Albert Einstein radically reshaped the understanding of gravity through his proposal of the General Theory of Relativity in 1916. The problem he tackled was to create a theory of gravity that was consistent with his Special Relativity Theory, including its radical transformation of the understanding of space and time measurements through the introduction of a unified concept of space-time. Special relativity is based on the invariance of the laws of physics under any constant change of velocity, and hence the equivalence from a physical viewpoint of all uniformly moving (non-accelerating) observers. General relativity extended these ideas to a special class of accelerating observers.

Einstein's insights

Einstein's first brilliant insight was that, in consequence of Galileo Galilei's (1564–1642) discovery in the seventeenth century that all falling bodies...