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Prism -    Anna Bjerger , 2021.

Swedish,b.1973-

This Is Why Einstein Knew That Gravity Must Bend Light

“This is the basis of Einstein's equivalence principle: the idea that an observer cannot distinguish between an acceleration caused by gravitational or inertial (thrust) effects. In the extreme case, jumping off of a building, in the absence of air resistance, would feel the same as being completely weightless.

The astronauts aboard the International Space Station, for example, experience complete weightlessness, even though the Earth is accelerating them towards its center with about 90% of the force we experience here on its surface. Einstein later referred to this realization, which struck him in 1911, as his happiest thought. It was this idea that would lead him, after four years of further development, to publish the General theory of Relativity.”

Imagine that you were inside a closed-off elevator, and that light came in through a tiny hole from the outside. If your elevator were accelerating, you’d see that light follow a curved, bent trajectory, as your changing motion would cause the path that light took to appear that way. But from inside the elevator, you have no way of knowing whether that acceleration was due to thrust, which is an inertial effect, or gravitation. An elevator accelerating at a constant 9.8 m/s^2 from a firing thruster would be indistinguishable from one stationary on Earth’s surface, where the acceleration due to gravity is 9.8 m/s^2. If they’re indistinguishable, then gravity must bend light, the same way any other acceleration does.