Category Archives: Metrology and fundamental constants

Periodic table gets updates for some atomic weights

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New Scientist: The order of the elements listed in the periodic table is dependent on the number of protons in the nucleus. However, each element has a variety of isotopes with varying numbers of neutrons in the nucleus, so each isotope has a slightly different mass. What’s more, the relative concentrations of each isotope can vary in different locations and environments. The International Union of Pure and Applied Chemistry (IUPAC) defines the standard atomic weight of the elements as listed on the periodic table based on the weighted mean of the stable isotopes of each element. Two years ago the IUPAC began listing a range of weights instead of a single value for elements that have varying concentrations depending on location and environment. And with the new change it is now listing both bromine and magnesium with ranges. Updates were also made to the weights of germanium, indium, and mercury.

Largest Mersenne prime discovered is 17 million digits long

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Ars Technica: Named for the French monk who studied them in the 17th century, Mersenne primes are prime numbers that can be written in the form Mp = 2p−1. The first three Mersenne primes are M2 = 3, M3 = 7, and M5 = 31. The Great Internet Mersenne Prime Search (GIMPS) is a distributed computing project that uses volunteers’ computers to calculate whether a prime number is also a Mersenne prime. On 25 January, after 39 days of calculations, a computer run by Curtis Cooper, a professor at the University of Central Missouri, discovered the 48th known Mersenne prime, 257,885,161−1. The discovery was confirmed independently by three different computers. GIMPS has been responsible for the discovery of 14 Mersenne primes, with the last discovery occurring in 2009. Cooper will be awarded $3000 for his help.

New clock may provide definition of the kilogram

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Nature: A clock that uses quantum mechanics to measure time also provides an alternative way to define the standard measurement of mass. Holger Müller of the University of California, Berkeley, created two matter waves from a cloud of ultracold cesium atoms. One half was given a set of precisely calibrated momentum kicks with a laser, after which it was recombined with the other, unaffected half. At 3 × 1025 Hz, the matter waves’ characteristic frequency is far too high to be measured directly. However, recombining the two halves creates an interference pattern whose frequency can be determined accurately with a laser. What’s more, because the frequency depends on the mass of an atom, the laser measurement allows for a direct calculation of a single atom’s mass. In principle, the exact number of atoms in 1 kg of different elements can therefore be determined. The current standard for mass is a 1-kg block of metal, whose mass is slowly changing because of microscopic contamination. The new single-atom clock joins two other options that are vying to replace the current definition of the mass standard.

Confirming a universal constant with alcohol

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Ars Technica: Some theories about the nature and history of the universe suggest that certain physical constants have varied in value over time. Observations of light from a galaxy 7 billion light-years away have shown that one of those constants has not changed for at least the past 7 billion years. The light that was observed showed methanol absorption lines that differed from Earth absorption lines by only 1 part in 10 million. Methanol’s chemical structure is heavily dependent on the ratio of proton mass to electron mass. If that ratio, which is considered one of the fundamental universal constants, had been different 7 billion years ago, then the absorption lines would have been significantly different. Although the observation doesn’t explain why the physical constants have the values they do—the fine-tuning calculations—it does help confirm that physical constants don’t vary in time or space.

Astronomical unit gets redefined

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Nature: A vote at a meeting of the International Astronomical Union has redefined the astronomical unit, the au, as exactly 149 597 870 700 meters. From Giovanni Cassini’s measurement in 1672 until late in the 20th century, the au was defined as the length of the semimajor axis of Earth’s elliptical orbit around the Sun; its value was determined by parallax calculations. The most recent, more precise definition was “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant).” That calculation has several flaws but had remained unchanged for many years because of concerns over how the change would affect software and other applications. The new, noncalculated value makes the unit much easier to explain to students, and no longer varies because of general relativity or the decreasing mass of the Sun.

Making the “real” kilogram obsolete

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IEE Spectrum: Last October delegates from the 55 member countries that define our basic measurement standards agreed unanimously on a tentative plan to base the kilogram on a fundamental constant of quantum mechanics instead of a lump of metal held in Paris. The move, which will also change the basis of three other core units—the ampere, the mole, and the kelvin—is the result of decades of work in trying to measure mass. One approach attempts to pin down the exact electromagnetic force needed to balance the gravitational tug on an object. The other counts the number of atoms in extremely round balls of ultrapristine silicon. For years the two approaches have produced starkly conflicting results. However, over the past few months, metrologists have been excited to find glimmers of convergence, and the effort to pin down mass once and for all is beginning to pick up steam, says IEE Spectrum‘s Rachel Courtland.

European satellite to test Einstein’s gravitational theory

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National Geographic: On Monday the European Space Agency launched a low-cost space probe into Earth orbit. LARES (Laser Relativity Satellite) is designed to measure frame dragging, or the distortion of spacetime caused by the rotation of a massive object, such as Earth. The probe, a solid metal sphere 35.5 cm wide and weighing 362 kg, is covered with reflectors. As the craft orbits the planet, an international network of laser-ranging stations will track its position. According to Einstein’s general theory of relativity, LARES‘s orbital plane should slowly precess over time. Although the shift will be small, measuring only about a few tens of millionths of a degree, the displacement should be about 4 m, enough for the laser-ranging system to record. At a cost of just $10 million, LARES may achieve greater accuracy measuring Earth’s frame dragging than did NASA’s Gravity Probe B, which cost $800 million.

The leap second’s fate will be determined in January

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Nature: At its meeting next January in Geneva, the International Telecommunication Union will decide whether to abandon the leap second. Thus the world’s official definition of the time of day would be decoupled from Earth’s rotation. First introduced in 1972, the leap second accounts for Earth’s slowing rotation rate and ensures that the Sun reaches its zenith over the prime meridian on average at exactly 12:00:00. Unfortunately for time keepers, Earth is spinning down at an erratic, unpredictable rate. Whereas seven leap seconds were added in the 1990s, only two were added in the 2000s. The unpredictability of leap seconds frustrates systems such as navigation satellites that require accurate timing. Indeed, the US GPS doesn’t use leap seconds at all. As Zeeya Merali reports for Nature, whether keeping track of leap seconds is a mild inconvenience or a serious problem is a matter of debate. Regardless of how January’s vote goes, Earth will continue to spin down. In her story, Merali quotes Peter Whibberley, a physicist at the UK’s National Physical Laboratory: “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.”

Metrologists make progress in redefining the kelvin

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Science Daily: Scientists at the National Metrology Institute in Berlin have succeeded in redetermining the Boltzmann constant, via acoustic gas thermometry, which may lead to a redefinition of the base unit of temperature measurement, the kelvin. The kelvin has traditionally been determined by the triple point of water, a chemico-physical material property that can vary depending on contaminants or different isotopes of water. Thus metrologists are seeking a more precise definition of the kelvin by using a fundamental constant, much like they redefined the base unit of length, the meter, using the speed of light. The scientists, who have published their findings in the scientific journal Metrologia, hope that within the next two years they will be able to refine their method and reduce the uncertainty of the result such that the redefinition of the kelvin will be cleared.