How We’re Redefining the kg

What do I have to push, sub-basement?
>>Woman: Sub-basement. [Buzzing safety alarm] I’m at the National Institute of Standards & Technology
in Washington D.C. and I’m going to the sub-basement. It’s getting dark down here. We’re going to find out how they’re going to
redefine a kilogram. The kilogram is in trouble. Since 1799, it’s been defined as
the mass of a metal cylinder, in a locked vault
in a basement in Paris. But over the last century, careful measurements
of this international prototype kilogram and in-theory-identical national standards
from around the world, have shown that their masses are diverging. The spread has grown to around
50 micrograms, or 50 parts per billion. And having a standard of mass that changes
is unacceptable. Plus, the kilogram is the last of the base SI units
to still be defined by a physical object. The metre, for example, used to be defined
as the length of a platinum bar in Paris, but in 1983 it was redefined
as the distance light travels in 1/299,792,458 of a second. This definition means that the speed of light
is set to exactly 299,792,458 point 00000… et cetera, metres per second. Note how this works:
first, you take the existing definition, say, the length of that metre bar, and you measure as carefully as you can
how it relates to a physical constant of the universe: the speed of light. Then you set the exact value of that constant and use *it* to redefine how long a metre is. I know this might seem circular,
but, importantly, it moves the point of truth off of the physical object,
and onto the unchanging constant of the universe. So, naturally, the thought is
to do the same thing with the kilogram. But… using which constant, and how? [Heavy mechanical noises] Well, there are a number of
different strategies that were attempted but the two that achieved
the greatest success were: 1) using a silicon sphere to determine and set
Avogadro’s number and 2) to use a Watt balance
to determine and set Planck’s constant.>>DEREK: Hi, how ya’ doin’? I’m Derek.
>>JON: Pretty good.
>>DEREK: Nice to meet you.>>DEREK: Where is the Watt balance?>>STEPHAN: The Watt balance is
behind these closed doors, and…>>DEREK: It’s in there?>>STEPHAN: It’s correct,
and right now the problem is that… We are in a crunch to get a number
by the end of May.>>DEREK: What’s the number?>>STEPHAN: The Planck’s constant.
This is what we measure with the Watt balance. In 2011,
the General Conference on Weights and Measures decided that the kilogram should be redefined
based on Planck’s constant, but that doesn’t mean that
the Avogadro approach was futile. I mean, you can use Avogadro’s number
to calculate Planck’s constant and vice-versa. So, ultimately, both approaches
are going to be used to redefine Planck’s constant and
Avogadro’s number simultaneously.>>STEPHAN: One good thing about
having silicon spheres, is that you only want to redefine if you have
agreement between different numbers, right? And the silicon sphere method is a method
in my mind that comes out of chemistry. You measure Avogadro’s constant, which is
a constant that comes out of chemistry. This method comes out of physics,
we measure Planck’s constant. So if they both agree, it’s a pretty strong sign, right?
Because you know chemistry and physics agree. Now, since I’ve already discussed the
Avogadro approach in a previous video, here I want to focus on the Watt balance. It’s actually now called a Kibble balance
in honor of its inventor, Bryan Kibble, who actually passed away in 2016. You know, traditional balances work by equating
the gravitational forces on objects in two pans. The Kibble balance looks kind of similar,
but all of the balancing happens on the left-hand side, where a mass pan is attached
to a coil of wire in a magnetic field. On the right-hand side is a motor. The whole apparatus is sealed
and operated in vacuum. The balance operates in two modes: Weighing mode and velocity mode, and both are required
to determine Planck’s constant. In weighing mode, a kilogram mass standard
is placed on the mass pan and then current is passed
through the coil in the magnetic field and adjusted until the weight of
the kilogram is equal and opposite to the electromagnetic
force on the coil. The equation for this is
Mass times the local gravitational acceleration is equal to the Magnetic field, times the length of wire in the coil, times the current flowing through it In this equation the variables that are difficult to measure exactly are the magnetic field strength, and the length of wire in the coil But luckily the Kibble balance allows us to get around this problem using velocity mode In velocity mode the kilogram mass is lifted off the mass pan and now the motor on the other side of the balance is used to Move the coil back and forth at constant velocity through the magnetic field. This motion induces a voltage in the coil which is equal to the magnetic field, times the length of wire in the coil, times its velocity. Now we have two equations which we can solve for B times L and so we can set them equal to each other and eliminate these variables without having to know precisely what their values are and if we rearrange a little bit you get voltage times current equals mass times gravity times velocity. on the left hand side, there is electrical power and on the right hand side, mechanical power, and that’s why this was called the Watt, the unit of power, balance But how do you go from this to Planck’s constant the number that relates a photon’s frequency to its energy? Well it turns out there’s actually a way of measuring voltage accurately using a macroscopic quantum effect that involves Josephson junctions so a Josephson junction consists of two superconductors separated by a thin piece of insulator Now if you apply a microwave radiation to that junction, you create a voltage across the device and its value is precisely known to be hf over 2 e. Where h is Planck’s constant, f is the frequency of the radiation, and e is the charge on an electron Now by tuning that frequency and stacking as many of these Josephson junctions as you want in series you can create virtually any voltage you like very very precisely. The way this is used in the Kibble balance is a stack of hundreds of thousands of Josephson junctions are put into the circuit with the coil as it is moved through the field and so you exactly balance the voltage which is induced in the coil using those Josephson junctions So you can measure that voltage very very accurately. But how do we measure current? Well it turns out this voltage measuring method is so good that instead of trying to measure current directly we instead measure V on R which is the same thing So this current is passed through a resistor, and we measure that voltage again using Josephson junctions And then to measure resistance we use another macroscopic quantum effect called the quantum hall effect. Which is Beyond the scope of this video but, suffice is to say that the resistance measurement will be an integer fraction, one over p times Planck’s constant divided by the charge on the Electron squared So if we sub all of this into our equation and solve for h, we have that Planck’s constant is equal to four over p n squared, those are all constant numbers that we know, times the local acceleration due to gravity times velocity divided by frequency squared times the mass which is one kilogram. So here we have a very precise equation for Planck’s constant in terms of the mass of one kilogram Now to get an answer that’s good to say, ten parts per billion You need to know all of these values very accurately So to measure V for example the velocity of the coil as it moves through the Magnetic field, we use a laser interferometer as the distance to the coil changes the interference Fringes pass over a detector And essentially by counting how many fringes go past in a certain period of time you can determine the speed of the coil very accurately To measure g, a device called a gravimeter was used to map out the local acceleration due to gravity in the balance room before it was built in there The gravimeter actually drops a corner reflector down a vacuum tube and measures its acceleration again through interferometry, counting the fringes as they pass This is a 3D printed map of the acceleration due to gravity in the Kibble balance room The bump is due to the mass of the powerful and very heavy permanent magnet that’s in the balance The acceleration due to gravity must continually be measured because it can be affected at this level of precision By the positions of the sun and moon and even the water table underneath the building In 2018 the kilogram will no longer be defined by an object in Paris Instead it will be defined based on the fixed value of Planck’s constant which is being finalized right now as a result of all these measurements from the Kibble balances and silicon spheres So right now what we do is, we put the mass in, and we get h out and in 2018, after redefinition, h will be fixed and you use that to realize the unit of mass>>STEPHAN: Easy
>>DEREK: Yeah, just that– just that easy.
>>STEPHAN: Yeah ->>DEREK: Just that simple.
>>STEPHAN: Simple Hey, this episode of Veritasium was supported in part by viewers like you on Patreon and by Audible, who, as you probably know because they’re longtime supporters of the channel are leading providers of spoken audio information including audiobooks original programming news comedy and more And for viewers of this channel they offer a free 30-day trial. Just go to You know, recently I’ve been traveling around the world to Israel, London, Mexico City And tomorrow I’m off to New Orleans because I’m shooting stuff for Netflix and What I’ll be listening to on the plane is Steven Pinker’s The Better Angels of Our Nature This is an awesome book that takes a scientific and statistical approach to the question of when is the best time to be alive as a human and his answer, is now. Whether it seems like it or not, violence and all the terrible things that humans have had to deal with has been on the decline for centuries and if you want the statistical proof for that you should check out this book it is fantastic And if you want you can download it for free by going to Or you can pick any other book of your choosing for a one-month free trial So I want to thank audible for supporting me, and I want to thank you for watching.

100 thoughts on “How We’re Redefining the kg

  1. Actually im confused why you use V instead of U for the Voltage… Like you also use m for mass and I for current… Just wondering

  2. But how do you measure the length of wire in the coil? And it seems highly suspectible to the unknown variable like faults which can only be known by being proved but to prove them youd need to know what the value should be you are trying to measure faultlessly.

  3. At 8:25 i could hear that the guy was german and i was right DR.Stephan Schlamminger greatings from germany

  4. doesn't seem any where near as elegant as the metre standard, if just one of those methods is found later to be in error it will throw things awry

  5. why not just measure the weight of the actual physical object using other units of weight then mark that measuement as the definition of kg

  6. why wouldn't we measures kilogram in force of earth atraccion in newtons, am in this quantity of newtons have the same quantity of newtons (that earth atraction gave) to a kilogram

  7. The one-time opportunity to define the meter so that the speed of light is exactly 300000 m/s, that would be nice but probably mess with other constants.

  8. Would 10 more or less 10 micrograms difference change our daily life buying products by weight? This good only to science not the consumers. TOO MUCH ADO OVER NOTHING FOR OUR DAILY NEEDS IS A WAIST OF TIME AND MONEY.

  9. Watching this on May 20, 2019.
    SI units have been officially redefined today, kg is defined by Planck’s constant.

  10. One day after the final episode of Game of Thrones what defines the kilogram is a method by defining the Planck constant to be exactly 6.626 070 15 × 10^−34 kg⋅m^2⋅s^−1. This approach effectively defines the kilogram in terms of the second and the metre, and took effect on 20 May 2019.

  11. so it wasn't 2018, here in paris the vote has finally been taken and the SI system is finally being updated next week i think and we're so excited !

  12. should've redefined the meter as 1/300,000,000 distance light travels in a second, or name that new unit >_>

  13. So the actual weight of a kilogram is not changing but the language or example we use to explain the weight of a kilogram is?

  14. Another scientific brain smash. P. S. The reason why the meter was redefined in terms of the speed of light was because scientists would get a different measurement every time they try determining the speed of light. Just saying..

  15. Ok.. So, I'm not gonna pretend to know anything about the mathematics involved… But, who makes the motor for the velocity mode? How do they know that all the parts in the motor are precise? Wouldn't a single flaw in a single peice of that motor ruin everything?

  16. 1:42 except "seconds" are not a fundamental constant as that is based on the earth's orbit which is changing slightly (Earth's Days Are Getting 2 Milliseconds Longer Every 100 Years) also that reference point is not universal.

  17. You still have to define "e" (elementary charge) somehow. Isn't it done through voltage measurement in the first place?

  18. The original meter was not based on the length of some platinum bar. It was based on some silly fraction of the distance from the equator, to the north pole. The platinum bar, I think, was based on that. The kilogram should be based on, "Get your thumb off the scale, you thieving bastard." So, what's so great about inches, feet, yards, rods, furlongs? Nothing really, as far as measurement. The Imperial system, in general, is a fair predictor of whether or not, someday, a third-grader might be able to learn to wipe his ass. The U.S. official measurements have used the metric system, for some time. The statutes defining feet and stuff all use metric measurements to do so.

  19. But how to we know the speed of light is constant? We've only been measuring it with precision for around 125 years, and the universe is on the order of billions of years old. This is like looking at the ground underneath your feet and then concluding the Earth must be flat.

  20. I dnt see the point of this …. how is this going to help us ?
    Is a kg still going to be the same as 1" new" kg ??

    Then whats the point of this ??

  21. This whole elektromagnetic machine looks cool and all but its boring really. I would use a monkey on a stationary bike as a motor. We could define a kilogram in number of bananas we need to feed the monkey for it to hold up 1kg for a minute using its bike.

  22. Let's not forget that Jupiter actualy affects earth gravitationaly, very weakly, but still measurably.

  23. Sooo… Interesting that they are doing this experiment in one place. Shouldn't it be done by multiple different teams in different parts of the world and the results compared andd verified? I thought that is how science works?

  24. can it be easy to standardise this unit mass definition anywhere in the world without investing so much on the setups?

  25. Our mortal enemies the flat earthers will dispute this new kg definition. Can't use gravity to discern Plank if gravity is fake?
    "Fecking Idiots, every last one of them."

  26. I wish they had just defined a meter as 1/300000000s instead of 1/299792458s of light travel

    I appreciate the precision they are trying to achieve, but since they are re-defining all the units anyway it's a good time to consider what makes things easiest going forward.
    A million years from now, no one will care how long the Platinum bar was. But they will notice how weird the defined numbers are and wonder why.

    Oh right, that ancient Platinum bar thing they'll say. Or at least some historian might.

    I'm sure they'll have a video on it. Why isn't a meter exactly 1/30000000s? What's up with that? With a picture of a meter and a confused looking scientist as click bait.

  27. me sitting here trying to understand the purpose of planck's constant in the first place ontop of everything else he said in the video

  28. Seconds are based on a physical object, aren’t they? A fraction of the earth’s solar day? And earth is a physical object

  29. How come does the speed of light CONSTANT change?
    Speed of Light going through Glass vs Speed of Light going through a Vacuum for example.

  30. I'm a fan of Veritasium but you look silly praising Steven Pinker's Better Angels.
    Steven Pinker, "the world's most annoying man".

  31. Thank you! I was looking for that video to explain me why h was better than the mass of carbon 12. Two thumbs up!

  32. You should do a video about the discrepancies between the Kibble balance and the Avogadro project. As you might have heard the measurements never had time to completely settle before the deadline to fix the constants so that left a discrepancy between the two constants with regards to the kilogram based on what method you use to realize it.

  33. Just curious how using gravity would help, in some places, such as hudsons bay, gravity is actually less then other places on the planet due to the compression of the bedrock from the massive ice sheet from the last iceage, I mean the ground is "bouncing back" but yea.

  34. Calling it now. When Quantum physics become more robust of a field we will encounter the same issue just with different variables. Speed of light is the speed of light except in fields of excessive gravity and other small forms of interference. So we really just kicked the kilogram a few millennia down the road…

  35. how can we Know that the speed of light is an unchangeable constant? what if it's accelerating or slowing down just a tiny little bit at a time?

  36. Measuring V/R to determine A is all so well and good but since you're measuring V with extreme accuracy and you require the value of A to an extreme accuracy, then R must be measured to an extreme degree of accuracy and this has always been very hard to do. I don't think you emphasized this enough. In fact, I'm sure you could make a whole video series on Ohms law and the difficulties of measuring R.

    Usually in electronics, we'd measure V and A to calculate R as measuring R is unreliable.

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