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# relativity

## Basic idea

Special relativity: physics is the same in all inertial frames; the speed of light is invariant. General relativity: gravity is the curvature of spacetime by mass-energy.

## Key formulas

* $E = mc^2$
* Lorentz factor: $\gamma = 1/\sqrt{1-v^2/c^2}$
* Spacetime interval: $ds^2 = -c^2 dt^2 + dx^2 + dy^2 + dz^2$
* Einstein field equations: $G\_{\mu\nu}+\Lambda g\_{\mu\nu}=\dfrac{8\pi G}{c^4}T\_{\mu\nu}$