I have been reading about the new SI units and specifically, want to get a better understanding of the definition of a kilogram. It was written that the kilogram will be defined in terms of Planck's constant $\mathrm{h}$, which has a newly accepted value of $6.626070150 × 10^{-34} \mathrm{kg⋅m^2s^{-1}}$. Now, I am also aware that Planck's constant relates the energy of a photon to its frequency.

But what does defining the kilogram in terms of Planck's constant mean, exactly? That is, when I measure $1\mathrm{kg}$ of a substance, what am I really measuring?

As an example, I read that whenever I measure the length of an object, I am really measuring the time it takes for light to travel between the two points of measurement.


marked as duplicate by John Rennie, Community May 14 at 8:45

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  • $\begingroup$ The linked question gives plenty of details. For a simple answer to your question here: whatever way the kilogram is defined, it is still used to measure a mass. $\endgroup$ – Stéphane Rollandin May 14 at 8:56