More than any other equation in physics, E = mc² is recognizable and profound. But what do we actually learn about reality from it?
For as long as humans had been conducting science, there was a seemingly immutable law of nature that was never violated: that in any reaction occurring in the Universe, mass was conserved. If you took a boulder and cleaved it into two, the two pieces would have the same mass as the initial, unbroken stone. If you devoured a massive dinner, your weight after you finished eating was the same as your weight before your meal plus the meal itself. And even with the advent of chemistry, whether you dissolved salts in a liquid, precipitated crystals out of a solution, or combusted a reactive substance with oxygen in the air, if you kept track of everything that went into the reaction with everything that came out of it, you’d find that mass was conserved.
In other words, no matter what you put in, what reacted, and what comes out of any before-and-after scenario, the sum of what you began with and the sum of what you ended with must be equal. Mass, as far as we could tell, was truly a conserved quantity. But once we uncovered the laws of special relativity, we realized that mass, alone, simply couldn’t be the…
