Why Isn’t a Kilogram a Kilogram?
100 Comments


The kilogram is the basic unit of mass in
the metric system — the system of measurement used by almost every country in the world. But there’s a serious problem: the standard
that defines how much mass a kilogram actually has, isn’t reliable anymore. After the French Revolution, the new government
of France proposed a unit of mass based on the weight of water, replacing older units
that were based on less reliable things like the weight of grain. In 1875, seventeen European countries signed
a treaty agreeing to use it, and the kilogram went international. The newly created International Bureau of
Weights and Measures, abbreviated the BIPM in French, got the job of creating a prototype
kilogram to be the worldwide standard and making a copy for each member nation. They made a metal cylinder out of an iridium-platinum
alloy chosen for its density, stability, and resistance to corrosion. The cylinder was
completed in 1899, and it’s still the official definition of the kilogram today. They stored it in a sealed vault in Paris,
and ever since, every few decades copies around the world have been weighed and compared against
the prototype to make sure everything is still accurate. Then, about 30 years ago, scientists realized
that something weird was going on: The masses of the original and the copies were starting
to drift apart, and they didn’t know why. It’s possible periodic washings are removing
tiny amounts of material from the surface of the prototype, or maybe pollutants from
the air were incorporated into the cylinder when it was originally made, and are gradually
escaping — no one really knows. We’re not even sure whether the original
is getting lighter or the copies are getting heavier. So far the change is only about the
weight of a single greasy fingerprint, but even that can be a problem for scientists
who rely on precise measurements. The best solution would be to redefine the
kilogram using universal constants, which the BIPM has already done for the other base
units that make up the International System of Units. A meter, for example, is now officially the
distance light travels in a specific fraction of a second. Today the race is on to do the same for the
kilogram using the Planck constant, a quantum physics concept that relates a photon’s
energy to its frequency. The most promising approach involves a watt
balance, a super-sensitive scale that measures an object’s weight using electric current
and voltage, which are already measured in units based on the Planck constant. The plan is to weigh an official kilogram
cylinder using this watt balance, and use that to establish the official mass of a kilogram
in terms of the Planck constant, forever. After that, scientists won’t need to use
unreliable physical kilogram weights anymore. The BIPM’s goal is to make a new definition
official at their next conference in 2018. So hopefully, we’ll soon be measuring mass
in terms of photons. Thanks for asking, and thanks especially to
all of our patrons on Patreon who keep these answers coming. If you’d like to submit
questions to be answered, or get some videos a few days early, go to patreon.com/scishow.
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100 thoughts on “Why Isn’t a Kilogram a Kilogram?

  1. I know mass/weight and such could mean different things and mass has different types but I find it funny the kilogram (a unit of mass) will be based on photons (a massless particle)

  2. I'm young and don't personally have a stable income so I can't ask on Patreon but could someone please ask "Why do we zone out?" for me? Thanks!

  3. I've read elsewhere that the mass of a kilogram is the mass of 2.15×10^25 Silicon 28 atoms, which I think is now a constant, meaning the kilogram no longer directly relies on the weight of a physical object.

    I'm not sure whether you guys know this, or whether the information I have is incorrect. Can anyone else confirm this? It might be useful for them to know as they could add this info the video as a citation (if it's correct of course.)

  4. A question that probably nobody really cares that much about: Are they going to redefine the kilogram, or are they going to redefine the gram? It's always bugged me that the defined start point is a prefixed unit, rather than a base unit.

  5. Why kilogram and not a gram? Kilogram is just thousand grams, so would not it make more sense to have the gram defined instead of KG?

  6. But why not make a meter the distance light travels in 1/300,000,000th of a second? I thought the metric system was supposed to have nice round numbers!

  7. Maybe it isn't the weights that are changing, maybe it is the scales that aren't accurate. If the scales are perfect, then why not just use scales to make objects to calibrate things?

  8. Why not have a specific volume of water at a certain purity at a certain pressure at a certain elevation….Yeah, that does seem harder than building a new EM based scale.

  9. What he won't tell you is that the US is included in that "almost", since pounds, etc are defined in terms of metric units

  10. I feel like this would fit in nicely with a video on the idea of significant digits and rounding, and how rounding can be dangerous.

  11. What happened to the silicon ball that was suppose to have a specific number of molecules that they could count them and then use that to define weight? Or is the watt balance easier to measure?

  12. According to Popular Science and PBS (links below) 60 tons of cosmic dust fall to Earth every day. The year 1875 was 141 years ago. 141 X 365 X 60 = 563 billion tons of dust added to the Earth's mass since the new standard in 1875. More mass equals more gravity. I'm guessing 563 billion tons isn't much mass compared to the total mass of Earth. But isn't that enough to make tiny bit more gravity, which would make 1 kilo have a little less mass?

    http://www.popsci.com/60-tons-cosmic-dust-fall-earth-every-day
    http://www.pbs.org/wgbh/nova/next/space/in-the-past-24-hours-60-tons-of-cosmic-dust-have-fallen-to-earth/

  13. Would worth noting that time is measured by the times a eletron circles around a Hidrogen or some shit like that

  14. thought there was something about all the atoms in a 1kg silicon sphere polished real good being used instead

  15. I think we all know what the solution is. Get rid of science all together and no more problem! At least that's what my Republican senator believes.

  16. Ok so.. focussing on the watt balance… (Team America). Why no mention of the other option?? (Not Team America).. Don't get me wrong, i like the Watt balance, but neither system has been settled on yet and both have merit, and the creation and the mechanics of both are interesting and cool, so why not, in the interests of education (And balance, what?), did you not cover both, and in a little more detail?

    Great channel Btw

  17. A kilogram is officially 1/4 of the mass of all the clothes you're wearing in this video that don't make sense in July.

  18. I'm a bit confused.
    I can see why, given potential minuscule differences in water, masses of various solutes and such that are very difficult to remove makes using water itself as the definition itself, but why are they weighing the kilogram weight itself and not the volume water it was supposed to be based on to establish the new standard? Said weight isn't any less arbitrary a mass than the water would be given its origins.

  19. Fine by me!!!! (as long as the office stays in Paris…) What about that silicon sphere that Veritasium talked about? that stuff relying on the number of ATOMS (????) in the sphere???

  20. "…we'll soon be measuring mass in terms of photons." Either I missed a few details or I've found a bit of an issue with this plan.

  21. As a blacksmith and machinist, and I'd hate to be right about this, what about using dissimilar metal corrosion as an explanation? A special lump of alloyed metal on a stainless steel platform for example. On a time scale of over a century, it could easily account for the weight of a greasy fingerprint.

  22. So, why can't we just take the volume of something that exists in nature (like water) and say that a kilogram is a cubic meter of water at STP, where water is defined as 1/3E8 light seconds?

  23. There is also an effort to define it as the mass of a specific number of molecules of a certain material.

  24. No, wait, what the actual fuck?! What's wrong with continuing to use water?
    It's not the water's fault the dumb hunk of platinum deteriorated!

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