Recall cards on Einstein's general relativity and its most extreme prediction. The theory itself: the equivalence principle, spacetime curved by matter, and gravity as that curvature. The classic tests: Mercury's perihelion precession, the 1919 eclipse deflection of starlight, gravitational redshift and time dilation, and the relativistic corrections that keep GPS accurate. Black holes: the event horizon, the Schwarzschild radius, the singularity, spaghettification, and the core mass needed to form one. The evidence: X-ray binaries, accretion disks, Cygnus X-1, and the supermassive black holes at galactic centers. And gravitational-wave astronomy: the binary pulsar PSR 1913+16, LIGO, and the 2015 black-hole and 2017 neutron-star mergers.
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