[tialaramex]

So, the thing about all the definitions now is there are no prototypes (the kilogram was the last one). So Wittgenstein's observation now becomes an actual claim about how our universe works.

It feels intuitively obvious/ redundant that a prototype metre is one metre long, but a statement that light in a vacuum travels a fixed distance in one second is not so obviously redundant. It doesn't matter what colour the light is? Nope. It doesn't matter when I measure? Apparently not.

[xgk]

You cannot measure how fast light travels in a fixed time unit without reference to time: you need to define length! Length is currently defined with reference to caesium time: the 2019 SI definition of metre takes "the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs." (From [1].)

As far as I can see, measuring time is the foundation of all definitions of other units. And the core reason why time is used to define everything else, is pragmatic: it's just technically easier to count (photon absorption) than to do anything else.

[1] https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

[AstralStorm]

It may matter how fast you're moving non-inertially and how massive nearby objects are. (General and Special Relativity) And then, there might be advanced quantum effects we don't know about yet.