News Article | October 26, 2016
Abolition would see 'official' time unmoored from the Sun. "The times," sang Bob Dylan, "they are a-changin'." His words could become literal truth in January, when the World Radiocommunication Conference of the International Telecommunication Union in Geneva, Switzerland, will vote on whether to redefine Coordinated Universal Time (UTC) and pull our clock time out of synchronization with the Sun's location in the sky. At issue is whether to abolish the 'leap second' — the extra second added every year or so to keep UTC in step with Earth's slightly unpredictable orbit. UTC — the reference against which international time zones are set — is calculated by averaging signals from around 400 atomic clocks, with leap seconds added to stop UTC drifting away from solar time at a rate of about one minute every 90 years. But "leap seconds are a nuisance", says Elisa Felicitas Arias, the director of the Time Department at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. They cannot be preprogrammed into software because they are typically announced only six months in advance by the International Earth Rotation and Reference Systems Service in Frankfurt, Germany. If the seconds get implemented inconsistently in different systems, clocks can briefly go out of synch, potentially leading to glitches that can stall computers and leave international financial markets vulnerable to attack. Still, some countries — principally China, Canada and the United Kingdom — want to keep leap seconds to maintain the link with solar time, in part for philosophical reasons. "Most Chinese scholars think it is important for timekeeping to have a connection to astronomical time because of traditional Chinese culture," says Chunhao Han of the Beijing Global Information Center of Application and Exploration, who adds, however, that China has yet to decide how it will vote in January. Last week, scientists and government representatives met at the Kavli Royal Society International Centre near Milton Keynes, UK, to discuss the issue, but they failed to reach a consensus, making the outcome of the January vote hard to predict. Arias, who co-organized that meeting, argues that leap seconds are obsolete now that global navigation systems, which set their own internal timescales, have replaced solar time for navigation and precision scientific measurements such as the motion of tectonic plates and how Earth's mass warps space-time. Adding an extra second inconsistently to multiple clocks across satellite networks could cause a system to fail for long enough to cause an air disaster, says Włodzimierz Lewandowski, a physicist at the BIPM. The US Global Positioning System ignores leap seconds for just this reason, and Russia's GLONASS system has had problems in the past incorporating the leap. Europe's Galileo system, which launched its first two satellites last month, and China's developing BeiDou system will also mark time with their own internal clocks. But Markus Kuhn, a computer scientist at the University of Cambridge, UK, says that most problems could be overcome by having a consistent prescription for adding extra seconds. Linux operating systems, for example, have experienced problems because they add the whole second in one abrupt jump at midnight, which confuses the software. In September, Google announced that it would use an alternative 'soft-leap' strategy, in which operating systems add portions of the second smoothly over an extended period. "This should be the standard approach," says Kuhn. Peter Whibberley, a physicist at the National Physical Laboratory in Teddington, UK, says that despite ten years of debate, "there's no convincing evidence that anything serious would happen if you made a mistake introducing a leap second into a system". Abolishing leap seconds only defers any problems, he adds. "A century down the line, we'll need to introduce a 'leap minute', and nobody has any sensible arguments for why that won't be a worse issue."
News Article | November 17, 2016
The universal constant of gravitation, G – affectionately known as "big G" to distinguish it from little g, the acceleration due to Earth's gravity – is a fundamental constant of nature. It completes the famous equation that describes the gravitational force of attraction between any two objects in the universe, whether they are planets or people or office supplies. Scientists have been trying to understand the strength of gravitation since Isaac Newton first identified the relationship between masses and gravitational force more than 300 years ago. But despite centuries of measurement, the constant is still only known to 3 significant figures, much less than any other constant of nature. The mass of the electron, for example, is known to about 8 digits. Furthermore, as G measurements become more and more sophisticated, rather than converging on a single value, the results diverge maddeningly from each other, with error bars that do not generally overlap. "Big G has been a frustrating problem," says Carl Williams, Deputy Director of NIST's Physical Measurement Laboratory (PML). "The more work we do to nail down it down, the bigger the divergences seem to be. This is an issue that no metrologist can be pleased with." Despite the lack of convergence, most of these disparate results are starting to cluster around one value. But there are some noticeable outliers, such as a pair of well-respected experiments conducted over the past 15 years by the International Bureau of Weights and Measures (BIPM), the intergovernmental organization that oversees decisions related to measurement science and standards. "There's sort of a big debate: Is it that we don't really understand gravity as a theory?" says NIST postdoctoral guest researcher Julian Stirling. "There's some small chance that maybe our understanding of gravity is wrong and there's something slightly different about these experiments that causes the value to be different from other big G experiments, which would be really interesting." The less exciting but more likely answer though, he says, is that systematic errors have crept into the BIPM measurements. So two years ago, the BIPM scientists and other leaders in the worldwide efforts to measure big G met and decided that these tests should be conducted again with the same equipment, but at a different facility and with a different team. NIST researchers took on the challenge and are currently preparing to repeat the BIPM experiment using the original apparatus, with a few upgrades. G is difficult to measure in part because it is extremely weak compared to other fundamental forces. Its value is tiny, about 6.67 x 10-11 m3 kg-1 s-2, a trillion trillion trillion times weaker than the electromagnetic force. "The gravitational force between two sedans parked one space apart is approximately 100 thousand times weaker than the force to separate two post-it notes," Stirling says. "There's a reason why this is the least well known of all the fundamental constants." To suss G out, the BIPM experiment used a torsion balance, a popular method for measuring G and one that was used in the very first measurements by English scientist Henry Cavendish in 1798. This type of device works by measuring the gravitational force between relatively small masses, typically metal spheres or cylinders that you could hold in your hand, by gauging the twisting or torqueing of a wire or strip of metal. BIPM's version is much more sophisticated than the original Cavendish balance. It uses eight masses, cylinders made of an alloy of copper and tellurium. Four are sitting on a round carousel that can be rotated between measurements. Inside the carousel, the other four masses, slightly smaller, sit on a disk suspended from the top of the balance by a strip of copper-beryllium 2.5 mm wide and 160 mm (approximately 6 inches) long, with about the thickness of a human hair. When the outer masses are placed so that they are exactly even with the inner masses, there is equilibrium. However, when the outer masses on their carousel are turned to a new orientation, the inner masses feel a net pull toward them. The gravitational force causes the inner masses to migrate toward the outer masses, twisting the strip that suspends them. Earth's gravity does not affect the measurements, since the attraction between the masses happens perpendicular to the planet's gravitational pull. The amount of force needed to twist the strip a certain amount is known. So by measuring the physical distance that the inner masses travel toward the stationary outer masses, using laser light and a mirror at the top of the strip, scientists can calculate how great the gravitational attraction is between them. And, with that information, they can fill in the gaps in Newton's gravity equation to calculate big G. Of course, to measure big G researchers also need to measure the other quantities in Newton's gravitational equation. That means knowing the exact mass and location of all of its parts, "every hole, every mass, and every screw," Stirling says. And that requires a coordinate measuring machine (CMM). CMMs are used to measure dimensions with high accuracy. This particular CMM is an immense granite table with an overhead touch probe, which will be used to detect the distances between points on an object in three dimensions with potentially half a millionth of a meter measurement uncertainty. The individual pieces of the torsion balance will be probed by a CMM before the experiments begin. But the CMM will also be used during the actual experiment, to ensure that the distances between the cylinders are known to high accuracy. Each big G measurement takes place in vacuum, so only the outer cylinders are accessible with the vacuum cap on. At the moment, the team is still preparing for their experimental run. This summer, a new CMM was delivered to NIST that was large enough to be used for the experiment. In fact, the CMM was so big that it had to be lowered in pieces through an air vent above the laboratory level, about four stories below ground, and a wall had to be removed to get it into the measurement room. Though the hardware is all from BIPM, there are a few updates. "We've had to replace a lot of the electronics," Stirling says. "And also computers have changed a bit over the last 15 years." "We are extremely excited, and also a little terrified, to see if we can sort out this discrepancy, and convincingly identify the measurement bias or unaccounted-for physics—or perhaps even new physics—that explains the existing results," says Jon Pratt, Chief of PML's Quantum Measurement Division. "The terrifying part is obvious: bias or unaccounted-for physics in this experiment is far and away the most likely explanation, yet they will be extremely hard to find, since some of the best measurement scientists in the world have already done their best to eliminate them! The exciting part for us is maybe less obvious: simply put, sorting out this type of discrepancy is what science is all about, and kind of what we live for at NIST." Explore further: What is the value of G?
News Article | November 29, 2016
NIST’s Patrick Abbott with one of the two smaller balances, used for the vacuum-to-air studies. Credit: National Institute of Standards and Technology When the kilogram, the world's basic unit of mass, gets a new definition in 2018, it will be based not on a physical artifact but a constant of nature. However, researchers will still need to "realize" the new definition, or translate it into a physical object, to make it possible to distribute the new standard to the laboratories and industries that need it. Of the two methods that are major contenders for this realization process – watt balances and silicon spheres – both require delicate measurements in vacuum. But most day-to-day mass measurements take place in regular air. This means that in order to disseminate the new kilogram, researchers must find reliable ways to compare a mass measured in vacuum to one measured in air. The world's national metrology institutes (NMIs) are each developing protocols to use in their own countries. But someone needs to check to make sure that their various methods are working well and getting comparable results. So the International Bureau of Weights and Measures (BIPM), an intergovernmental organization that has custody over the current official kilogram standard, asked a few NMIs to perform a dry run of their proposed methods of dissemination, as part of a pilot study to ensure that the plans for distributing the new definition are feasible. NIST just completed its dry run this month. "For this pilot study, each NMI has done a primary realization of a kilogram using either a watt balance or silicon sphere," says Patrick Abbott of the Mass and Force Group in NIST's Physical Measurement Laboratory. "The idea was: How well can we take that primary realization and pass it on?" Presently, the U.S. standard for mass is a plum-sized cylinder of platinum-iridium called K20, which is regularly calibrated against the world's current definition for the kilogram – the International Prototype Kilogram (IPK), housed at BIPM headquarters in Paris. After redefinition, K20 will be replaced by a new U.S. standard: the NIST-4 watt balance. NIST staff began the pilot study by calibrating a sample mass, made of platinum-iridium, in their watt balance. But the next step – transferring the calibration to masses in air – was a bit tricky. Air contains water and other impurities that are adsorbed by the surfaces of the masses used in the calibration process. So a mass measured in air will be slightly heavier than that same mass measured in vacuum. The nagging question for metrologists is, by how much? NIST researchers have prepared a couple of ways to overcome this problem. The first involves a room-sized double-decker instrument that uses magnetic levitation to float a mass in the air, to balance it against a mass in vacuum, and do a direct comparison of the two. Eventually, this instrument – called the Magnetic Suspension Mass Comparator – will be the preferred method of disseminating the kilogram. But it is still being constructed and tested, so it was not used in the dry run. The second method involves using a set of smaller instruments at NIST. These balances are able to compare the masses of two objects at a time in either regular air or in vacuum. Prior to the dry run, NIST staff used one of these apparatus to conduct a study gauging exactly how much mass is added to an object when it goes from vacuum to air, based on its material and the smoothness of its surface. With this information, the NIST researchers took the mass that had been calibrated using the watt balance, removed it from vacuum, and compared it – in air – to a pair of stainless steel working standards, of the type that might be used to calibrate customers' weights. The team applied the corrections that it gathered from its adsorption studies to make the jump from vacuum to air. To connect these findings to the current definition for mass, the team also measured all of these test masses against one of the official U.S. mass standards, whose definition is tied to the IPK. Abbott says he expects the BIPM will be ready to share results from the pilot study by early next year. Other participating NMIs include the National Research Council of Canada (NRC Canada) and France's Laboratoire National de Métrologie et d'Essais (LNE), each of which has its own watt balance, as well as the National Metrology Institute of Germany (PTB) and the National Metrology Institute of Japan (NMIJ), which use silicon spheres. Explore further: Vacuums provide solid ground for new definition of kilogram
News Article | November 19, 2015
The world will hold onto its leap second, at least until 2023. The International Telecommunication Union, a part of the United Nations, decided on Thursday at the World Radiocommunication Conferences that it would delay its decision on whether to abolish the leap second so that potential effects of its elimination can be studied. The leap second is an extra second of time added periodically to the world’s clocks, but some experts have raised fears it could throw off modern technology, including banking systems, GPS, flight operators, and more. The adjustment is meant to compensate for the differences between traditional solar time, based on the rate that the earth spins, and time kept by the world’s hyper-accurate atomic clocks, which measure time by cesium atomic frequency. Without adding a leap second every few years, the time on our computers and phones would very slowly drift out of sync with the rising and setting cycle of the sun. An extra second has been added every few years since 1972—the last one happened this summer—but the practice has been increasingly under scrutiny. Demetrios Matsakis, Chief Scientist for Time Services at the US Naval Observatory, told Motherboard earlier this year the leap second puts untold numbers of systems at risk for failure. "We should eliminate leap seconds because of the real-world practical impossibility of reliably implementing them, due to either their inherent nature or to general lack of knowledge of their very existence," he said at the time. Other countries, including France, Italy, and Japan also oppose the leap second, while Russia and Britain support it. All countries were supposed to come to a mutual decision on a course of action during this year’s ITU meeting, said Sanjay Acharya, a spokesperson for the ITU. “If not everybody agrees, systems won't function in any case, so we do strive for consensus in decisions like these,” he said. “There are very strong positions on either side of this so therefore the decision at this time was to delay the decision until 2023.” The next ITU World Radiocommunication Conference is in 2019, but the decision will be delayed until the following meeting, pending studies. These studies will be conducted by the ITU with other organizations, including the General Conference on Weights and Measures (CGPM), the International Committee for Weights and Measures (CIPM), the International Bureau of Weights and Measures (BIPM), the International Organization for Standardization (ISO), and the World Meteorological Organization (WMO). Leap seconds always fall at the end of June or December, but the next leap second has not yet been announced. Time will tell how many more of them remain in our future.
News Article | January 30, 2017
Geneva, Switzerland, 30 January 2017 – What are the correct symbols for bits and bytes? How do you accurately measure the spectrum of light? How do you quantify airborne sound? Consistency in quantities and units is essential for accurate measurement and can only be achieved if everyone is using the same language. The ISO/IEC 80000 series of Standards does just that, and it is currently under revision. The ISO/IEC 80000 quantities and units series is referenced in the Bureau International des Poids et Mesures (BIPM) guide on the International System of Units (SI), known as the SI brochure, and thus provides important support for the definition of quantities and units. It consists of 13 different parts, featuring two from the IEC (International Electrotechnical Commission) and 11 from the ISO (International Organization for Standardization), some of which are approaching a crucial and final stage of their revision. This series of Standards underpins the international harmonization of terms, definitions and symbols used in science and engineering and thus guarantees a unified language and writing of formulae. It reduces the risk of error and facilitates and encourages communication between scientists and engineers of many disciplines. Dr. Michael Krystek, Chair of IEC TC 25, states “The unification of measurements on a global scale, for all domains of activities, is vital to industries and global trade. This harmonization will ease the development of innovative products and services. The ISO/IEC 80000 series is periodically revised in order to stay relevant to today’s market demands.” The series gives terms, definitions, recommended symbols, units and any other important information related to quantities used in science, engineering, metrology and industry. It is a reference for those writing scientific or technical documents, textbooks, standards and guides. A number of the parts are currently at Draft International Standard (DIS) stage and approaching publication. They are: In addition, two parts have recently reached DIS stage, meaning interested parties can once more submit feedback on the draft before final publication. They are: The other parts in the series on quantities and units are: The IEC parts of the ISO/IEC 80000 series of Standards are developed by IEC Technical Committee (TC) 25: Quantities and units, whose secretariat is located in Italy. IEC TC 25 maintains a close collaboration with IEC TC 1: Terminology, and is in constant liaison with ISO/TC 12: Quantities and units, the International Organization for Legal Metrology (OIML), the International Telecommunication Union (ITU) and the BIPM. On top of the two parts that are under direct IEC responsibility, the Commission also contributes significantly to the other parts of the ISO/IEC 80000 series of Standards. IEC Subcommittee (SC) 34A: Lamps, IEC SC 62B: Diagnostic imaging equipment, IEC TC 76: Optical radiation safety and laser equipment, IEC TC 100: Audio, video and multimedia systems and equipment, and IEC TC 110: Electronic display devices, all play an integral role in the development of ISO 80000-7: Light and radiation. Furthermore, ISO-80000-8: Acoustics specifically refers to Standards developed by IEC TC 29: Electroacoustics. The Standards are expected to be used by metrology and technical institutes, academia, technical book writers and translators, standards developers and many areas of industry. Copies of IEC International Standards are available from the IEC Webstore and from IEC National Committees. About the IEC The IEC (International Electrotechnical Commission) is the world’s leading organization that prepares and publishes globally relevant International Standards for all electric and electronic devices and systems. It brings together 171 countries, representing 99.1% of the world population and 99.2% of world electricity generation. More than 20 000 experts cooperate on the global IEC platform and many more in each member country. They ensure that products work everywhere safely and efficiently with each other. The IEC also supports all forms of conformity assessment and administers four Conformity Assessment Systems that certify that components, equipment and systems used in homes, offices, healthcare facilities, public spaces, transportation, manufacturing, explosive environments and during energy generation conform to them. IEC work covers a vast range of technologies: power generation (including all renewable energy sources), transmission, distribution, Smart Grid & Smart Cities, batteries, home appliances, office and medical equipment, all public and private transportation, semiconductors, fibre optics, nanotechnology, multimedia, information technology, and more. It also addresses safety, EMC, performance and the environment. www.iec.ch
News Article | November 25, 2015
A leap second is gone in the blink of an eye. But a long-awaited decision on whether to ditch these occasional time insertions — which ensure that official time is synced with Earth’s rotation — has been delayed for at least eight years. After representatives who gathered this month at the World Radiocommunication Conference in Geneva, Switzerland failed to agree on whether the costs of the leap second outweigh its benefits, the International Telecommunication Union (ITU) announced that it would defer a decision until 2023, when it will have more information on the impacts of getting rid of the leap second. The union did, however, make a decision that could shift the responsibility of defining the official Coordinated Universal Time (utc) — and in turn the leap second — to the body that is already responsible for defining the second, along with the other SI units. Leap seconds are necessary because Earth’s rotation is slowing in an unpredictable way. Without them, the time of day when the Sun is at the highest point in the sky would drift by about one minute over about 100 years. However, these extra seconds have to be programmed into electronic systems manually and can upset systems that depend on accurate timings. Most countries, including China, the United States and many in Europe, favour scrapping the leap second and basing utc on the continuous tick of atomic clocks. Official time would slowly move out of sync with Earth’s rotation, but — given that it would take thousands of years to accumulate a difference that is greater than the kinds of shifts already caused by changing the clocks backwards and forwards for daylight savings time — many argue that this would cause few problems. “If we have an offset from solar time, it is not extremely important,” says Elisa Felicitas Arias, director of the Time Department at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, who wants to scrap the leap second. “We are already shifted by one hour in summer compared to winter time. Are we affected because of that?" Once the drift is appreciable, the argument goes, a correction could be added much further down the line, perhaps by adding a leap minute or hour. A small number of countries however, including Russia and the United Kingdom, want to keep the leap second. Russia is concerned about how GLONASS, its Global Navigation System — the only one to incorporate leap seconds — would cope, says Vincent Meens, from France’s National Centre for Space Studies, and the chair of the ITU subgroup tasked with debating the topic. Britain’s argument is largely based on the desire to keep a link between official time and Earth’s rotation, says Peter Whibberley, a metrologist at the National Physical Laboratory in Teddington, UK. Astronomers are among those who would be affected if the leap second were scrapped. Their software would need to cope with Earth's rotational time — which defines when stars and galaxies are seen in the sky — being offset by more than a second from universal time, says Meens. Historically, the ITU has borne responsibility for the definition of utc, through an international treaty that also governs how nations share radiowaves. But at the Geneva conference, the ITU announced that it would modify the treaty. Rather than having a stand-alone definition of utc, the treaty will only cite an SI definition — and mention of the leap second will be moved from the utc definition-proper to a mere ‘description’ in a subsidiary resolution, which expires in 2023. Whibberley says that that the biggest effect of these seemingly subtle changes will be to remove responsibility for defining UTC, and therefore the leap second from the ITU. The General Conference on Weights and Measures (CGPM), which already has ultimate responsibility for defining SI units, including the second, is most likely to become the authority in the future, he adds. BIPM, a subsidiary of the CGPM, is responsible for generating International Atomic Time, on which utc is based, from the results of 500 clocks distributed around the world. “In effect, therefore, the BIPM will ‘own’ the definition of utc," says Whibberley, "even if there is no formal process to transfer responsibility.” The CGPM's involvement is unlikely to mean a decision on whether to scrap the leap second will come sooner than 2023, however: the organization's next chance to even propose a change would not come until 2018.
Quinn T.,BIPM |
Parks H.,BIPM |
Speake C.,University of Birmingham |
Physical Review Letters | Year: 2013
This Letter describes new work on the determination of the Newtonian constant of gravitation, G, carried out at the BIPM since publication of the first results in 2001. The apparatus has been completely rebuilt and extensive tests carried out on the key parameters needed to produce a new value for G. The basic principles of the experiment remain the same, namely a torsion balance suspended from a wide, thin Cu-Be strip with two modes of operation, free deflection (Cavendish) and electrostatic servo control. The result from the new work is: G=6.67545(18)×10-11 m3 kg-1 s-2 with a standard uncertainty of 27 ppm. This is 21 ppm below our 2001 result but 241 ppm above The CODATA 2010 value, which has an assigned uncertainty of 120 ppm. This confirms the discrepancy of our results with the CODATA value and highlights the wide divergence that now exists in recent values of G. The many changes made to the apparatus lead to the formal correlation between our two results being close to zero. Being statistically independent and statistically consistent, the two results taken together provide a unique contribution to determinations of G. © 2013 American Physical Society.
Metrologia | Year: 2011
Since 1954 when the definition of the second first came under the authority of the intergovernmental organization of the Metre Convention, the range and complexity of time metrology have increased far beyond anything envisaged in those days. Today, the essential international coordination of this domain of metrology is through the organs of the Convention with the exception of the definition of Coordinated Universal Time, UTC. In this short article I suggest that this also should now come under the authority of the Metre Convention. © 2011 BIPM & IOP Publishing Ltd.
Milton M.J.T.,BIPM |
Davis R.,BIPM |
Metrologia | Year: 2014
In 2011, the General Conference on Weights and Measures (CGPM) confirmed its intention to adopt new definitions for four of the base units of the SI based on fixed numerical values of selected constants. These will be the kilogram, the ampere, the kelvin and the mole. The CGPM was not able to adopt the new definitions at that time because certain experimental and coordination work was not complete. This paper reviews criteria proposed by the Consultative Committees of the CIPM for such a 'new SI' to be adopted and reports on recent progress with work to address them. We also report on work being undertaken to demonstrate that the most important technical aspects of realizing such a new system are practicable. The progress reported here confirms the consensus developing amongst the Consultative Committees and the National Metrology Institutes that it will be possible for the CGPM to adopt these new definitions in 2018. © 2014 BIPM & IOP Publishing Ltd.
Proceedings of the International School of Physics "Enrico Fermi" | Year: 2013
This article outlines the origins and history of the Metre Convention, the BIPM and the International System of Units (SI), with particular reference to the historical development of units based on fundamental constants or invariants of nature. In the past, the ideas and the intention to proceed towards a unit system based on invariants of nature had existed but it has only recently become a practical possibility. The adoption by the 24th General Conference on Weights and Measures in October 2011 of a Resolution outlining the principles of such a system is the culmination of more than two hundred years of advances in physics and metrology. © Società Italiana di Fisica.