Is There A Fundamental Reason Why E = mc²?

Ask anyone — even someone with no background in science — to name something that Einstein did, and odds are they’ll come back with his most famous equation: E = mc². In plain English, it tells us that energy is equal to mass multiplied by the speed of light squared, teaching us an enormous amount about the Universe. This one equation tells us how much energy is inherent to a massive particle at rest, and also tells us how much energy is required to create particles (and antiparticles) out of pure energy. It tells us how much energy is released in nuclear…