Equivalence Principle
The foundational postulate of general relativity asserting that the physical effects of a uniform gravitational field are locally indistinguishable from uniform mechanical acceleration.
Governing formula
m_inertial = m_gravitational
SI unit
Fundamental Relativistic Principle
In depth
Einstein realized that an observer inside a closed elevator cannot distinguish whether the elevator is accelerating in deep space at 9.8 m/s² or resting on Earth's surface. Consequently, free-falling observers experience weightlessness.
Examples in the real world
Astronauts floating weightlessly in the International Space Station because both station and crew are in continuous gravitational free fall.