Ciro Santilli Unifies both special relativity and gravity.
Not compatible with the Standard Model, and the 2020 unification attempts are called theory of everything.
One of the main motivations for it was likely having forces not be instantaneous, but rather mediated by field to maintain the principle of locality, just like electromagnetism did earlier.
Subtle is the Lord by Abraham Pais (1982) page 22 mentions that when Einstein saw this in 1915, he was so excited he couldn't work for three days.
Combination of electromagnetism and general relativity. Unlike combining quantum mechanics and general relativity, this combination was easier.
TODO any experiments of interest at all?
In 2020 physics, best explained by general relativity.
TODO: does old Newtonian gravity give different force results than general relativity?