An interactive simulation demonstrating Einstein's Elevator thought experiment. The learner toggles between an accelerating rocket in deep space and a stationary elevator on Earth to observe the indistinguishability of acceleration and gravity.
Learn MotionBlack Holes: Physics, Structure, and Theory
Foundations of General Relativity
The Equivalence Principle
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Einstein's greatest insight was the Equivalence Principle. Imagine you are in a windowless elevator. If you drop a pen, it falls to the floor. Is this because Earth is pulling it down? Or is the elevator accelerating upwards in deep space? According to the principle, these two situations are physically identical. You cannot tell the difference by any local experiment. This implies that gravity is not a traditional force, but rather a feature of the reference frame itself.
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