Rest in Peace: Stephen Hawking, Scientist, Physicist(1942–2018)

Stephen Hawking's equation: Black Holes

${\displaystyle S_{\text{BH}}={\frac {kA}{4\ell _{\text{P}}^{2}}},}$

where ${\displaystyle A}$ is the area of the event horizon, calculated at ${\displaystyle 4\pi R^{2}}$, ${\displaystyle k}$ is Boltzmann's constant, and ${\displaystyle \ell _{\text{P}}={\sqrt {G\hbar /c^{3}}}}$ is the Planck length. This is often referred to as the Bekenstein–Hawking formula. The subscript BH either stands for "black hole" or "Bekenstein–Hawking". The black-hole entropy is proportional to the area of its event horizon ${\displaystyle A}$. The fact that the black-hole entropy is also the maximal entropy that can be obtained by the Bekenstein bound (wherein the Bekenstein bound becomes an equality) was the main observation that led to the holographic principle.
He died earlier today, Pi day.