Why gravity and acceleration are indistinguishable — Einstein's happiest thought
In 1907, Einstein realized that a person in free fall feels no gravity. Conversely, an accelerating rocket in empty space feels exactly like gravity. This equivalence principle — that gravitational and inertial mass are identical — became the foundation of general relativity. It implies that gravity isn't a force at all, but a manifestation of curved spacetime.
No experiment performed inside a closed elevator can distinguish between uniform gravitational acceleration and rocket acceleration. Drop a ball, throw a projectile, shine a light beam — the results are identical.
Try it: Toggle between the gravity and acceleration scenarios. Drop objects and watch their trajectories — they're indistinguishable from inside the elevator.
A photon climbing out of a gravitational field loses energy — its frequency decreases and its wavelength increases (shifts toward red). This is a direct consequence of the equivalence principle: in an accelerating frame, photons traveling "upward" are redshifted.
If gravity and acceleration are equivalent, then light must bend in a gravitational field. A horizontal light beam in an accelerating elevator curves downward — and by the equivalence principle, the same must happen near a massive object. This was confirmed by Eddington's 1919 solar eclipse observations.
In 1959, Pound and Rebka measured gravitational redshift directly by sending gamma rays up a 22.5-meter tower at Harvard. The tiny frequency shift of 2.46 × 10⁻¹⁵ matched Einstein's prediction precisely.