General Relativity theory, developed by Einstein in 1907-1915, states that being at rest in the gravitational field and accelerating are identical physically. For example, an observer can see the ball fall the same way on the rocket and on Earth. This is due to the rocket’s acceleration, which equals 9.8 m/s 2.

Gravity defines macroscopic behaviour, and so general relativity describes large-scale physical phenomena such as planetary dynamics, the birth and death of stars, black holes, and the evolution of the universe.

Black holes and the expanding universe are two concepts that have their roots in general relativity. Even GPS satellites must account for general relativistic effects to deliver accurate position measurements to people on Earth.