Special relativity is more fun

Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula E = mc², where c is the speed of light in vacuum

Applies to nature, can't go faster than nature, we are band limited.  Here is Einstein:

The insight fundamental for the special theory of relativity is this: The assumptions relativity and light speed invariance are compatible if relations of a new type ("Lorentz transformation") are postulated for the conversion of coordinates and times of events... The universal principle of the special theory of relativity is contained in the postulate: The laws of physics are invariant with respect to Lorentz transformations (for the transition from one inertial system to any other arbitrarily chosen inertial system). This is a restricting principle for natural laws...^[7]

What he says is that any invented coordinate system that has nothing to do with nature must obey some simple rules of translation. Space and time are engineering units, so this is all about telling engineers to be careful, and make sure some rules of symmetry and orthogonality are maintained.

Experiments suggest that this speed is the speed of light in vacuum

This statement came from considerations of homogeneity of any coordinate system that has right angles must have a light speed, or an equivalent. They get this if a fairly easy manner by showing one has to have an invertable multiply in your coordinate system, because reversing things must be possible.

None of this has anything to do with nature, but it is all about instructions to engineers using coordinates with a right angle.  E=MC^2 simply states that the engineer has to count uniformly, the C being the counter, the E being a 'divide by zero' if his counter does not work.

But, here is a hint.  These rules apply to nature as well.  Nature has to count uniformly, nature needs a simple multiply, and has to have a speed of light. There is one simple proof of this, the proton with a lifetime of 10E35 years. For all practical purposes, that means that nature is a counting system.