Weight, what? A new definition of a kilogram

Apparently, our species took a giant step forward on Friday and nobody realized it, at least until today when the Washington Post reported on the work of John Pratt, the chief of quantum measurement at the National Institute of Standards and Technology, which oversees weights and measures in the United States.

He and his colleagues published a new measurement standard of weight on Friday.

Hard to believe that this page-turner study, described in the abstract, didn’t get the world’s attention.

A detailed analysis of these measurements and their uncertainties has led to the value h = 6.626 069 79(30) × 10−34 J s. The relative standard uncertainty is 45 × 10−9. This result is 141 × 10−9 fractionally higher than h90. Here h90 is the conventional value of the Planck constant given by $h_{90}\equiv 4 /( K_{{\rm J\mbox{-}90}}^2R_{{\rm K\mbox{-}90}})$ , where KJ-90 and RK-90 denote the conventional values of the Josephson and von Klitzing constants, respectively.

Bottom line? A kilogram on Thursday wasn’t what a kilogram is today.

“It’s not obvious that it’s a big deal, but it’s a big deal,” Pratt tells the Washington Post. “We could switch from a 19th-century definition of mass to a more 21st- or 22nd-century definition of mass. We could get it based on an idea more than an object. And that’s just beautiful, and I’m proud of our species for getting to this place.”

There is a cylinder in France. It is a kilogram. More precisely, it is the kilogram which defines what a kilogram is.

And it’s been losing weight mass for reasons that are unclear, perhaps impurities in the metal. Who knows?

But it threw the world into disarray, even if you didn’t realize it.

“There’s a lot that rides on these sorts of things that people take for granted,” Pratt tells the Post. “Like breathing.”

It took 16 months for Pratt and his team to come up with a new way of determining the weight of a kilogram.

Don’t be alarmed by this small, strange number. The most important thing about the NIST measurement isn’t so much the number (though that’s also a big deal) as the uncertainty: just 13 parts per billion. This means that the NIST scientists think their measurement of Planck’s constant is within 0.0000013 percent of the correct number.

When the International Committee for Weights and Measures announced that it would reconsider the kilogram definition, it said it would require three measurements with uncertainties below 50 parts per billion, and one below 20 ppb. But with the new NIST measurement, the world now has at least three experiments below 20 ppb — another was conducted by a Canadian team using a Kibble balance, the third by an international group that calculates the Planck constant based on the number of atoms in a sphere of pure silicon.

“I can’t stress enough how impressed I am at humanity for being able to pull this stuff off,” Pratt said.

Weigh to go, humanity!

I have been corrected that the kilogram is a measurement of mass, not weight. Change all puns accordingly. I’m sure they’ll still work.

  • Gary F

    I thought my pants felt snugger today.

  • Matt Black
    • Gary F

      Living large……

    • Josh Ruhnke

      I am going to Paris in 2 weeks. Definitely adding this to the itenerary.

  • Jeff C.

    I always enjoy the opportunity to re-post a Tom Scott video. Here is the one he made about this a year ago.


  • jon

    Well that’s it then.. All SI units are now defined in terms of a universal constant.

    Though I personally still like the definition of a kilogram as the 1000 times the weight of 1 cubic centimeter of liquid water at 0°C and for all practical purposes if I ever need to recreate SI measurements on my own that’s what I’m going to use… (after I reinvent the meter, and the second, by measuring the radiation from a cesium 133 atom, and then use that time period to measure the speed of light in a vacuum…)

  • Guest

    THIS is what Leonard does on the Big Bang Theory, experimental physicist.

    Side fact, the weight of an object depends on the density of the air around it. Think weighing a bowling ball in water. Not much of a change, but 13 billionths isn’t much either.
    By using such weird definitions, a well-equipped lab can tell another EXACTLY how much time, volume, weight, distance was involved. Instead of saying it was Exactly X times our object that we think weighs a kilogram…..which has to be re-checked every few years.
    These guys just made more of an improvement to society than all the MVPs in championship games.

    • jon

      Mass, would be the correct term for a Kg.
      I used the wrong word myself in the comments here.

      Weight varies based a number of factors, and is essentially the static downward force on an object, but mass is constant.

      Pounds are historically a unit of force (pounds per square inch, etc) but have recently been co-opted to be a measurement of mass that is proportional to a Kg as well… (what could go wrong using one name for two units of measurement? Also where did that mars probe go?)

      This might be confusing here on earth, though when we start settling other planets things will get really confusing as some one tosses a 50 kg of something around easily on mars/the moon, but it’s a struggle to lift here on earth, at that point we are going to need to start clarifying these, currently, pedantic points more clearly…

  • Guest

    Side joke, one guy looking at another who is pulling a barrel up via a pulley.

    That is a splendid invention you have there, Mr. Pulley. What are you going to call it.

    • jon

      I first saw this in the book “infinite jest” so I will credit that author, David Foster Wallace, for it though I suspect he lifted the story from somewhere else.
      (the version I found on the internet says “amature radio operator” infinite jest I believe it was a bricklayer…)

      “I am writing in response to your request for additional information for block number 3 of the accident reporting form. I put ‘poor planning’ as the cause of my accident. You said in your letter that I should explain more fully and I trust the following detail will be sufficient. I am an amateur radio operator and on the day of the accident, I was working alone on the top section of my new 80 foot tower.

      When I had completed my work, I discovered that I had, over the course of several trips up the tower, brought up about 300 pounds of tools and spare hardware. Rather than carry the now un-needed tools and material down by hand, I decided to lower the items down in a small barrel by using a pulley, which was fortunately attached to the gin pole at the top of the tower. Securing the rope at ground level, I went to the top of the tower and loaded the tools and material into the barrel. Then I went back to the ground and untied the rope, holding it tightly to ensure a slow descent of the 300 pounds of tools. You will note in block number 11 of the accident reporting form that I weigh only 155 pounds. Due to my surprise of being jerked off the ground so suddenly, I lost my presence of mind and forgot to let go of the rope. Needless to say, I proceeded at a rather rapid rate of speed up the side of the tower. In the vicinity of the 40 foot level, I met the barrel coming down. This explains my fractured skull and broken collarbone. Slowed only slightly, I continued my rapid ascent, not stopping until the fingers of my right hand were two knuckles deep into the pulley.

      Fortunately, by this time, I had regained my presence of mind and was able to hold onto the rope in spite of my pain. At approximately the same time, however, the barrel of tools hit the ground and the bottom fell out of the barrel. Devoid of the weight of the tools, the barrel now weighed approximately 20 pounds. I refer you again to my weight in block number 11. As you might imagine, I began a rapid descent down the side of the tower. In the vicinity of the 40 foot level, I met the barrel coming up. This accounts for the two fractured ankles, and the lacerations of my legs and lower body. The encounter with the barrel slowed me enough to lessen my injuries when I fell onto the pile of tools and, fortunately, only three vertebrae were cracked. I am sorry to report, however, that as I lay there on the tools, in pain, unable to stand and watching the empty barrel 80 feet above me, I again lost my presence of mind. I let go of the rope . . .”

      Edit: This version also leaves out the lacerations on his hand from the pulley itself… that was mentioned in the book as I recall.

  • Bob Sinclair

    //”But with the new NIST measurement, the world now has at least three experiments below 20 ppb — another was conducted by a Canadian team using a Kibble balance…” – Was that a dog, cat, or some other animal kibble? And if the kibble is eaten, than does that invalidate the experiment?

  • imp

    I think you are making fun of the abstract, but it’s LaTeX code that has not been compiled. Of course it is unreadable to everyone (except people who use that typesetting language.) If you need me to compile it for you and make it into an image or something, get in touch.