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Illustration of Einstein's principle of equivalence - there is a elevator moving up with a big acceleration, inside, there is a man with an electric torch pointed horizontally, but because of the acceleration, the man is distorted like smashed down in the direction of the floor under very high gravity, and even the beam of light of the torch is bending down, hand drawing in pencil with nervous thin lines and vivid colors, no text
Einstein’s elevator starts with a suspiciously simple idea: blindfolded inside a closed elevator, you cannot tell whether gravity is pulling you down, or you are in free space and some maniac has attached a rocket to the bottom of the cabin and is accelerating you upward. In either case, an experiment will show that objects you let go of inside the cabin accelerate downwards. Now shine a beam of light horizontally across the elevator. As the elevator accelerates upwards, the beam appears to bend downwards. And if the effects of acceleration and gravity are locally equivalent, Einstein reasoned, gravity must bend light too. Observations during the solar eclipse of 1919 by expeditions led by British astronomer Arthur Eddington famously found starlight deflected by the Sun, turning Einstein into a scientific celebrity. Today we see the same trick on a gigantic scale: galaxies and galaxy clusters bend and distort the light of objects behind them - this is called gravitational lensing. Not bad for a thought experiment involving an elevator with extremely questionable safety standards.
As a little aside, Sir Arthur Eddington was a seriously enthusiastic long-distance cyclist (so: very sympathetic to me!) - and he invented the notorious “Eddington number.” Check it out if you are curious! - But watch out: there are two Eddington numbers... You do not want the one involving protons, but the one involving bicycles. ;-)
