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Adam Brown – A deep but accessible introduction to general relativity
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The three ways we know black holes are real
At 1:23:00 · chapter starts 1:18:51
The distant observer sees something strange: as the infaller approaches the event horizon, gravitational time dilation makes them appear to move slower and slower, their light redshifts from visible to infrared to radio, and eventually they just fade — the last photon arrives, and then silence. The observer never crosses, as seen from outside. But from the infaller's own perspective, their clock ticks normally, the horizon is unremarkable, and for a galactic-mass black hole the tidal forces at the horizon are too weak to feel [1] — Adam Brown "From outside, you never see someone cross the event horizon — they slow, redshift, and fade. From inside, the infaller notices nothing unus…" 1:13:03 . Brown emphasises that the event horizon is 'teleological' — it is not a locally measurable feature but a fact about your future. For a large enough black hole you could have descendants born inside the event horizon, live out a normal life, and only die at the singularity.
Two eclipse expeditions failed before 1919 — Argentina got clouded out, and a German team was arrested when WWI broke out. That was lucky: Einstein's pre-GR prediction was wrong, predicting only the Newtonian bending. He corrected it to double the Newtonian value during the quiet of the war, just in time for Eddington's successful 1919 expedition.
General relativity predicts that gravity bends light passing the Sun at exactly double the amount predicted by Newtonian physics, confirmed by Eddington's 1919 eclipse expedition.
Sir Arthur Eddington's 1919 British eclipse expedition confirmed Einstein's corrected light-bending prediction, launching Einstein as a global celebrity and making GR the consensus view.
