Measurement of separate events must satisfy these conditions and match the theory's conclusions.

Relativity is a falsifiable theory: It makes predictions that can be tested by experiment.

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Around 1960, general relativity became central to physics and astronomy.

New mathematical techniques to apply to general relativity streamlined calculations and made its concepts more easily visualized.

The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example, when standing on the surface of the Earth) are physically identical.

The upshot of this is that free fall is inertial motion: an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics.

It seemed to offer little potential for experimental test, as most of its assertions were on an astronomical scale.

Its mathematics of general relativity seemed difficult and fully understandable only by a small number of people.

These are the Michelson–Morley experiment, the Kennedy–Thorndike experiment, and the Ives–Stilwell experiment.

Einstein derived the Lorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.

It introduced concepts including spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction.