Richard Feynman made the following statement:
the relationship between the gravity forces and electrical forces remains unknown
In context, he made this statement partly as to contrast the well-known relationship between magnetism & electricity.
As I have read, the energy in the electromagnetic field contributes to gravity by affecting spacetime curvature, which surely Feynman would have understood. So what might he have been referring to instead?
the energy in the electromagnetic field contributes to gravity by affecting spacetime curvature, which surely Feynman would have understood
That's for sure.
I think he meant if gravity can be derived from the e.m. field, or the other way round. In the twenties (of last century, obviously), Kaluza and Klein proposed a five-dimensional spacetime to include electromagnetism (not knowing they laid the foundations for modern string theory). You can Google for this theory.
