Why dont we use measurements based on natural constants?

Question

For example, why isnt the unit of electric charge the same charge found in 1 electron/proton? Or speed/distance/time based on the speed of light, and some fundamental measure of distance/time?

It seems like this would help in finding relationships between things, but maybe thats just me.

Don't tell me that "the size of these units isn't convenient". This is why we use prefixes. (kilometer, kilogram, centimeter, etc.)

"Actually, the metre is defined as "the distance in which light travels in 1/299792458 of a second in a vacuum".
The second itself is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom".
So, evidently, we do use constants present in nature for our measurements. Americans on the other hand..."

What are you talking about? What is so significant about cesium and 1/299792458 of a second?

Answer 1

Actually, the metre is defined as "the distance in which light travels in 1/299792458 of a second in a vacuum".
The second itself is defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom".
So, evidently, we do use constants present in nature for our measurements. Americans on the other hand...

Answer 2

We do. All scientific units, except for mass, are based on natural constants. The reason for defining the metre, for instance, as the distance light travels in 1/299792458 second, rather than say 1/300000000 second, is that each new definition has to agree with the old one.

1 kg is still the mass of a lump of metal in Paris. There are plans to change this definition and relate it to natural constants, such as the mass of a certain number of atoms, but that will only happen when we have the technology to make the new definition more accurate and stable than the existing one.

Answer 3

When the units themselves were first defined to be convenient at the time. For example:
One meter used to be defined as: "one ten-millionth of the length of the meridian through Paris from the pole to the equator". For measuring traveled distances, this would be a very convenient unit, easily comparable on any standard map.
The second, also, fits very well into the base-12 and base-60 number systems of ancient civilizations (60s/min, 60min/hr, 12hrs/day or night), to whom time was a measure only of the progress of a day. If we were to invent a new unit of time, would it not make sense to define it such that some multiple of 10 seconds would fit a relevant time scale to us?

It's important to consider that when many of these units were originally standardized, modern natural references were not available, or even known.

Answer 4

You make a good point.
The only reason is history. People just did not know better when the units were developed. Now the SI system is popular and it is not likely that it will change.

It only will make constants go away like universal gas constant or the permittivity constant. Nothing really to write home about.