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What does Einstein’s equivalence principle say about gravity and acceleration?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

Einstein’s equivalence principle is a foundational concept of general relativity that asserts, on a local scale, it is impossible to distinguish between the physical effects of gravity and those of acceleration.12

This principle suggests that the forces experienced in a gravitational field are of a similar nature to those experienced in an accelerated frame of reference. For example, a person inside a sealed, falling elevator cannot perform any experiment to determine if they are in a state of free-fall within a gravitational field or if they are in an inertial state of uniform motion. Conversely, the effects observed in an elevator being accelerated upward are indistinguishable from those produced by a stationary elevator resting in a gravitational field.321

By establishing this equivalence, Einstein concluded that gravity must influence light, causing its path to curve just as it would in an accelerated system. This insight led to the realization that gravity should be treated as a geometric phenomenon, where the presence of mass curves space-time.124