Feature: The new SI
Ton Kinsbergen/Science Photo Library
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Metrology in the balance
The world’s leading metrologists are keen to redefine the seven SI units in what is billed as the system’s
biggest overhaul since the French Revolution. Robert P Crease reports from a recent meeting that
discussed the proposed changes
The event at the Royal Society in London in January
began precisely on time. But after the final delegates
had taken their seats, Stephen Cox, the society’s executive director, noted sheepishly that the wall clock was
running “a little slow” and promised to reset it. Cox
knew the audience cared about precision and would
appreciate his vigilance. As the world’s leading metrologists, they had gathered to discuss a sweeping reform
of the scientific basis of the International System of
Units (SI), in the most comprehensive revision yet of
the international measurement structure that underpins global science, technology and commerce.
If approved, these changes would involve redefining
the seven SI base units in terms of fundamental physical
constants or atomic properties. The most significant of
these changes would be to the kilogram, a unit that is
currently defined by the mass of a platinum–iridium
cylinder at the International Bureau of Weights and
Measures (BIPM) in Paris, and the only SI unit still
defined by an artefact. Metrologists want to make these
changes for several reasons, including worries about
Physics World March 2011
the stability of the kilogram artefact, the need for
greater precision in the mass standard, the availability
of new technologies that seem able to provide greater
long-term precision, and the desire for stability and elegance in the structure of the SI.
Over the two days of the meeting, participants expressed varied opinions about the force and urgency of
these reasons. One of the chief enthusiasts and instigators of the proposed changes is former BIPM director Terry Quinn, who also organized the meeting. “This
is indeed an ambitious project,” he said in his opening
remarks. “If it is achieved, it will be the biggest change Robert P Crease
is chairman of the
in metrology since the French Revolution.”
Department of
Philosophy,
The metric system and the SI
Stony Brook
The French Revolution did indeed bring about the University, and
single greatest change in metrology in history. Instead of historian at the
merely reforming France’s unwieldy inherited weights Brookhaven National
and measures, which were vulnerable to error and Laboratory, US,
abuse, the Revolutionaries imposed a rational and or- e-mail rcrease@
ganized system. Devised by the Académie des Sciences, notes.cc.sunysb.edu
39
Feature: The new SI
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BIPM
1 The unstable kilogram
Housed in the basement of the International Bureau of Weights and
Measures in Paris is the International Prototype of the Kilogram (IPK).
Made from an alloy of platinum and iridium, it is the only remaining
artefact used to define a base unit in the SI. Comparisons with the six
identical copies kept next to it suggest that the IPK is unstable and
losing mass. Metrologists therefore want to create a new definition of
the kilogram that does away with the IPK and is based instead on
fundamental constants.
The kilogram
has stubbornly
resisted all
attempts to
redefine it
in terms of
a natural
phenomenon
40
it was intended “for all times, for all peoples” by tying
the length and mass standards to natural standards: the
metre to one-forty-millionth of the Paris meridian and
the kilogram to the mass of a cubic decimetre of water.
But maintaining the link to natural standards proved
impractical, and almost immediately the length and
mass units of the metric system were enshrined instead
using artefacts deposited in the National Archives in
1799. The big change now being championed is to
achieve at last what was the aim in the 18th century – to
base our standards on constants of nature.
Despite its simplicity and rationality, the metric system took decades to implement in France. Other nations eventually began to adopt it for a mixture of
motives: fostering national unity; repudiating colonialism; enhancing competitiveness; and as a precondition for entering the world community. In 1875 the
Treaty of the Meter – one of the first international
treaties and a landmark in globalization – removed
supervision of the metric system from French hands
and assigned it to the BIPM. The treaty also initiated
the construction of new length and mass standards –
the International Prototype of the Metre and the
International Prototype of the Kilogram – to replace
the metre and kilogram made by the Revolutionaries
(figure 1). These were manufactured in 1879 and officially adopted in 1889 – but they were calibrated against
the old metre and kilogram of the National Archives.
At first, the BIPM’s duties primarily involved caring
for the prototypes and calibrating the standards of
member states. But in the first half of the 20th century,
it broadened its scope to cover other kinds of measurement issues as well, including electricity, light and
radiation, and expanded the metric system to incorporate the second and ampere in the so-called MKSA system. Meanwhile, advancing interferometer technology
allowed length to be measured with a precision rivalling
that of the metre prototype.
In 1960 these developments culminated in two farreaching changes made at the 11th General Conference
on Weights and Measures (CGPM), the four-yearly
meeting of member states that ultimately governs the
BIPM. The first was to redefine the metre in terms of
the light from an optical transition of krypton-86. (In
1983 the metre would be redefined again, in terms of
the speed of light.) No longer would nations have to go
to the BIPM to calibrate their length standards; any
country could realize the metre, provided it had the
technology. The International Prototype of the Metre
was relegated to a historical curiosity; it remains in a
vault at the BIPM today.
The second revision at the 1960 CGPM meeting was
to replace the expanded metric system with a still
greater framework for the entire field of metrology.
The framework consisted of six basic units – the metre,
kilogram, second, ampere, degree Kelvin (later the
kelvin) and candela (a seventh, the mole, was added in
1971); plus a set of “derived units”, such as the newton,
hertz, joule and watt, built from these six. It was baptized the International System of Units, or SI after its
French initials. But it was still based on the same artefact for the kilogram.
This 1960 reform was the first step towards the current overhaul, which seeks to realize a centuries-old
dream of scientists – hatched long before the French
Revolution – to tie all units to natural standards. More
steps followed. With the advent of the atomic clock and
the ability to measure atomic processes with precision,
in 1967 the second was redefined in terms of the hyperfine levels of caesium-133. The strategy involved
scientists measuring a fundamental property with precision, then redefining the unit in which the property
was measured in terms of a fixed value of that property.
The property then ceased to be measurable within the
SI, and instead defined the unit.
The kilogram, however, stubbornly resisted all
attempts to redefine it in terms of a natural phenomenon: mass proved exceedingly difficult to scale up
from the micro- to the macro-world. But because mass
is involved in the definitions of the ampere and mole,
this also halted attempts for more such redefinitions of
units. Indeed, in 1975, at a celebration of the 100th
anniversary of the Treaty of the Meter in Paris, the then
BIPM director Jean Terrien remarked that tying the
kilogram to a natural phenomenon remained a “utopian” dream. It looked like the International Prototype
of the Kilogram (IPK) was here to stay.
Drifting standard
A pivotal event took place in 1988, when the IPK was
removed from its safe and compared with the six identical copies kept with it, known as the témoins (“witnesses”). The previous such “verification”, which took
Physics World March 2011
Feature: The new SI
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The sphere…
The Avogadro approach (figure 2) defines the mass
unit as corresponding to that of a certain number of
atoms using the Avogadro constant (NA), which is
about 6.022 × 1023 mol–1. It would, of course, be impossible to count that many atoms one by one, but instead
it can be done by making a perfect enough crystal of
a single chemical element and knowing the isotopic
Physics World March 2011
2 Principle of the Avogadro method
CSIRO, Australia
place in 1946, had revealed slight differences between
these copies, attributable to chemical interactions between the surface of the prototypes and the air, or to
the release of trapped gas. The masses of the témoins
appeared to be drifting upwards with respect to that of
the prototype. The verification in 1988 confirmed this
trend: not only the masses of the témoins, but also those
of practically all the national copies had drifted upwards with respect to that of the prototype, which differed in mass from them by about +50 µg, or a rate of
change of about 0.5 parts per billion per year. The IPK
behaved differently, for some reason, from its supposedly identical siblings.
Quinn, who became the BIPM’s director in 1988,
outlined the worrying implications of the IPK’s apparent instability in an article published in 1991 (IEEE
Trans. Instrum. Meas. 40 81). Because the prototype is
the definition of the kilogram, technically the témoins
are gaining mass. But the “perhaps more probable”
interpretation, Quinn wrote, is that “the mass of the
IPK is falling with respect to that of its copies”, i.e. the
prototype itself is unstable and losing mass. Although
the current definition had “served the scientific, technical and commercial communities pretty well” for
almost a century, efforts to find an alternative, he suggested, should be redoubled.
Any artefact standard will have a certain level of uncertainty because its atomic structure is always changing – in some ways that can be known and predicted
and therefore compensated for, in others that cannot.
Furthermore, the properties of an artefact vary slightly
with temperature. The ultimate solution would be to
tie the mass standard, like the length standard, to a natural phenomenon. But was the technology ready? The
sensible level of accuracy needed to replace the IPK,
Quinn said, was about one part in 108.
In 1991 two remarkable technologies – each developed in the previous quarter-century, and neither
invented with mass redefinition in mind – showed some
promise of being able to redefine the kilogram. One
approach – the “Avogadro method” – realizes the mass
unit using a certain number of atoms by making a
sphere of single-crystal silicon and measuring the
Avogadro constant. The “watt balance” approach, on
the other hand, ties the mass unit to the Planck constant, via a special device that balances mechanical with
electrical power. The two approaches are comparable
because the Avogadro and Planck constants are linked
via other constants the values of which are already well
measured, including the Rydberg and fine-structure
constants. Although in 1991 neither approach was close
to being able to achieve a precision of one part in 108,
Quinn thought at the time that it would not be long
before one or both would be able to do this. Unfortunately, his optimism was misplaced.
One way of relating the kilogram to a fundamental constant is to count the number of atoms in
an object that is big enough to be weighed. With current technology this cannot be done directly,
but the number of atoms within a perfectly spherical single crystal of silicon can be measured.
The diameter of the sphere is measured using optical interferometry and the spacing between
the atoms is measured by combined X-ray and optical interferometry. These results let the
volume of the sphere and the volume occupied by one atom be found, from which the number of
atoms in the sphere can be calculated. Weighing the sphere then allows the Avogadro constant
to be determined. The latest work has used spheres made from pure silicon-28, which is created
using techniques similar to those used to separate uranium-235 from natural uranium.
abundances of the sample, the crystal’s lattice spacing
and its density. Silicon crystals are ideal for this purpose as they are produced by the semiconductor industry to high quality. Natural silicon has three isotopes –
silicon-28, silicon-29 and silicon-30 – and initially it
seemed that their relative proportions could be measured with sufficiently accurately. Although measuring
lattice spacing proved harder, metrologists drew on a
technique using combined optical and X-ray interferometers (COXI), which was pioneered in the 1960s
and 1970s at the German national-standards lab (PTB)
and the US National Bureau of Standards (NBS) – the
forerunner of the National Institute of Standards and
Technology (NIST). It relates X-ray fringes – hence
metric length units – directly to lattice spacings. For a
time in the early 1980s the results of the two groups
differed by a full part per million (ppm). This disturbing discrepancy was finally explained by an alignment
error in the NIST instrument, leading to an improvement in the understanding of how to beat back systematic errors in the devices.
The source of uncertainty that turned out to be much
more difficult to overcome involved determining the
isotopic composition of silicon. This appeared to halt
progress toward greater precision in the measurement
of the Avogadro constant at about three parts in 107.
Not only that, but the first result, which appeared in
2003, showed a difference from the watt-balance
results of more than 1 ppm. There was a strong suspicion that the difference stemmed from the measurements of the isotopic composition of the natural silicon
used in the experiment. The leader of the PTB team,
Peter Becker, then had a stroke of luck. A scientist from
the former East Germany, who had connections to the
centrifuges that the Soviet Union had used for uranium
separation, asked Becker if it might be possible to use
enriched silicon. Realizing that a pure silicon-28 sample would eliminate what was then thought to be the
leading source of error, Becker and collaborators
jumped at the opportunity. Although buying such a
41
Feature: The new SI
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NIST
3 Principle of the watt balance
B
M
beamsplitter
laser
Josephson
junction array
R
voltage
standard
fixed
reflector
integrating
voltmeter
coil position
and velocity
measurement
feedback
adjustable
current
source
The watt balance consists of a coil of wire hung from one arm of a conventional balance. The coil is placed in a strong magnetic field and an
electric current, i, is passed through it generating a force, F, that can be adjusted to equal the weight of a mass placed on the same arm of the
balance (mg). Although i can be measured accurately, its relationship to F is determined by the product of the length of the wire, l, and the
strength of the magnetic field, B, both of which are difficult to measure accurately. However, a technique invented by Bryan Kibble at the UK’s
National Physical Laboratory avoids the problem by making the system act as an electric generator. The mass is removed and the coil is moved at
speed u in the magnetic field to generate a voltage, V (not shown). By measuring u and V, the product Bl can be found and used to calibrate the
weighing phase of the experiment. This results in an expression equating electrical power (Vi) to mechanical power (mgu). As these are
measurements of virtual power, real energy-loss mechanisms do not affect the result. By using exquisitely precise quantum standards for the
voltage and resistance, the electrical power can be related to Planck’s constant, h, and the second, while the mechanical power is related to the
kilogram, the metre and the second. Currently, the apparatus measures h in SI units, but if the definition of the kilogram is changed to fix the
numerical value of h, then the apparatus will provide a route to the mass unit in the new SI.
sample would be too costly for a single lab – an eyewatering 72m for 5 kg of the material – in 2003 representatives of Avogadro projects from around the world
decided to pool their resources to buy the sample and
form the International Avogadro Coordination (IAC).
Becker at the PTB managed the group, parcelling out
tasks such as characterization of purity, lattice spacing
and surface measurements to other labs.
The result was the creation of two beautiful spheres.
“It looks like what we’ve made is just another artefact
like the kilogram – what we are trying to get away from,”
Becker said at the January’s Royal Society meeting. “It’s
not true – the sphere is only a method to count atoms.”
What is
remarkable
about the
watt balance
is how it relies
on several
astonishing
discoveries,
none of which
were made
by scientists
attempting
mass
measurements
42
…and the balance
The second approach to redefining the kilogram involves an odd sort of balance. Whereas an ordinary balance compares one weight against another – a bag of
apples, say, versus something else of known weight –
the watt balance matches two kinds of forces: the mechanical weight of an object (F = mg) with the electrical force of a current-carrying wire placed in a strong
magnetic field (F = ilB), where i is the current in the
coil, l is its length and B is the strength of the field. The
device (figure 3) is known as a watt balance because
if the coil is moved at speed u, it generates a voltage
V = Blu – and hence, by mathematical rearrangement
of the above expressions, the electrical power (Vi) is
balanced by mechanical power (mgu). In other words,
m = Vi/gu.
In modern watt balances, the current, i, can be determined to a very high precision by passing it through a
resistor and using the “Josephson effect” to measure
the resulting drop in voltage. Discovered by Brian
Josephson in 1962, this effect describes the fact that if
two superconducting materials are separated by a thin
insulating material, pairs of electrons in each layer
couple in such a way that the microwave radiation of
frequency, f, can create a voltage across the layer of
V = hf/2e, where h is Planck’s constant and e is the
charge on the electron. The resistance of the resistor,
meanwhile, is measured using the “quantum Hall
effect”, which describes the fact that the flow of electrons in 2D systems at ultralow temperatures is quantized, with the conductivity increasing in multiples of
e2/h. The voltage, V, is also measured using the Josephson effect, while the speed of the coil, u, and the value
of g can also be easily obtained.
What is remarkable about the watt balance is how it
relies on several astonishing discoveries, none of which
were made by scientists attempting mass measurements. One is the Josephson effect, which can measure voltage precisely. Another is the quantum Hall
effect (discovered by Klaus von Klitzing in 1980), which
can measure resistance precisely. The third is the concept of balancing mechanical and electrical power,
which can be traced back to Bryan Kibble at the UK’s
National Physical Laboratory (NPL) in 1975, who had
actually been trying to measure the electromagnetic
properties of the proton. These three discoveries can
now be linked in such a way that the kilogram can be
measured in terms of the Planck constant. By “bootstrapping”, the process could now in principle be reversed, and a specific value of the Planck constant used
to define the kilogram.
In Michael Faraday’s famous popular talk “A chemPhysics World March 2011
Feature: The new SI
physicsworld.com
ical history of the candle”, he called candles beautiful
because their operation economically interweaves all
the fundamental principles of physics then known,
including gravitation, capillary action and phase transition. A similar remark could be made of watt balances.
Though not as pretty as polished silicon spheres, they
nevertheless combine the complex physics of balances
– which include elasticity, solid-state physics and even
seismology – with those of electromagnetism, superconductivity, interferometry, gravimetry and the quantum in a manner that exhibits deep beauty.
Towards the “new SI”
The Avogadro approach and watt balances each have
their own merits (see box right), but as the 21st century
dawned, neither had reached an accuracy of better
than a few parts in 107, still far from Quinn’s target of
one part in 108. Nevertheless Quinn, who stepped
down as BIPM director in 2003, decided to pursue the
redefinition. Early in 2005 he co-authored a paper entitled “Redefinition of the kilogram: a decision whose
time has come” (Metrologia 42 71), the subtitle derived
from a (by then ironic) NBS report of the 1970s heralding imminent US conversion to the metric system.
“The advantages of redefining the kilogram immediately outweigh any apparent disadvantages,” Quinn
and co-authors wrote, despite the then apparent inconsistency of 1 ppm between the watt balance and silicon
results. They were so confident of approval by the next
CGPM in 2007 that they inserted language for a new
definition into “Appendix A” of the BIPM’s official
SI Brochure. Furthermore, they wanted to define each
of the seven SI base units in terms of physical constants
or atomic properties. In February 2005 Quinn organized a meeting at the Royal Society to acquaint the scientific community with the plan.
The reaction ran from lukewarm to hostile. “We were
caught off guard,” as one participant recalled. The case
for the proposed changes had not been elaborated, and
many thought it unnecessary, given that the precision
available with the existing artefact system was greater
than that of the two newfangled technologies. Not only
were the uncertainties achieved by the Avogadro and
watt-balance approaches at least an order of magnitude away from the target Quinn had set in 1991, but
there was also still this difference of 1 ppm to be
accounted for. Nevertheless, the idea had taken hold,
and in October 2005 the BIPM’s governing board, the
International Committee for Weights and Measures,
adopted a recommendation that not only envisaged the
kilogram being redefined as in Quinn’s 2005 paper, but
also included redefinitions of four base units (kilogram,
ampere, kelvin and mole) in terms of fundamental
physical constants (h, e, the Boltzmann constant k and
NA, respectively). Quinn and his colleagues then published a further paper in 2006 in which they outlined
specific proposals for implementing the CIPM recommendation, with the idea that they be agreed at the 24th
General Conference in 2011.
In recent years both approaches have realized significant progress. In 2004 enriched silicon in the form of
SiF4 was produced in St Petersburg and converted into
a polycrystal at a lab in Nizhny Novgorod. The polycrystal was shipped to Berlin, where a 5 kg rod made
Physics World March 2011
A tale of two approaches
Avogadro method
Best measurement
The lowest uncertainty is the 30 parts per billion measurement made by the
International Avogadro Coordination (IAC)
Advantages
● The definition of the kilogram using a fixed value of the Avogadro constant is
reasonably intuitive, but requires auxiliary defining conditions
Disadvantages
● It is a very difficult experiment and requires a worldwide consortium (the IAC) to
make the measurements on the two silicon-28 spheres currently in existence
● The measurements are unlikely to be repeated, so the existing spheres will become
a form of artefact standard subject to questions about their long-term stability
Watt-balance approach
Best measurement
The lowest uncertainty is the 36 parts per billion measurement made by the National
Institute of Standards and Technology (NIST) in the US
Advantages
● It is not an artefact standard; the technique measures a range of masses and is not
limited to multiples and sub-multiples of 1 kg
● A watt balance can be built and operated by a single measurement laboratory
● The results from a worldwide ensemble of watt balances can be compared and
combined. This would provide the world with a robust mass standard, which is better
than the individual contributions
● A fixed value of h, combined with a redefined ampere that fixes the value of
the elementary charge, e, will be a big plus both for physics and high-precision
electrical measurements
Disadvantages
● Although the principle is simple, the implementation is not, and requires significant
investment of money, time and good scientists
● The definition of the kilogram using a fixed value of Planck’s constant is less
intuitive than the current definition
Ian Robinson, National Physical Laboratory, UK
from a single crystal of silicon-28 was manufactured in
2007. The rod was sent to Australia to be fashioned into
two polished spheres, and the spheres were measured
in Germany, Italy, Japan and at the BIPM. In January
this year the IAC reported a new measurement of its
results with an uncertainty of 3.0 × 10–8, tantalizingly
close to the target (Phys. Rev. Lett. 106 030801). The
result, the authors write, is “a step towards demonstrating a successful mise en pratique of a kilogram definition based on a fixed NA or h value” and claim it is
“the most accurate input datum for a new definition of
the kilogram”.
Watt-balance technology, too, has been steadily developing. Devices with different designs are under development in Canada, China, France, Switzerland and
at the BIPM. The results indicate the ability to reach
an uncertainty of less than 107. Their principal problem, however, is one of alignment: the force produced
by the coil and its velocity must be carefully aligned with
gravity. And as the overall uncertainty is reduced, it gets
ever harder to make these alignments. The previous
difference of 1 ppm has been reduced to about 1.7 parts
in 107 – close but not quite close enough.
Still, these results have led to a near-consensus in the
metrological community that a redefinition is not only
possible but likely. One BIPM advisory committee has
proposed criteria for redefinition: there should be at
least three different experiments, at least one from
43
Feature: The new SI
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Towards a new SI
Base unit
Reference constants used
Reference constants used to define
Exact value of constant in the
to define the unit in the current SI
the unit in the proposed “new SI”
“new SI”
s, second
hyperfine splitting in Cs-133
hyperfine splitting in Cs-133
9192 631 770 Hz
m, metre
speed of light in vacuum, c
speed of light in vacuum, c
299 792 458 m s–1
kg, kilogram
mass of International Prototype of the Kilogram, m(κ)
Planck constant, h
6.626 068...× 10–34 J s
elementary charge, e
1.602 176…× 10–19 C
A, ampere
permeability of free space, μ0
Boltzmann constant, k
1.380 65...× 10–23 J K–1
K, kelvin
triple point of water, Ttpw
mol, mole
molar mass of carbon-12, M(12C)
Avogadro constant, NA
6.022 141…× 1023 mol–1
cd, candela
luminous efficacy of a 540 THz source
luminous efficacy of a 540 THz source
683 lumen W–1
The seven SI units with the reference constants that are currently used to define the unit and those in the proposed “new SI”. The second, metre and
candela remain unchanged, while the kilogram, ampere, kelvin and mole will all now be related to fundamental constants.
each approach, with an uncertainty of less than five
parts in 108; at least one should have an uncertainty of
less than two parts in 108; and all results should agree
within a 95% confidence level. The proposal drafted
by Quinn and colleagues, for what is called the “new
SI”, is almost certain to be approved. It does not actually redefine the kilogram but “takes note” of the intention to do so. The redefinition is now in the hands of
the experimentalists, who are charged with meeting the
above criteria.
The greatest change of all?
These developments gave Quinn the confidence to
organize another meeting. This time, he and his fellow
organizers devised a careful strategy. The 150 participants at January’s Royal Society meeting included
three Nobel laureates: John Hall of JILA (whose work
contributed to the redefinition of the metre); Bill Phillips of NIST; and von Klitzing himself. Under the new
proposals, no longer will these physical constants be
measured as, within the SI, their numerical values
are fixed (see box above). Furthermore, the definitions
are similar in structure and wording, and the connection to physical constants made explicit. The language
makes it clear what these definitions really say – what
it means to tie a unit to a natural constant – giving them
a conceptual elegance. The proposed new definition
for the kilogram, for example, is “The kilogram, kg, is
the unit of mass; its magnitude is set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 068…× 10–34 when it is expressed in the unit
s–1 m2 kg, which is equal to J s.” (The dots indicate that
the final value has not yet been determined.)
The metrological community is vast and diverse, and
different groups tend to have different opinions about
the proposals. Those who make electrical measurements tend to be enthusiastic; h and e now became
exactly determined and much easier to work with.
Moreover, the awkward split between the best available
electrical units introduced by Kibble’s device and those
available in the SI is eradicated. The only outright objection from this corner at the meeting, by von Klitzing,
was tongue-in-cheek. “Save the von Klitzing constant!”
he protested, pointing out that his eponymous constant,
RK = h/e2, which has been conventionally set (outside
the SI) at 25 812.807 Ω exactly for two decades, now
becomes revalued in terms of e and h, making it long
and unwieldy rather than short and neat. Yet he went
on to express sympathy with the redefinition, citing
44
Max Planck’s remark in 1900 that “with the help of fundamental constants we have the possibility of establishing units of length, time, mass and temperature, which
necessarily retain their significance for all cultures, even
unearthly and non-human ones” (Ann. Phys. 1 69).
The mass-measurement community tends to be less
sanguine. Mass measurers can currently compare masses with about an order of magnitude greater precision
– one part in 109 – than they can achieve by directly
measuring a constant. The new definitions thus appear
to introduce more uncertainty into mass measurements
than exists at present; in place of careful traceability
back to a precisely measurable mass, you now have
traceability back to a complicated experiment at various national laboratories. As Richard Davis, the recently retired head of the BIPM mass division, remarked
about the SI, “It’s got to be like a piece of Shaker furniture: not just beautiful but functional.” Advocates, however, point out that comparison measurements conceal
the uncertainty present in the kilogram artefact itself,
so that ultimately no new uncertainty is introduced.
“Uncertainty is conserved,” as Quinn remarked.
One group not present at the meeting are the students, educators and other members of the public interested in metrology. As the Chicago Daily Tribune
complained after the SI was created in 1960, “We get
the feeling that important matters are being taken out
of the hands, and even the comprehension, of the average citizen.” Woe to the colour-blind seamstress, it continued, who can use a tape measure but cannot tell an
orange–red wavelength. The half-joke concealed the
worry that measurement matters, which should be simple for the average person to understand, were about
to become too complex for anyone except scientists.
One of the attractions of science for students is that the
concepts and practices are perspicuous, or aim to be –
but the new SI seems to put the foundations of metrology out of reach of all but insiders. Woe to the butcher
and grocer, someone may jest when the new SI takes
effect, who is not proficient in quantum mechanics.
Still, each age bases its standards on the most solid
ground it knows, and it is appropriate that in the 21st
century this includes the quantum. None of the attendees at the Royal Society objected in principle to the
idea that the kilogram should eventually be redefined
in terms of the Planck constant. “It’s a scandal that we
have this kilogram changing its mass – and therefore
changing the mass of everything else in the universe,”
Phillips remarked at one point. A few people were
Physics World March 2011
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It’s a scandal that we have
this kilogram changing its mass
– and therefore changing
the mass of everything else in
the universe
bothered that it now appeared impossible for scientists
to detect whether certain fundamental constants were
changing their values, though others pointed out that
such changes would be detectable by other means.
Many participants, however, were troubled that the
Avogadro and watt-balance teams have produced two
measurements that are still not quite in sufficiently
good agreement, throwing a spanner, if not perhaps a
large one, into the attempt to pick a single value. “The
person who has only one watch knows what time it is,”
said Davis, citing an ancient piece of metrology wit.
“The person who has two is not sure.”
Quinn appears confident that the discrepancy will be
resolved in a few years. The only clear controversy on
show at the meeting concerned what would happen if it
is not. Quinn wants to plunge ahead with the redefinition anyway, given that the level of precision was so
remote that it would not affect measurement practice.
Some objected, fearing that there would be secondary
effects, such as in legal metrology, which concerns legal
regulations incorporated into national and international agreements. Others worry about the perception
of metrology and metrological laboratories if a value
is fixed prematurely and then the mass scale must be
changed in the light of better measurements in the
future. NPL director Brian Bowsher, referring to the
current climate-change controversy in which sceptics
leap on any hint of uncertainty in measurements,
stressed the importance of being “the people who take
the time to get it right”.
Calling the new SI the greatest change since the
French Revolution may be hyperbole. The advent of
the SI in 1960 was possibly just as important in that it
introduced new units and tied existing ones to natural
phenomena for the first time. The new changes will also
have scarcely any impact on measurement practice, and
are largely for pedagogical and conceptual reasons.
Nevertheless, the change is breathtaking in its ambition – the most extensive reorganization in metrology
since the creation of the SI in 1960 – and the realization of a centuries-old dream.
The new SI also represents a transformation in the
status of metrology. When the SI was created in 1960,
metrology was regarded as something of a backwater
in science – almost a service occupation. Metrologists
built the stage on which scientists acted. They provided
the scaffolding – a well-maintained set of measuring
standards and instruments, and a well-supervised set
of institutions that cultivated trust – that enabled scientists to conduct research. The new SI, and the technologies that make it possible, connect metrology much
■
more intimately with fundamental physics.
Physics World March 2011
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