this post was submitted on 03 Apr 2026
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It’s not about the units i used. It’s about using something to define itself. The same problem happens when you use c to define empty space since empty space can define c.
Once you decide which units are used in maxwells equations then the electromagnetic permeability and permissivity pops out as a proportions of c.
Read more Feynman if you don’t believe me.
That may be, and I've been meaning to dig into my copy of the Lectures, but that's moving the goalposts. You said that it was a tautology because it was defined by the meter, and the meter was defined on it. That statement is demonstrably false.
Everything in physics is defined by relative properties. Scale all fundamental units by the same factor and we can not detect any change in behavior whatsoever
Speed of light may be constant, but we can not make measure it through any other means then by measuring it in terms of ratios against other constants
I used the meter because that’s generally what is used for measurement in scientific endeavors. There was no goal post moving if the statement applies for all SI measurements.
Literally the entire point of the comment that you're responding to is that it isn't true for the metre, and it isn't true for any SI units.
Your entire claim of tautology rests on the assertion that the speed of light is defined by something external to light itself. That's false. It remains false irrespective of which SI measurements you swap in.
Just because the speed of light can be expressed in terms of SI units, doesn't mean its definition depends on them. Which is the point that wolframhydroxide was making.
This directly disproves your original assertion of tautology.
Every metric of speed of light is necessarily relative to other things. Even if you define as 1, now you must be able to know what one unit of time is relative to one unit of distance, and if you do not know that then you do not know that your speed of 1 means.
All fundamental units are defined relative to each other in physics, and all other units are defined relative to the fundamental units.
https://en.wikipedia.org/wiki/SI_base_unit
Even the Cesium time standard is defined relative to electric fields which are defined by time and distance and charges, and charges are defined by energy defined by force defined by time and distance and more....
But isn't the measurement of the speed of light our own proportion derived from the constant that is 1g of water at 1ATM?