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Revised definitions for four scientific units — the kilogram, the kelvin, the ampere and the mole — come into force today. The change, decided last year, means that all the base units of the International System of Units (SI) are now defined according to fixed fundamental constants of nature, rather than by a physical object or arbitrary reference.

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Murray Gell-Mann

Murray Gell-Mann, theoretical physicist who won a Nobel for codifying fundamental particles. 


Nobel laureate Murray Gell-Mann once described himself as “a character out of Damon Runyon”. Like the novelist’s gritty characters (they inspired the musical Guys and Dolls), Gell-Mann had flair, and peppered his exacting speech with slangy cracks and sarcastic put-downs. He mocked the scientific establishment’s jargon by giving his inventions jokey names. He worked incredibly diligently, but often claimed to be simply tossing off ideas. He attributed his first major insight to a slip of the tongue, and wrote the equations for two other seminal breakthroughs on napkins. He died on 24 May 2019.

Gell-Mann was a theorist in elementary particle physics. When he entered the field in the late 1940s, powerful accelerators were starting to make particles beyond the familiar proton, neutron and electron. Researchers badly needed the equivalent of a periodic table to map the relationship between all of these. Gell-Mann’s most noted contribution was to create one, and to use it to show where more new particles could be found.

Born in 1929 in Manhattan, New York City, Gell-Mann went to Yale University in New Haven, Connecticut, at 15, to study physics. In 1948, he entered the Massachusetts Institute of Technology in Cambridge, determined to earn a PhD in two years. He castigated himself for taking an extra six months to write up his thesis. In 1951, he moved to the Institute for Advanced Study at Princeton, New Jersey, to work with Robert Oppenheimer.

The next year, as a 22-year-old postdoc with Enrico Fermi at the University of Chicago, Illinois, he tackled a knotty problem. A class of recently discovered particles — produced in abundance in cosmic-ray collisions — had unexpectedly long lifetimes. “Easy come, easy go” was a tenet of the particle world. These were therefore labelled “strange” particles. In a talk at the Institute for Advanced Study, the famous slip of the tongue gave him an idea that the particles had a previously unknown fundamental property — he labelled it “strangeness”. (Others independently had a similar idea.) Strangeness was a new quantum number: something that expresses the values that certain kinds of particle can have.

At first, he was afraid to commit himself to print. A conscription notice at the end of the Korean War — his exemption paperwork had not been filed properly — prompted him to send an article to the Physical Review. It referred to the new “Curious Particles”, and the editors objected. The published version concerns “New Unstable Particles” — a phrase he deemed “sufficiently pompous”.

In 1955, Gell-Mann moved to the California Institute of Technology (Caltech) in Pasadena. That year, he also married the archaeologist Margaret Dow. A few years later, he devised a scheme to codify particles, grouping all those known into eight families — the Eightfold Way, he named it in joking homage to Buddhism. Given all the other theoretical ideas flying around, few physicists paid heed. One family in Gell-Mann’s scheme had a glaring hole. At a conference in July 1962 at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, he urged experimenters to find the missing particle, naming it the Omega Minus.

At lunch with two of the experimenters, attending from Brookhaven National Laboratory in New York, he sketched out on a napkin how the particle might be found by indicating the particles into which it would decay. The two — Nicholas Samios and Jack Leitner — took the napkin back to Brookhaven, and used it to convince their director to give them high priority for running time on the lab’s accelerator. They then found the Omega Minus. It was a triumphant discovery, and it vindicated the soundness of Gell-Mann’s entire scheme. Gell-Mann called Samios and said, with his usual nonchalance: “Nick, I hear you have found something very interesting.”

The second napkin episode took place the next year, in the faculty dining room at Columbia University in New York City. Over lunch, his host Robert Serber asked Gell-Mann if the particles of the Eightfold Way were formed by mixing and matching subunits. “So I showed him why I hadn’t considered it,” Gell-Mann said. Scribbling equations on what was to hand, he explained that such subunits would have to have fractional charges. By the time of his talk the next day, however, he’d thought, “What the hell, why not?” and proposed the idea. Reacting against “pretentious scientific language”, Gell-Mann called the subunits quarks, after a passage in James Joyce’s Finnegans Wake.

Meanwhile, Richard Feynman, whose office at Caltech was next to Gell-Mann’s, proposed a similar idea, calling the subunits partons. This led to a long feud in which the two vied over the name, Gell-Mann mocking Feynman’s idea as “put-ons”. When members of the physics community tried to conciliate by calling the subunits “quark–partons”, Gell-Mann prevailed.

In 1969, Gell-Mann was awarded the Nobel Prize in Physics “for his contributions and discoveries concerning the classification of elementary particles and their interactions”. For the next few decades, he continued to be a leader in developing the theory of particle physics. In those years, Samios recalled, “when I’d ask a particle theorist why they were working on something like current algebra or group theory, the answer was invariably, ‘Because Murray is working on it!’”

Given his writing habits and hypercritical temperament, few expected him to undertake a popular book. In 1994, he published one. The Quark and the Jaguar, he called it; the quark represented the simple side of nature, the jaguar the complex.

In talks, Gell-Mann liked to dwell, not on the triumphs of himself and others, but on the confusions, mistakes and vacillations that blocked their way. The practice was closet self-congratulation, some carped. But there was more to it. Gell-Mann would sometimes recite to audiences a ditty that he had seen on the wall of a doughnut shop:

As you ramble on through life, Brother, / Whatever be your goal, / Keep your eye upon the doughnut, / And not upon the hole.

Gell-Mann would add: “I try to keep my eye on the hole.”


Nature 570, 308 (2019)

doi: 10.1038/d41586-019-01907-y


Thanks to a quirk of quantum theory, subatomic particles can emit light as they travel through a seemingly empty vacuum.

The speed of light varies in water, ice and other media. In some cases, an electron or other charged subatomic particle passing through a medium travels more quickly than light moving through the same medium. Such a speedy particle creates a cone of compressed waves as it zips through its surroundings. These waves emit light, called Cherenkov radiation, that has a bluish tinge.

Alexander Macleod, Adam Noble and Dino Jaroszynski at the University of Strathclyde in Glasgow, UK, find that this phenomenon can also occur in a vacuum. According to quantum theory, a vacuum is actually filled with pairs of ephemeral particles that spontaneously come into being before cancelling each other out again. The team’s calculations show that these particles can momentarily reduce the speed of light around them. As a result, a particle travelling at near the speed of light might emit Cherenkov radiation.

Under certain conditions, Cherenkov radiation in a vacuum should be detectable. If so, observations of the radiation could verify some interactions between light and matter predicted by quantum theory.

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LaTeX or Word? For physicists and mathematicians, the choice is obvious. But for scientists in other fields the merits of LaTeX have largely gone unnoticed.

The open-source software system — used to create and precisely format scientific manuscripts — is more akin to coding than writing. Since its development in 1985, LaTeX has become popular in disciplines such as mathematics, physics and computer science.

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‘Time to crop’ for 137Cs in the surface soil and its long-term effects to population based on model assessment

Van Thang Nguyen, Ngoc Ba Vu, Nguyen Phong Thu Huynh, Cong Hao Le



Long-term behavior of artificial radionuclides in surface soils is very important to assess the radiological effects to population. Among artificial radionuclides, 137Cs is most important because of its long half-life and its biggest abundance in the environment. In this study, the fate of 137Cs in the surface soil layers was assessed by the Canadian Centre for Environmental Modelling and Chemistry (CEMC) soil model which is well known as a simple assessment of the relative potential for degrading reaction, evaporation, and leaching of a pesticide applied to a surface soil. The total decrease rate of 137Cs activity concentration in the surface soil (Te1/2) was 10.4 years found in the top 0–5 cm of the soil layer. The activity decrease of 137Cs and the corresponding Te1/2 values under the different depths of surface soil layer were investigated. The influences of soil organic material, soil water content and soil porosity to the losing rate of 137Cs were considerable. Long-term effects of 137Cs to population were assessed through activity concentrations of 137Cs in any parts of the food chain. Soil-to-plant transfer factor (TF), transfer coefficients Fm (transfer to animal milk) and Ff (transfer to animal meat) collected from many literatures were used for activity calculation. The effective doses to population due to ingestion of edible parts of plants, milk and meat was evaluated. The incorporation of four terms: radiological doses, soil-to-plant transfer factors (TF), plant-to-animal coefficients (Fm and Ff) and the total decrease rate of 137Cs is the new approach. The new concept ‘Time to crop’ (TC) based on the effective doses to population was explored and first used for agricultural proposals in the topsoils of the 137Cs exclusion zone. TC was calculated for many scenarios of radioactive exposures, many type of plants, animals and plant groups. TC values were found various in order fruits < leafy vegetables < tubers < cereals < grasses. The highest TC was found for grasses group as a result of the long-term accumulation of radionuclides in animals.


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 Validation of gamma scanning method for optimizing NaI(Tl) detector model in Monte Carlo simulation

Huynh Dinh Chuong, Nguyen Quoc Hung, Nguyen Thi My Le,  Vo Hoang Nguyen, Tran Thien Thanh


The aim of this study is the validation of gamma scanning method for optimizing NaI(Tl) detector model in Monte Carlo simulation. The experimental procedure involved: scanning on front and lateral surfaces of the detector with collimated low-energy photon beam; calibrating the efficiency with energies between 31-1408 keV for point sources at distances of 0 cm and 30 cm from source to the detector. The Monte Carlo code used for the simulations was MCNP6. The diameter and the length of crystal were determined according to the measured results of gamma scanning with a collimated 241Am radioactive source. The distance from window to crystal was estimated using transmission measurement recorded on a second detector. The density of reflector was adjusted to obtain the match between measured and simulated values of efficiency ratio of 81 and 31 keV from a 133Ba radioactive source. The optimized model was applied in Monte Carlo simulations to determine the efficiency and energy spectrum response function of NaI(Tl) detector for point source measurements in two configurations. Good agreement was obtained between measured and simulated results.


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