Professor Michael Merrifield, from The University of Nottingham, UK, explains why we need to move on to new ways to define a kilogram. Derek Mueller presented the silicon sphere from Australia, which should be used as a successor to the kilogram from France, but these have limitations as they can lose or gain mass throughout the tens of years by handling them or by simply letting them be in a closed room.

The race to redefine the kilogram is on. Right now the scientists want to redefine the kilogram in such a way that the Planck constant will have the fixed value of:

6.62606957(29)×10^−34 Js

The Planck constant, h is:

h = ν / E = E * t, where E is energy and ν is the frequency and t is time

Other units of measurement have been changed, like the meter which is the distance the light travels in 1/ 299 792 458th of a second. A second is precisely the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. These units, seconds and meters, have now been defined using absolute measurements. The kilogram will soon follow.

If we take the frequency of the transition between the two hyperfine levels of the ground state of the caesium 133 atom which we use to define the second and want to get the mass of the photon emitted that way, we now have E = h * ν = m * c^2.

We know h, c, and v and thus we get the measurements for the “mass” of that photon: m = h * ν / c^2. When we compute the numbers get m photon = 6.777265 * 10^-41 kg. That means that a kilogram would have these many “photon masses”:

1 kg = 1,4755214677307143810962091640212e+43 such masses

And here it goes: the kilogram is now defined using absolute measurements and there is no need for the, now obsolete, prototype from France. Well done.

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