This 1934 photograph shows Einstein in front of a blackboard, deriving Special Relativity for a group of students and onlookers. Although Special Relativity is now taken for granted, it was revolutionary when Einstein first put it forth, and it isn’t his most famous equation; E = mc² is. (Credit: public domain)

The true meaning of Einstein’s most famous equation: E=mc²

More than any other of Einstein’s equations, E = mc² is the one that most people can name. But what does it all mean?

For hundreds of years, there was an immutable law of physics that was never challenged: that in any reaction occurring in the Universe, mass was conserved. That no matter what you put in, what reacted, and what came out, the sum of what you began with and the sum of what you ended with would be equal.

But under the laws of special relativity, mass simply couldn’t be the ultimate conserved quantity, since different observers would disagree about what the energy of a system was. Instead, Einstein was able to derive a law that we still use today, governed by one of the simplest but most powerful equations ever to be written down, E = mc².

A nuclear-powered rocket engine, preparing for testing in 1967. This rocket is powered by Mass/Energy conversion, and E=mc². (Credit: ECF (Experimental Engine Cold Flow) Experimental Nuclear Rocket Engine, NASA)

There are only three parts to Einstein’s most famous statement:

1. E, or energy, which is the entirety of one side of the equation, and represents the total energy of the system.
2. m, or mass, which is related to energy by a conversion factor.