This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so.
To resolve this difficulty Einstein first proposed that spacetime is curved.
Maxwell's equations—the foundation of classical electromagnetism—describe light as a wave that moves with a characteristic velocity.
The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond.
Special relativity is based on two postulates which are contradictory in classical mechanics: The resultant theory copes with experiment better than classical mechanics.
For instance, postulate 2 explains the results of the Michelson–Morley experiment.
As such, it employs an analytic method, which means that the elements of this theory are not based on hypothesis but on empirical discovery.
By observing natural processes, we understand their general characteristics, devise mathematical models to describe what we observed, and by analytical means we deduce the necessary conditions that have to be satisfied.
Its mathematics of general relativity seemed difficult and fully understandable only by a small number of people.
In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation.
The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 18 were critical to its validation.
It introduced concepts including spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction.
In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age.
The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.