What does E=mc2 really mean, and how are holograms made?

We explore the curious questions that science can answer

Out of the equation: Einstein’s theory of relativity changed the way we see the universe (Granger/Shutterstock)

What does E=mc2 actually mean?

That energy is the same kind of “thing” as mass, except it’s expressed in a different way. (This really is a very profound fact about the universe.) It also means that energy can be turned into mass and mass can be turned into energy.

E stands for energy, the m stands for mass, and c is the speed of light – already a large number which gets even bigger (90,000,000,000, 000,000 J/kg or so) when you square it. That is the exchange rate between mass and energy: the power released in an atomic explosion comes from that conversion. The Hiroshima bomb, with just 10kg of plutonium, only actually converted about 1g into pure energy – but it was enough to wipe out a city.