The kilogram used to be defined by a specific chunk of platinum-iridium.
But now it’s defined using the second and the Plank constant, somehow.
Assuming you already have a very accurate measurement of the second, and the Plank constant, how do you combine those two things to get the kilogram?
In: 1
The Planck constant gives the relation between the frequency and the energy of a photon. One way energy can manifest itself is in accelerating heavy objects; one Joule is the energy needed to accelerate a mass of one kilogram by one m/s². The Planck constant being 6.62607015×10^(−34) Joules per Hertz thus means that a single photon of a frequency of one Hertz fired at a mass of one kilogram will accelerate that mass by 6.62607015×10^(−34) m/s² (note: this is a thought experiment, not a viable experimental setup).
So, the kilogram being defined through the Planck constant means that one kilogram is however much mass can be accelerated by 6.62607015×10^(−34) m/s² by one single photon of a frequency of one Hertz.
The meter, by the way, is defined in a similar way through the speed of light; one meter is defined as “however far light travels in 1/299792458 second in a vacuum”.
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