# Lesson 18: <br>Special Relativity, 4

> Einstein's Mass and Energy

```{admonition} &nbsp; What's Coming:
Here we go. The T-shirt equation. Nothing is the same after this fourth of Einstein's 1905 papers...that one that said $m=\dfrac{E}{c^2}$. We'll develop it without calculus (although if you want to see a full derivation, it's included for those with that background). Energy now simplifies! Mass becomes interesting! Particle collisions become understandable! Matter gets made! And I get my white board on the Big Bang Theory!
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