When two black holes or neutron stars spiral together they radiate spacetime ripples — gravitational waves. LIGO detected GW150914 on 2015-09-14: two 36+29 solar-mass black holes merging ~400 Mpc away. The strain h~10⁻²¹ stretches a 4-km arm by less than a proton diameter. Chirp mass M_ch = (m₁m₂)³/⁵/(m₁+m₂)¹/⁵ fully characterizes the inspiral. The merger time follows from energy loss by gravitational radiation (post-Newtonian approximation).
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