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tags: #physics #relativity #gravity

The Principle of Equivalence is the central insight that led Einstein from Special to General Relativity. It states that, locally, the effects of gravity are indistinguishable from the effects of acceleration.

Einstein's thought experiment:
Imagine you are in a windowless room. You drop a ball, and it falls to the floor. You cannot tell whether you are:

1. In a room at rest on the surface of the Earth, experiencing gravity.
2. In a rocket ship in deep space, accelerating "upwards" at 9.8 m/s².

This equivalence between gravitational mass and inertial mass (an observation first noted by Galileo's Gravity Experiments) has a profound implication: gravity is not a force. If it were a force, it would be a remarkable coincidence that it produces the exact same effects as pure acceleration.

Instead, this principle suggests that gravity is a feature of spacetime itself. This idea is the foundation for the theory of Spacetime Curvature.

Reference

Coursera, "Question Reality: Matter"