# What do the current SI definitions of kilogram and Kelvin actually mean?

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As per [Wikipedia](https://en.wikipedia.org/wiki/International_System_of_Units), the new definition of kilogram is, “The kilogram is defined by setting the [Planck constant](https://en.wikipedia.org/wiki/Planck_constant) *h* exactly to 6.62607015×10−34 J⋅s (J = kg⋅m2⋅s−2), given the definitions of the metre and the second.”, while that for Kelvin is, “The kelvin is defined by setting the fixed numerical value of the [Boltzmann constant](https://en.wikipedia.org/wiki/Boltzmann_constant) *k* to 1.380649×10−23 J⋅K−1, (J = kg⋅m2⋅s−2), given the definition of the kilogram, the metre, and the second.”

These are the only two, in the list of 7 which don’t help form an idea of what that quantity actually is.

In: Physics If you look up mathematical constants and physical constants and getting a formula from a relatively non changing or constant figure.

Since May 2019, all of the SI base units have been defined in terms of physical constants. As a result, the fundamental physical constants: the speed of light in vacuum, c; the Planck constant, h; the elementary charge, e; and the Boltzmann constant, kB, have known exact numerical values with a finite number of digits when expressed in SI units. The Planck constant is energy times time, which is momentum times distance, and momentum is force times velocity, and force is how you accelerate mass.

So, with energy, distance, and time, you know mass. Originally, the base SI units were defined using physical object. There was a reference kilogram for example. But as our understanding physics and ability to measure things developed, it was discovered that the reference objects were not the sizes we thought they were and that they would slowly change over time. Reference kilogram’s mass would unavoidably change over time. Instead, the units were redefined using fundamental constants, things that would never change. However, it is still useful to visualize the units using the old definitions outside of technical contexts. For example, a gram being the mass of cubic centimeter of liquid water is much easier to visualize and is closer enough to the real value that you can use it most of the time. There is a bit of debate in the interpretation of a unit in the first place. The whole thing of measuring is rather arbitrary. Historically we have always just used what’s convenient (hands, feet for length, stones for weight for example). In SI, we want to define very specifically what these things are, so that we can be precise about what we mean when we say kilogram or second or kelvin.

It’s quite natural to define things in terms of physical constants, since these are numbers that are pretty universal. If you found extraterrestrial life, you could explain to an alien exactly how a kilogram is defined using this language, and the *only* assumption is that physics has not changed.

For example, you mention the definition of a second, that’s very intuitive. Say “take the atom with 55 protons and 78 neutrons and start counting” instead of “ok come to earth and measure one full revolution of the sun then divide by 3600”. One of which is far more universal than the other, and also much more specific

The physical constants you mention actually are also quite intuitive as well. For example, the planck constant can be found by saying “take the atom that is just one proton and one electron and put it in a magnetic field, h is the difference between the lowest and second lowest energy”.

These are numbers that exist everywhere, and things that have “real” meaning, since they correspond to universal physical phenomena. Things like meters, kilograms, etc are arbitrary scales that don’t really have meaning on its own, so in order to give them meaning we use the physical constants to define them, in a super roundabout way One way of looking at it is that if you convert a kilogram of mass into a single photon of energy, it would have a frequency equal to the reciprocal of the Planck constant.