Whitehead explained relativity to Einstein. Philosophy was a second career for Whitehead, after he retired from mathematics. He is hard to read, but he has to be read first hand since translations suffer from the usual failure to transmit meaning.
Whitehead's Principles of Relativity - published almost a decade after Einstein's theory of relativity - is based on a fixed geometry of space/time. As Gibbons and Will note in the first article linked below:
The principal motivation behind Whitehead’s alternative to Einstein’s theory was the desire to retain fixed, non-dynamical, background-independent, causal relations between spacetime events which do not depend upon one’s location in spacetime.
IOW, Whitehead was mathematically and philosophically protesting the “observer problem” which Einstein's theory made so clear. The measurements of the CMB from the 1960's and myriad other observations since show that space/time is not fixed, indeed does not pre-exist, the universe (space/time) is expanding, there was a beginning of real space and real time.
As a philosophy, it retains merit – but as science, it is debunked.
On the Multiple Deaths of Whitehead's Theory of GravityModern Cosmology and Process Philosophy