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The kilometer is dead. Long live the kilogram

2018 CGPM noted that SI units are based on absolutes … because Planck's constant is absolute, the kilogram, unlike Le…

2018 CGPM noted that SI units are based on absolutes … because Planck’s constant is absolute, the kilogram, unlike Le Grand K, will never fall due.

Today I did something I sent to do now and then: bought a kilo of basmatiris. It came nicely packed in a sealed bag. It feels like what I think a kilo should feel, and it is actually labeled “1

kg”, and that was enough for me. I did not ask for it to be weighed. Even when a supplier actually weighs a kilo of something for me-apples or onions, say-I do not think much about it. There is a weight on one side of the balance th marked “1 kg”, and it suits me.

Nothing important in this couple, I’m sure. But now consider this: What happens if I used that bag of rice, or apples, to “define” the kilogram? It’s probably you asking me, “What’s a kilo?” What happens if I dug out my unopened basmati or bag the apples and said, “This is a kilo!” What would you think?

Continue: What happens if we mandate that each transaction involving kilos had to use either my rice or my apples to fix? In other words, when you try to buy a kilo carrot, say how would you react if the seller pulls out my apples and puts them on the scales as a measure? My guess is that you would be scared. You ask for a more credible measure, probably one of the six-page weights with “1 kg” stamped on it in Hindi and English. Reasonably enough. But, how are you sure that the weight is 1 kg? You have to measure it against another, maybe a dumbbell from your nearest gym, labeled “1 kg”, but in turn, how do you know the weight really is 1 kg?

This leads somewhere, I promise. This leads all the way to a certain glass of glass sitting in a glass jar somewhere in Paris: International Prototype Kilogram, lovingly called “Le Grand K” (“The Big K”). Since 1889, the kilometer has been defined as Le Grand K’s weight. This means that every single action you drive over which says “1 kg” – whether a hexagonal piece of black metal or the scale you check your swallow yourself daily – was once calibrated by another weight, and it was confirmed by another and so on: An unbroken chain of weights extending from my base food packet through your vegetable supplier’s hexagonal kilogram blocks all the way to the ingots in Paris.

The score is, you know you bought a kilo of carrots just because “Le Grand K” weighs one kilo. So if you ask, “what’s a kilo?”, Is the real answer “the block of platinum in Paris weighing one kilo”. But it can also be “so much carrots weighing a kilo”. In other words, we have this vague unsatisfactory definition: one kilogram is something weighing one kilo. Reminds me of something that British plant researcher Jonathan Drori told me recently: A tree is defined effectively as something that looks like a tree.

This still leads somewhere, I promise. If you, like me, find it unsatisfactory that the kilogram is defined as follows, and it has been since 1889, you will be happy about what happened near Paris on November 16, a week ago. It was the last day of the 26th CGPM-French Acronym for the General Conference on Weights and Measures, which happens once every four years.

CGPM brings together delegates from around the world to review measurement standards and science. They always strive to improve and sometimes expand what are called the SI devices – what you know as the metric system. Thus, in 1907, the fourth CGPM defined the karate as 200 mg or one-fifth of one gram. The ninth CGPM, 1948, defined a whole range of devices: ampere, ohm, volt, watts, joule, bar and more. On 17th, 1983, the definition of the meter changed. Like the kilogram, it had also been defined in 1889, as the length of a one-meter platinum bar (so you might have said “one meter is the bar in Paris that is one meter long”). However, in 1983, the CGPM defined the meter with reference to the speed of light, called “absolute” in the sense that it did not change.

The light travels 299,792,458 meters in one second. A meter is thus the distance the light travels in exactly 1 / 299,792,458 of a second. The meter, defined.

Why do you need to refer to an absolute? Because even platinum ingots are stored under glass cans in a vault decay over time. Le Grand K is now thought to weigh about an eyelash less than when it was made. It may not seem like something worth caring about, but we live in an age where even the degree of irregularity can be critical: think about miniature electronics or laser operations or carefully calibrated doses of drugs. So yes, an eyelash in kilos is really worth taking care of.

Perhaps the 1983 redefinition of the meter gives you an idea of ​​what happened to the kilogram on November 16: it was also redefined. In fact, momentum has been built for several years to redefine the kilogram based on an absolute. The 1987 CGPM discussed what an alternative definition might look like. At CGPM 2011, there was an overall agreement to define it in the form of Planck’s constant, but exactly how it was left to work out. The conference in 2014 noted that progress had been made towards neglecting the new Planck-based definition, but it was not yet finished. Maybe next time.

Fairly, the 2018 CGPM had drawn up a resolution containing this clause: “The definition of kilograms applicable since 1889 … based on the mass of the international prototype of the kilogram, is suspended.”

It also had this clause: “The kilogram, the symbol kg, is the mass SI unit. It is defined by the fact that the fixed numerical value of Planck Constant h is 6.662607015 × 10 -34 kg m 2 / s. “(It is 662.607015 trillion trillionths kg m 2 / s, no matter what the units mean).

On November 16, CGPM passed the resolution, which means that the world on May 20 will have a new definition for the kilogram. Which leaves the questions I imagine asking you: What is Planck’s constant? What’s up with kilot?

Most of us studying physics at school or college enter Planck’s constant when we learn about Heisenberg’s famous uncertainty principle. There are limits, according to the principle, how carefully we can measure both the object’s position and speed. The more precisely you nail down its position, the more uncertain (or uncertain) becomes your measurement of its speed – or actually its momentum, which is its mass multiplied by its speed. The less the uncertainty in the position, the greater is the uncertainty in the momentum and vice versa. The seesaw between these two uncertainties is captured in a single relationship: multiply them, and the answer can never be less than Planck’s constant.

Of course, there is little in our everyday lives. I can tell you quite exactly, for example, I’m sitting in a chair right now and my speed is zero. I can be so accurate because I measure my weight in kilos and my speed in meters per second, and in that sense Planck’s constant disappearance is small. But go down to an electron scale, which weighs one billion trillion pounds, and it will be difficult. Measure an electron’s position only in an atomic size, and Planck’s constant says that the uncertainty of speed is going to be thousands of miles per second, which is quite seriously uncertain.

The role of weight in this uncertain dance between speed and position is the reason why Planck can constantly define the kilogram. Let’s say you had an item whose speed you knew within 1 meter per second and whose position you knew within a small fraction of a meter-to be accurate to within 662.60701550 billion trillions of a meter. Planck constantly tells us that such an item weighs 1 kg.

The kilometer is defined there.

It is obviously hard to imagine the measurement in this way. But we actually have an instrument that can be so accurate. The Kibble balance is essentially a road scale that measures how much power we need to balance one kilo; and thus it gives us a value for Planck’s constant. Last week’s CGPM noted that the SI units are based on absolutes such as the velocity of light and Planck’s constant, and noted their values ​​as part of their resolution. So because we have a default value for Planck’s constant, and because the meter and the other are also defined, we can define the kilogram.

And because Planck’s constant is absolute, this kilo, unlike Le Grand K, will never be due. Raise an entire eyebrow to it.

Once a computer scientist lives Dilip D & # 39; Souza now in Mumbai and writes for his dinners. His Twitter handler is @DeathEndsFun

First Published: Tuesday, November 22, 2018. 09 2:00 PM IST

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