When Albert Einstein first published the Special Theory of relativity in 1905, he was ridiculed. People thought it was just too weird and radical to be real. Einstein wasn’t satisfied with his theory either, because the theory did not apply if Gravity was present or if the observer was accelerating.
One day, while observing a window washer on a ladder near his patent office, he had a thought experiments. He imagined what would happen if the worker were to fall. He put himself in the window washer’s perspective, and imagined what he would experience as he was falling.
He realized that if he was falling, gravity would be the only force acting on him. He would be accelerating towards the ground, but since the ground would not be pushing up on his body, he would feel no weight.
And this would be no different than being weightless in space. In a way gravity and acceleration were different ways to describe the same thing.
The way to connect gravity in the theory of relativity was through the idea of acceleration, and this became the basis of general relativity. Einstein imagined being in a room with no windows, and a bathroom scale.
It would weigh 80 Kgs, What if the room was on a space ship accelerating in an upward direction at 9.8 m/s/s. He would feel the same weight. There would be no difference He imagined what would happen if he took a flashlight and pointed it from one side of the room to the other, as the space ship was accelerating upwards. And many more examples.
Then he realized, maybe the shortest path between two points is not a straight line but a curved line near gravity. This was the key insight that Einstein had about gravity. But in order to express this mathematically, it required very complicated mathematics that even a genius like Einstein could not easily figure out.
General relativity does not answer everything. Although it tells us how gravity works, it doesn’t really tell us what exactly it is.
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