eli5: Why was E=mc2 so revolutionary?

Question

eli5: Why was E=mc2 so revolutionary?

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Answer 1

Weirdly enough, it wasn’t. Kind of.

The idea of mass-energy equivalence had been floating around for a while by 1905. It was understood that there was some link between matter and energy, and *c* had been brought into that (in some cases using ideas about the ether), and ideas about the “fixed” kinetic energy of particles. So things had generally settled down to E = mc^2 or E = 1/2 mc^(2).

In particular, Poincaré was playing around with electromagnetic radiation and in 1900 reached E = mc^2 in that context, but not for all things.

Einstein’s 1905 paper on E=mc^2 is only a couple of pages long. It was an interesting afterthought to his main paper on special relativity, where he does a thought experiment and shows that if a body gives off *L* energy as electromagnetic radiation, its mass must go down by *L/c^2*.

That led him down the path of mass-energy equivalence, and helped explain violations of conservation of mass.

Others built on this, and Felix Klein provided a generalised proof of E = mc^2 in a 1918 paper.

Anyway. As for its uses, E = mc^2 can be used to predict the energy released from certain nuclear reactions. This is pretty important for nuclear energy (both nuclear power and nuclear weapons). The equation doesn’t really explain the huge energies that come out of radioactive decades, but it does let you calculate them if you know the mass changes. But it wasn’t until the 30s that this was put into practice.

In terms of public perception, E = mc^2 became famous due to the [July 1946 issue of Time](https://en.wikipedia.org/wiki/File:Einstein_-_Time_Magazine_-_July_1,_1946.jpg), which featured Einstein, a nuclear explosion, and the equation. The nuclear bombs (both tests and uses in war) were a major demonstration that E = mc^2 calculations could have real-world implications. Einstein used the attention the nuclear bombs (which he wasn’t particularly involved in) received to achieve a level of fame outside the scientific community, which he leveraged for political goals. E = mc^2 – due to its simplicity (and relative importance) became the symbol of that.