Around 1870, William Clifford published a short note in The Proceedings of the Cambridge Philosophical Society describing all forces as an aspect of the curvature of space.
This is the basis of General Relativity as well as the gauge theory of Electromagnetism. Whilst it's well known that curvature is the manifestation of gravity, it is less well-known that this is also the case for electromagnetism (also for the weak and strong force).
(It's also possible to read Cliffords note as also stating that matter is also an aspect of curvature, and this is how supergravity theorises matter).
Q. Is there any documented evidence that Einstein knew of Cliffords bold speculation?