What is the equivalence principle?
Quick Answer
The equivalence principle says that gravitational and inertial mass are equivalent, and that free fall is locally indistinguishable from weightlessness. It is a cornerstone of general relativity.
The Full Story
The equivalence principle is the idea that all objects fall the same way in a gravitational field if other forces are absent, regardless of their mass or composition. Einstein used this insight as a starting point for general relativity, published in 1915. It implies that gravity and acceleration are deeply related, which leads to the concept that gravity is geometry rather than just a force in the Newtonian sense. The principle has been tested to extremely high precision in modern experiments.
Key Facts
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"Why gravity and acceleration can look the same"