Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B So what is the new definition of the kilogram?

  1. Nov 13, 2018 #1
    So this article "Quantum leap for mass as science redefines the kilogramme" said that there is a new definition of the kilogram coming. But they neglected to mention what that new definition is exactly. All they said was that it's now based on Planck's Constant. So I worked my way backwards trying to figure out what that is. First I divided the Planck by the kilogram, and came up with this:

    h / 1 kg = 6.62607×10^-34 m^2/s

    So that unit (m^2/s) looks like I can use the standard constants the speed of light and the metre.

    h / (1 kg * 1 m * c) = 6.62607×10^-34 m^2/s / (1 m * c)
    = 2.2102191×10^-42

    Consequently after rearranging, we get:

    1 kg = h / (2.2102191×10^-42 m * c)
    ~ 4.5244383E+41 h / (c * 1 m)

    Is that all there is to it? Just some weird huge number multiplied by the Planck divided by the speed of light and the meter? Do I need to throw some Pi's or Euler's numbers in there too?
     
  2. jcsd
  3. Nov 13, 2018 #2

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The new definition I found is Planck's constant divided by 6.62607015e-37 m-2s
     
  4. Nov 13, 2018 #3

    gneill

    User Avatar

    Staff: Mentor

  5. Nov 13, 2018 #4

    Dale

    Staff: Mentor

    The new definition will be:

    “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νCs.”
     
  6. Nov 13, 2018 #5

    Buzz Bloom

    User Avatar
    Gold Member

    Is the speed of light still defined in SI units as exactly 299,792,458 meters per second?
    If so, this would imply that the second would no longer be defined in terms of the cesium clock. It would then instead be defined as the time it takes for light to travel 299,792,458 meters in a vacuum.
     
  7. Nov 13, 2018 #6

    Dale

    Staff: Mentor

    The speed of light is unchanged and the second is still defined in terms of the cesium hyperfine transition. I am not sure what makes you think this is implied.

    The new definitions can be seen here:
    https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf

    You can see that the wording of the definitions of the second and meter have been changed, but not their meaning.
     
  8. Nov 13, 2018 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Changing the kilogram doesn't change the meter or the second.
     
  9. Nov 13, 2018 #8

    Buzz Bloom

    User Avatar
    Gold Member

    Hi Dale:

    I apologize for my senior moment brain lapse and careless reading. I somehow got it into my head that the meter was being redefined.

    Regards,
    Buzz
     
  10. Nov 13, 2018 #9

    Dale

    Staff: Mentor

    Ah, makes sense.

    The excitement is all about getting rid of the international prototype kilogram.
     
  11. Nov 15, 2018 #10
    So this video states that not only is kg changing, but they are also now fixing the values of Planck's constant, Avagadro's number, and even the Ampere and the Kelvin!

     
    Last edited by a moderator: Nov 15, 2018
  12. Nov 15, 2018 #11

    Dale

    Staff: Mentor

    Yes, there will no longer be any physical prototypes and also they are harmonizing all of the definitions to be of the “defined constant” type. The units will no longer be defined either by a prototype or by a specific experiment. The experiments will serve to realize a unit with a given precision, but will not be the definition.
     
  13. Nov 16, 2018 #12

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Last edited by a moderator: Nov 16, 2018
  14. Nov 16, 2018 #13

    Dale

    Staff: Mentor

    Yay! It was expected, but still it is good to have it official
     
  15. Nov 16, 2018 #14

    kith

    User Avatar
    Science Advisor

    There was a live stream of some lectures and the final vote at the 26th General Conference on Weights and Measures today. There's a recording on youtube .
     
  16. Nov 16, 2018 #15
    So, if I have some matter, and I want to know (as best I can) what the mass is, I have to put it in something like a Watt balance. That is, I have to put it in something that allows me to relate it to Planck's constant, the meter and the second. There is no set way to do this, the Watt balance is just one option. Correct?
     
  17. Nov 16, 2018 #16

    Dale

    Staff: Mentor

    That is correct. There is no special definitive measurement technique.
     
  18. Nov 16, 2018 #17

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Many will interpret this statement to mean they will have to do something differently when they weigh something.

    That of course is not the case. In fact, even for the government regulators, the process they use to calibrate the standards will not change.

    The only thing that will change is the standard itself, and that change is of such a small magnitude as to be totally negligible for the purposes stated above.
     
  19. Nov 16, 2018 #18
    Well, suppose I want to explain this to a bunch of high school students. I think that what I'd say is something like "to find the mass of an object precisely as possible, one has to place the object in a device that allows you to relate it as best you can to Planck's constant, the second, and the meter, such as a Watt balance. The Watt balance, if it uses the quantum hall effect, Josephson junctions, etc. will give you an equation where m = hp(n^2)(f^2)/(4gv) where h is planck's constant, p and n are whatever the hell they are, f is the frequency from the JJunctions, g is local gravitational filed strength, and v is the speed that the mass went at in the Watt balance. But a different device will relate the mass to Planck's constant, the meter, and the second differently."

    That's not great, but I think it gives a student a better idea of what is going on than “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νCs.”
     
  20. Nov 16, 2018 #19
    There seems to be a rather significant underlying issue here, notable in that it has been touched on but not really explored in the coverage of this redefinition.

    I understand that there has been some observed drift in the mass of the Reference Kilogram, and this addresses that rather directly, BUT---

    One thing that is going on here is the shift from an empirically based definition to one that exists as a defined term. This strikes me as pretty major (I know it isnt the first quantity to be so redefined in the last century). Any thoughts on the implications or simply the evolution (pro? con? indifferent?) of moving from empirical to defined standards for metrological quantities?

    diogenesNY
     
  21. Nov 16, 2018 #20

    gneill

    User Avatar

    Staff: Mentor

    Defined standards based on (presumably constant) universal constants would seem to be a win. Constants don't ablate, rust, wear, dent, absorb or outgas material or do other sneaky things as physical objects tend to do over time.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted