It must be a baker's worst nightmare: a world where a kilo ain't a kilo anymore. Next May 20, the universal standard for a kilogram gets its first upgrade in 130 years. There'll be screams a la patisserie.
Image: Reuters/J. Cabeazas
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Imagine being called "Le Grand K" and, having served the world for 130 years, being told that you're no longer so grand after all. Well, that's exactly what's in store for a platinum-iridium cylinder stored under three glass bell jars in France. It has stood as the world's standard for the measurement of one kilogram since 1889. And it's so precious it's kept about as securely as the proverbial (and real) crown jewels, with keys stashed in different locations.
In all that time, Le Grand K has been a solid, physical representation of science's endeavors to measure and calculate the world around us. But that world is changing as in a year's time; on World Metrology Day, May 20, 2019, the International System of Units will be revised, with a set of new measurements that are anything but physical. The new standards are based on fixed numerical values derived from pure physics.
"Our measurement system started on a local level, where the size of a local leader's foot or hand or the weight of grains determined standards of length and mass. This was followed by a global system, where the Earth's properties played an important role in establishing an internationally accepted system of measures. Prototypes for the meter and the kilogram were meant to guarantee the stability of these units."
That's how Klaus von Klitzing, a German physicist and Nobel Laureate, put it in an article in Nature Physics in February 2017. His very own work, which includes the Quantum Hall effect and the von Klitzing constant (what a name?!) has, by von Klitzing's own account, had a profound impact on metrology, the science of measurement.
But of all our international metric standards, "the kilogram in particular turned out to be unstable with time," wrote von Klitzing.
Basically, this international standard is losing weight. Well, that's the common conception anyway. The US National Institute of Standards and Technology (NIST) says Le Grand K will, in fact, have "a mass of slightly less or slightly greater than one kilogram, to within 10 parts per billion in certainty" when it's relieved of its duties next year.
Goodbye physical world
The International System of Units holds seven "base units." And over time, each of these has - in a sense - left the physical world. That is to say that these standards are no, or will, no longer be based on physical objects such as Le Grand K.
Hey, Mr. Baker, is that an exact kilogram you've got there, measured with the Planck constant?!Image: DW/T. Walker
Instead, our physical world will be dictated by constants of nature - arguably still in the physical world, but allow me this minor conceit.
A—ampere (electric current)
K—kelvin (temperature)
s—second (time)
m—meter (length)
kg—kilogram (mass)
cd—candela (luminous intensity)
mol—mole (amount of substance)
Temperature has been defined by the so-called triple point in a sealed glass cell of water. The triple point is "the temperature at which water, ice and water vapor exist in equilibrium," according to NIST. That, you might think, was constant enough. But the water can contain chemical impurities, and those impurities can shift the triple point to create inaccuracies.
It's just like tap water in a pot on a stove. Common knowledge says the water will boil at 100 degrees Celsius (212 Fahrenheit; 373.15 Kelvin). But as soon as you add solute, like salt, the boiling point goes up. The salt is an impurity.
But soon we will have new constants that include an exact numerical value for the Boltzmann constant to define temperature. Then there are the Planck and Avogadro constants, which will also improve our sense of accuracy.
Whether it's hot or cold, temperatures can shift with impurities, but not the Boltzmann constantImage: picture alliance/AP Photo/sakhalife.ru
So to get back to weights - or to be precise, mass - the Planck constant will become a fixed, exact value that we'll use to define the mighty kilogram.
It will be used in the same way that the speed of light is used to define the meter - the unit of length. The speed of light has been assigned an exact numerical value. So if we want to know the distance of the Earth to the Moon, we measure how long it takes for light to travel between the two.
Getting these measurements, and keeping them stable with exact numerical values, is becoming increasingly important in our hyper-connected digital world - it all has to align somehow. And just think of the scale over which things are measured these days. Mass, for instance, is measured over an almost unimaginable scale - from that of an atom to that of planets. Precision is more than a plus.
All this has to go through a final stage of approval at the General Conference on Weights and Measures in November 2018.
No matter which way it goes, rest assured, your local baker will sleep tight. Most of us won't even be able to tell the difference - unless, of course, you weight every loaf you buy. For now, head on down to the bakery and treat yourself to a kilo of cake this World Metrology Day.
The two-time Nobel prize winner Marie Curie was born 150 years ago
She is the only woman to have won two Nobel prizes, one for physics and one for chemistry. After World War I, the scientist and radiologist stood up for international science cooperation in the League of Nations.
Image: imago/United Archives International
Growing up in a family of teachers
Marie Sklodowska (here in the middle of her siblings Zosia, Hela, Josef and Bronya) was born on November 7, 1867 in Warsaw. Her father Wladyslaw Slodowski was a maths and physics teacher. Her mother Bronislawa was head teacher of a girls' boarding school.
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All about education
Marie Curie's mother, Bronislawa Slodowska, spent her whole education at a prestigious girls' boarding school in Warsaw's Freta Street. Afterwards she herself worked there as a teacher before becoming the head teacher. When Bronislawa died Marie was only 13 years old.
Image: imago/United Archives International
Studies only for boys
In 1883, at the age of 15, Marie completed her secondary education at the top of her class. But as a girl she was not allowed to go to university in Poland. As her father could not fund studies for her abroad, Marie worked as a private tutor for wealthy families and taught farmers' children reading and writing. Meanwhile, she attended secretly organized classes.
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Studies in Paris and the discovery of radioactivity
In 1891, as a young student, Marie went to Paris. There, she could register in physics at Sorbonne University. She was one of 23 girls among 1,825 students. Though the French language was hard for her, she passed her exams. In 1896, with her colleague Henri Becquerell, she discovered that uranium calium sulfate dyes photographic plates black.
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Scientist colleague Pierre Curie becomes Marie‘s husband
When Marie first met him in 1894, Pierre Curie was leading the research laboratory of the municipal technical college for industrial physics and chemistry (ESPCI). Their common passion for science soon brought them closer together and they married on July 26, 1895.
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Experiments on radioactive substances
Marie continued to explore radioactivity, among other things with this machine she developed with Pierre. It is a piezoelectric electrometer that can measure the elctric conductivity of air containing radium. In 1898, Marie and Pierre discovered polonium. It was named after Marie's home country Poland.
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Doctorate
Marie's doctorate dissertation about radioactive substances caused a big furor among scientists. Within one year, it was translated into five languages in 17 editions. By this time, Marie and Pierre were already starting to show the first symptoms of radiation sickness.
Image: gemeinfrei
Nobel Prize in Physics
In the same year of Marie‘s doctorate, 1903, she and her husband received the Nobel Prize for Physics of the Swedish Academy of Sciences, which noted "the extraordinary achievements" they had acquired with their research on radiation.
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Children without a father
Marie Curie gave birth to her first daughter Irène in 1897. Seven years later she had her second daughter Ève. Ève barely got to know her father, who died in 1906 after he suffered an accident with a horse and cart. After that, the faculty recommended Marie to become the new head of the laboratory. She was the first woman to teach at the Sorbonne University.
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Breaking new ground
Marie Curie became a full physics professor in 1908, the first woman to achieve the distinction. She lectured at the Paris Radium Institute, founded previously by her husband. The institute came to establish international measuring standards for radioactivity. In honour of both Marie and Pierre, the unit of measurement was named "Curie".
Image: Getty Images/Three Lions
More medicine in World War I
During World War I, Marie Curie‘s work at the Paris Radium Institute focussed more on medicine. She developed a mobile x-ray car that first-aid attendants could take to the front. This photo shows Marie at the Institute with a delegation of the American Expeditionary Corps. The other woman standing at the desk is Marie‘s daughter Irène.
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United States of America
In 1920, Marie Curie travelled to the United States. The media celebrated her more as a healer than a scientist. Besides visiting the White House (the photo shows her with US President Warren Harding) and doing a touristic programme, Marie Curie also gave lectures to female academics and visited research facilities, as well as chemical companies.
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Standing up for international research cooperation
During stays at diFferent US universities, Marie Curie was awarded nine honorary doctorates. After her return she used her fame to argue for more intense international cooperation at the newly founded League of Nations. Amongst other things, Marie wanted to achieve reliable guidelines for publications, copyright and scholarships.
Image: imago/United Archives International
The interest in physics runs in the family
Marie's older daughter lrène became a famous physicist herself. This photo shows her and her husband Jean-Frederic Joliot-Curie in the laboratory. In 1935, both received a Nobel Prize for the discovery of artificial radioactivity.