[Physics] Why didn’t we replace our SI units with a better system?

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It seems to me that the SI units we use today are nothing but the result of a historical 'coincidence'.

I recently began researching about natural (absolute) systems of units, which are defined in such a way that selected universal physical constants are normalized to unity. These are very convenient, since almost all equations in physics are simplified.

Planck Units

Take for example the Planck units, where five fundamental physical constants take on the numerical value of 1.

To quote from Wikipedia:

Planck units have a profound significance since they elegantly simplify several recurring algebraic expressions of physical law by nondimensionalization. They are particularly relevant in research on unified theories.

Planck units are even dubbed "God's units", since Planck units are free of anthropocentric arbitrariness.


I know that many 'everyday' quantities for a physicist in terms of Planck units would be very small numbers (for instance) but that shouldn't be a problem if we use scientific notation. In fact, it's a great thing since it can give us a better picture about those quantities being now in a unit system which is conceptually linked at a fundamental physical level. Besides, many such 'everyday' quantities are also expressed by very small / very large numbers in SI units. Think Planck's constant.

Frank Wilczek even argues that using Planck units would help us re-frame important questions in physics:

We see that the question is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in [Planck] units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number [1/(13 quintillion)].

AI am aware that it would be a big upheaval if we are to change our SI units but if it is to be done it better be done right now before more physicists grow up using the old unit system. The change is not likely to render old written material useless since, for instance, Newton's Principia Mathematica remains a fundamental text in mechanics even though it uses outdated notions such as "quantity of motion" and "quantity of matter".


Given all that I explained, is there a reason we choose to stick with the old and weary unit system?

Why didn't we leave the kilogram, defined by an arbitrary French rod, to ladies gossiping about Kim Kardashian's weight and choose something that makes more sense scientifically?

Best Answer

The short answer is that it is simply not possible to design a "one size fits all" unit system. The staggeringly large range of mass, time and length scales that appear in the Universe prevent this. The Planck unit system you mentioned is mainly useful for people who will never touch an experimental apparatus. The vast majority of scientists and engineers do not even do physics, let alone theoretical quantum or cosmological physics, and need a standard unit system that reflects the magnitude of quantities that are most likely to be found in their everyday work.

Thus, professional physicists use whatever unit system is most convenient for the problem at hand. There is no danger that "more physicists grow up using the old unit system", as if that would somehow obscure people's understanding. Actually the more redundant and bizarre unit systems that trainee physicists are exposed to, the better. This teaches you fluency in converting between these systems, and helps you to communicate with people from different subfields.