest art of of the units: measurement: a fundamental revision of the Inter- national System of Units (Système international “If, then, we wish to obtain standards of length, d’unités, or SI for short). time, and mass which shall be absolutely perma- The new definition of the SI (base) units is so nent, we must seek them NOT in the dimensions, fundamental in nature that it is not an exagger- or the motion, or the mass of our planet, but in ation to call it a paradigm change. Instead of a the wave-length, the period of vibration, and the small, selected number of base units (with all of absolute mass of these imperishable and unalter- their obsolete, arbitrary and idealized notions), it able and perfectly similar molecules.” is a set of fundamental constants that will deter- mine the world’s measurements from that point Address to the Mathematical and Physical on. In contrast to all material measures, such Sections of the British Association constants represent those “objects” that are truly immutable. In the system that comprises the units currently in use, the values of the fundamental constants are specified – leading to the remarkable situation that the values of the fundamental constants are in a permanent state of flux, since our measurement capabilities are reflected in these values. Begin- ning in the autumn of 2018, this relationship will become inverted: from the specified values of the fundamental constants, the units will be derived as an inference. Provided that the fundamental constants are truly constant, our system of units will then have the most solid and reliable foun- dation conceivable. The units will be universal, in the literal sense of the word: in principle, it will be possible to apply them throughout the entire universe. To put it simply, even a Martian could then understand what a kilogram is (a feat that, currently, is not possible, unless we send James Clerk the Martian the prototype of the kilogram – the Maxwell, 1870 3 Experiments for the New SI Special Issue ▪ PTB-Mitteilungen 126 (2016), No. 2 After Maxwell, it was above all the great Max In certain scientific circles, particularly the field of Planck who made use of “Constanten” (constants) theoretical physics, Planck’s natural units actually did when he formulated his law of radiation: gain prominence, albeit more as a theoretical pursuit and not as practical units. “On the other hand, it should not be without By contrast, the revision of the International interest to say that the use of the two [...] constants System of Units that is currently under negotiation a and b offers the possibility of establishing units does indeed envisage being suited for daily use; it for length, mass, time and temperature which will also live up to this aspiration, not least because – necessarily maintain their significance for all times on the basis of the new definitions – the realization and for all cultures (this also includes extraterres- and dissemination of the units can (in principle) be trial and non-human cultures) – independent of continuously improved. No technological barriers of special bodies and substances – and which can, any kind will be inherent in the new system of units. therefore, be called “natural mass units.” In this respect, the new system of units that is planned represents a milestone in the history of Ann. Physik 1, 69 (1900) science – as well as in the history of technology in the foreseeable future after the new definition takes effect. Yet, due to its universal validity, the new system represents considerably more than this: it is a milestone in the history of civilization itself. From the Middle Ages until well into the 18th and 19th centuries, the units were decreed by the sovereign of a country, and used regionally for the most part. With the French Revolution at the close of the 18th century came the abandonment of feet, ells and miles, lines, fathoms and rods in favor of a measure wrested from the planet Earth – the meter was born, and with it came the kilogram. The Metre Conven- tion and its Member States put these units into use throughout the world. Today, our life on this planet is characterized by a single, uniform measuring system (with a few exceptions). In 2018, the step will be taken that reaches beyond our little planet: The units are about to cast aside their anthropomorphic Max Planck, 1901 mantle, and I am certain that they will not freeze, but instead enjoy their new freedom. Planck Units If fundamental constants are multiplied and divided by each other in such a way that a dimensional observation yields length, time and mass, then Planck units are thereby defined: c Planck mass m = = 2.176 ∙ 10–8 kg P G h = Planck’s constant G Planck length l = = 1.616 ∙ 10–35 m P c3 c = speed of light l G = gravitational constant (“Big G”) = P –44 Planck time tP = 5.391 ∙ 10 s c k = Boltzmann constant mc4 2 Planck temperature T P = 1.417 ∙ 1032 K P k Conversely, if the natural constants are expressed in these Planck units, they assume the numerical value of 1. 4 Special Issue ▪ PTB-Mitteilungen 126 (2016), No. 2 Fundamental constants are to define the units A Paradigm Change in the International System of Units (SI) Rainer Scharf*, Thomas Middelmann** The significance of measurement selecting these base units are primarily practical * Dr. Rainer Scharf, in nature, as the SI serves to facilitate communi- science journalist, e-mail: r.scharf@ Measurement is one of the foundations of our cation between different groups such as retailers, rz-online.de modern-day civilization. It is a key prerequisite of consumers, engineers and researchers. Thus, ** Dr. Thomas Middel- trade, technology and science – fields that demand the selection of a particular base unit may seem mann, Semiconduc- ever more precise and reliable measurement useful to one group and strange to another, but tor Physics and Mag- netism Department, procedures. During measurement, the existing the SI endeavors to reconcile these divergent e-mail: thomas. state of a quantity is compared to a reference state interests. [email protected] that is made available by means of a measurement At present, the base units are defined in various standard such as the prototype of the kilogram. In ways. The reference quantities used include order to ensure that different measurements can be artifacts (the kilogram), idealized measurement compared to one another, a binding agreement on regulations (the ampere), material properties a suitable reference quantity is necessary. (the kelvin and the mole), defined factors (the These reference quantities are the units in which candela) and physical constants (the second and measurements take place. The “binding agree- the meter). If one of these quantities changed ment” illustrates the legal nature of the units, as (as, for example, appears to be happening with they can fulfill their purpose only if they are used the international prototype of the kilogram), the in the same way at all times. correlations given by the physical constants would The result of the measurement of a quantity Q seemingly have to change as well – leading to is the product of a numerical value {Q} and a unit preposterous consequences.