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A key idea in the Theory of Relativity. The upshot of "everything is relative".

If "all inertial observers are equivalent" (Einstein), it follows that two inertial observers, moving relative to each other, must be able to write down their physics formulae, their laws of physics, in the same way. If observers had to use different formulae depending upon their state of motion they could not all, with justice, say: "I'm still the other observers are moving." Perhaps it might be said the observer with the simplest formulae was at absolute (not relative) rest the others were in absolute motion.

Thus, for example, if one observer looks at a piece of light and writes vlight = 1/sqrt(eoµo) then the other should be able to look at the same piece of light and write v'light = 1/sqrt(e'oµ'o) where the primed quantities are the corresponding measurements made with the other's measuring apparatus - measuring apparatus at rest relative to the other observer. Both formulae should work - once the arithmetic has been done - the number to the left of the equals sign should equal the number to the right. The same should be true for all the formulae expressing "general" laws of physics. (eo and µo are the electric and magnetic permittivity of free space respectively.)

Many scientific formulae in everyday use are not form invariant. They work perfectly well in most circumstances. The form invariant version might well be over elaborate for everyday use.