gravity

• one of the four fundamental interactions, by far the weakest (between two protons, ~10⁻³⁶ of the electric force)
• always attractive, infinite range → dominates at large scales
• Newton: $F=\frac{G{m}_1{m}_2}{r^2}$, with $G=6.674\times 10^{-11}$ N m² kg⁻²
• general relativity: mass and energy curve spacetime, free bodies follow the straightest paths (geodesics)
• clocks run slower deeper in a gravity well (man what even is time?)
• GPS: satellite clocks gain ≈ 45 μs/day from weaker gravity and lose ≈ 7 μs/day from orbital speed (net ≈ +38 μs/day)
• hypothetical carrier: graviton (never detected), no complete quantum theory of gravity yet
• gravitational waves: first detected by LIGO in 2015 (two merging black holes of ~30 solar masses)