Scientists Are About to Redefine the Kilogram | Science

Locked in a vault that requires three keys to open, in the town of Sèvres just to the southwest of Paris, there is a kilogram. Actually, it's the Kilogram, the International Prototype of the Kilogram (IPK), the kilogram against which all other kilograms must take their measure, The Grand K. This cylinder of platinum-iridium alloys is in a temperature-and-humidity-controlled environment, in a safe with six official copies, in the underground vault of Sèvres.

"If you were to drop it, it would still be a kilogram, but the mass of the world would change," says Stephan Schlamminger, a physicist with the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

The IPK only emerges from its vault every 40 years or so, when golf ball-sized ingot, exactly a kilogram by definition since 1889, is used to calibrate copies that are shared with countries around the world. But there is a problem. In the vault with the IPK are six witnesses, or "witnesses" -the official copies. Over the years, as evidenced by the rare occasions when The Grand K and his witnesses have been measured, the mass of the IPK has "drifted."

Most of the heads of micrograms, or millionths of a gram-than-IPK. You could say that the IPK is losing mass, only you can not say that, because the IPK is immutably and unwaveringly one kilogram. Besides, physicists do not even know if the IPK is losing mass or gaining mass in the long run, just that it is slowly drifting due to imperceptible amounts of material aggregated from the air, or rubbed off during a weighing, or smudged on the silvery surface of the IPK during one of its meticulous baths.

As you can imagine, this minute drifting causes a lot of headaches-not to mention industries that rely on small and accurate mass measurements, such as pharmaceutical companies.

"At the moment, the kilogram is defined in the mass of a particular thing," says Ian Robinson of the National Physical Laboratory (NPL) in South London. "And if that thing is destroyed, or whatever, it's awkward."

Fortunately, the metrologists of the world have a solution: redefine the kilogram in terms of a natural, universal constant. Most of the units in the International System of Units (SI) are already defined according to universal constants, such as the meter, which is officially limited to the speed of light in a vacuum in 1 / 299,792,458th of a second. Of course, this definition relates to the second, which is defined as 9,192,631,770 periods of a specific frequency of electromagnetic radiation (microwaves in this case) that causes the outer electron of a cesium-133 atom to transition (switch from a quantum measurement of "spin up" to "spin down," or vice versa).

But the kilogram, the last remaining unit defined by an artifact, has stubbornly resisted redefining-until now. On November 16, at the 26th meeting of the General Conference on Weights and Measures, delegates from 60 member states will be gathered in the following plenary sessions: "Planck's constant-a number that relates the frequency of a wave of light to the energy of a photon in this wave. And according to Richard Davis, a physicist with the International Bureau of Weights and Measures (BIPM), "they're expecting a substantial majority."

Max Planck and Albert Einstein

In 1879 the Johnson Matthey in London, a 20-year-old Max Planck defended his thesis On the second law of thermodynamics, and Albert Einstein was born. Though the two scientists did not know it during the course of their lives, their collective work on the fundamental physics of gravity and quantum mechanics would come to the foundation for a 21st century definition of the kilogram.

So what is Planck's constant? "At a fundamental level, it's hard to say," Davis says.

Planck's is a very small number: 6.62607015 x 10^-34, to be exact, will be officially defined at the November 16 meeting. In 1900, Max Planck calculated the number of stars, matching the energy and temperature of the stars to their spectrums of electromagnetic radiation (collectively known as blackbody radiation). At the time, experimental data suggests that energy is not free flowing at any value, but rather contained in gold bundles quanta-Which quantum mechanics takes its name-and-planck.

Five years later, Albert Einstein published his theory of special relativity, which would come to be expressed in the infamous equation E = mc² (en anglais, une article en anglais). The term "energy equals mass times the speed of light squared" is an epiphany which is fundamentally bound up in all the matter of the universe. He also calculated the theoretical value of a single, fundamental quantum of electromagnetic energy-a known photon-which resulted in the Planck-Einstein relation, E = hv. The equation states that the energy of a photon equals Planck's constant (h) times the frequency of electromagnetic radiation (v which is the Greek symbol bare rather than a "v").

"You know the energy of a photon, which is hv, but you also know the energy of a mass, which is mc². [So], E = hv = mc². Right there you can see how you can get a massage from h [Planck’s constant], v [the wave frequency] and c [the speed of light], "Says David Newell, a physicist at NIST.

But this is not only place Planck's constant shows up. The number is needed to describe the photoelectric effect which solar cells are based on. It is also used in Niels Bohr's model of the atom, and it appears in the Heisenberg uncertainty principle.

"It's like saying, well, what about Pi?" Davis says. "What's Pi? Well, it's the circumference of the circle divided by the diameter of the circle. But then Pi shows up everywhere in mathematics. It's all over the place. "

The key connecting Planck's constant to the kilogram is its unit, the joule-second, or J · s. This constant is measured in Hz and is measured in Hertz (Hz), or cycles per second. A joule is equal to a kilogram multiplied by meters squared divided by seconds squared (kg · m²/ s²), with a few clever measurements and calculations, one can arrive at the kilogram.

But before you can convince the world to change the standard of the mass unit, your measurements will be better in the history of science. And as Newell puts it, "measuring something absolute is damn hard."

Measure for Measure

We often take a second look at a meter. But for the majority of human history, such measures of time and length are rather arbitrary, defined according to the whims of local customs or rulers. One of the first decrees that national measurements must be standardized from the Magna Carta in 1215, which states:

"Let there be one measure for the whole of our kingdom, and one measure for ale, and one measure for corn, namely the London quarter"; and one width for both dyed and russet halberget, namely two ells within the selvedges. Let it be the same with weights.

Objective following the Enlightenment, as it was introduced to a constraint on the physical constraints of the universe, it became apparent that varying standards of measurement were presented to the impediment to the advancement of the species. Scientists spread across the globe in the 18th and 19th centuries, measuring everything from the precise shape of the Earth to the distance to the sun and every day to German Lachter (about two meters, depending on region), unequalities and miscommunications abounded.

The French finally had a revolution-not just of politics, but also of measures. As the 18th century drew to a close, the Kingdom of France was estimated to have some million million units, making it impossible to keep track of them all. Urged by the National Constituent Assembly, which formed during the outset of the French Revolution, the French Academy of Sciences, was set to become a new member of the world. from the North Pole to the Equator.

A mathematical expedition led by French mathematicians and astronomers Jean Baptiste Joseph Delambre and Pierre Méchain triangulated the distance of a portion of that length, stretching from Dunkirk to Barcelona, ​​in order to calculate the new meter. The survey was completed in 1798, and the new standard was soon adopted in France.

The meter came to represent a fundamental unit of measure, defining the liter (1,000 cubic centimeters) and even the kilogram (the mass of one liter of water). By 1875, the world was ready to adopt the metric system, and the Meter Convention of the Year of the Representatives of 17 Nations sign the Treaty of the Meter, creating the International Bureau of Weights and Measures cast in platinum-iridium alloy, defining the meter and the kilogram for the world.

Planck and Einstein began to poke and prod the Newtonian structure of physics, discovering new laws among the vastness of the cosmos and the fundamentals of the atom, the system of measurement needed to be updated accordingly . By 1960, the International System of Units (SI) was published, and countries around the world established metrology institutions to continually refine the official definitions of our metric units (kilogram (mass), second (time) ), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminosity).

From these bases, all other units may be calculated. Velocity is measured in meters by one or two; the volt is measured in terms of amps of current and resistance in ohms; and the definition of the yard is now proportional to 0.9144 of a meter.

Today, as during the 18th century, the subject of refining such measurements is at the forefront of scientific capability. The definition of the kilogram is unlikely to change your daily life.

Take, for example, the second. Since 1967, the definition of a GPS-based technology, and without this precision, GPS technology would be impossible. Each GPS satellite carries an atomic clock, which is critical to the fact that time passes infinitesimally but measurably Slower Einstein's theory of relativity. Without a new definition, we could not get a better understanding of these two things, GPS measurements would drift farther and farther off course, making everything from Google Maps to GPS-guided munitions nothing but science fiction.

The relationship between the second and the GPS reveals the fundamental implications of metrology and science: advancing research requires and permits for new standards of measure. Where this cycle is ultimately going to be, but following the death of the meter bar and the abandonment of the second half of the day, one thing is clear: the IPK is next up to the guillotine.

The Kibble Balance

Planck's constant, but it was not until recently that metrology advanced to the extent that the world would accept a new definition. By 2005, a group of scientists from NIST, NPL and the BIPM, "Newell calls" the gang of five, "started to push the issue. Their paper on the matter is titled, Redefinition of the kilogram: a decision whose time has come.

"I consider it a milestone paper," Newell says. "It was very provocative-it annoyed people."

One of the key technologies to measure the Planck constant in the paper is a watt balance, first conceptualized by Bryan Kibble at NPL in 1975. (After his death in 2016, the watt balance was renamed the Kibble balance in Bryan Kibble's honor.)

The Kibble balance is, at a fundamental level, the evolution of a 4,000 years ago: balance scales. But instead of weighing an object against the other, a Kibble balance allows physicists to weigh a mass against the amount of electromagnetic force required to hold it up.

"The balance works by passing through a strong magnetic field," says Ian Robinson of NPL, who worked with Bryan Kibble on the first watt scales from 1976 onward.

The balance operates in two modes. The first, weighing or force mode, mass balances against an equal electromagnetic force. The second mode, velocity or calibration mode, uses a motor to move the magnets while the mass is not on the balance, generating an electric voltage which gives you the strength of the magnetic field. As a result, the force of the mass in weighing mode is equal to the electrical force generated in velocity mode.

Nobel-winning physicists, Brian Josephson, and Klaus von Klitzing. Planck's constant thanks to the work of two Nobel-winning physicists. In 1962, Josephson described a quantum electrical effect related to voltage, and von Klitzing revealed a quantum effect of resistance in 1980. The two discoveries make it possible to calculate the electrical strength of the Kibble balance in terms of quantum measurements (using Planck's constant) , which, in turn, equates to the mass of a kilogram.

In addition to the Kibble balance, the "gang of five" paper addresses another way to calculate Planck's constant-by-crafting spheres of virtually pure silicon-28 atoms, the most perfectly round objects ever created by humanity. The volume and mass of a single atom in the sphere can be measured, which allows metrologists and chemists to refine the constant Avogadro (the number of entities is one mole), and from Avogadro's number, one can calculate Planck's via already-known equations.

"You need two ways of doing this so that you are not in a hiding place," Robinson says.

In order to reduce the number of kilograms, a change in the rate of change which must calculate the value to within an uncertainty of 20 parts per billion. The international silicon sphere effort has become more precise and achieves an uncertainty of only 10 parts per billion.

And as a result of all these measures, much more than the kilogram is about to change.

The New International System of Units

More than redefining the kilogram, the 26th meeting of the General Conference on Weights and Measures (CGPM) is a fixed value for the constant Planck, and as a result, enacting the largest transformation of the International System of Units since its inception in 1960 Previously, Planck has been measured incessantly, averaged with other measurements across the world, and has been evaluated every few years.

"No one will measure the Planck constant once this [vote] has passed, because it's value will have been defined, "Davis says.

In addition to the constant Planck, the constant Avogadro will be at a fixed value,e, the charge of one proton), and the triple point of water (273.16 degrees Kelvin, or 0.01 degrees C).

By choice the constant Planck as an absolute value, scientists are turning away from mechanical measurements and adoption of quantum electrical measurements to define our fundamental units. Once the constant is defined, it can be used to calculate a range of masses from the atomic level to the cosmic, leaving behind the need to scale up.

"If you have an artifact, you only anchor your scale at one point," Schlamminger says. "And a fundamental constant does not care about the scale."

The new value for Planck is also changing the definitions of our electrical units, such as the 1948 definition of the ampere. Physicists have long used the Josephson and von Klitzing effects to calculate electrical values ​​with precision, but these measurements can not be part of the Planck constant-is a fixed value.

"It 's always grated on me that if I wanted to get my SI volt or my SI ohm, I had to go through the kilogram. I had to go to a mechanical unit to get my electrical units, "Newell says. "That seemed very 19th century, and it was."

Now, the electrical units will be used to get the kilogram.

"People talk about, oh it's redefining the kilogram, but I think this misses an important point," Schlamminger says. "We're going to get these electrical units back into the SI."

For All People, For All Time

There are more than a half-dozen Kibble scales around the world, and many countries from South America to Asia are building their own nature. No longer going to be a kilogram, where is it going to be, and everyone is so afraid it is not used but once in a while.

"It means now, what we can do is spread the mode of mass determination around the world," Robinson says.

For the scientists who are working this change affects, the new International System of Units is nothing short of a historic opportunity.

"I'm still kind of worrying that this is a dream, and tomorrow I wake up, and it's not true," Schlamminger says. "I think this is finishing the arc that people started thinking about before the French Revolution, and the idea was to make measurements for all times for all people."

"This is one of the highlights of my life," says Klaus von Klitzing of the Max Planck Institute, who will be able to see the results of the new SI. "This is wonderful. We have the unification of these quantum units … with the new SI units, and this is a wonderful situation. "

Such changes to our fundamental values ​​to describe the universe are not always easy, and it is hard to imagine when one will occur again. The meter was redefined in 1960 and then again in 1984.

The second was redefined in 1967. "Now that was quite a revolutionary change," Davis says. "People for eternity had the time of the revolution of the Earth, and all of a sudden we changed to a vibration in an atom of cesium."

Whether the definition of the second is a more fundamental change to the understanding of the definition of the kilogram is not to say, but, like the second, the redefined kilogram is undoubtedly a significant moment in the advancement of our species.

"Getting rid of the last artifact … that's the historic thing," Davis says. "Measurement standards have been based on these artifacts, really, since anyone knows. Neolithic times excavations show standard-standard lengths, standard masses-that are little pieces of chert or rock or something. And so that's how people have been doing it for millennia, and this is the last one. "

The SI will change again, but primarily as a matter of just infinitesimal uncertainties, or switching to a different wavelength of light or chemical measure that is ever-so-slightly more precise. In the future, we can add SI to the values ​​we define. But we can never again do what we do now, to leave behind the understanding of our ancestors, and embrace a new system of measure.