When Einstein was seeking his Unified Field Theory only two forces were involved i.e. the electromagnetic and gravitational.
Subsequently two additional forces have been recognised i.e. the electroweak and the (strong) nuclear.
When one looks at it, these appear in holistic mathematical terms very much as internal counterparts to the other two forces (which operate extensively throughout nature).
Indeed the weak force (which for example can be used to explain the radioactive disintegration of certain atoms) offers itself as the internal (negative) counterpart to the electromagnetic; likewise the nuclear force readily suggests itself as the internal (negative) counterpart to the gravitational.
I have already drawn attention to the natural complementary relationship as between the electromagnetic and gravitational forces. Whereas we would - literally - see the first as "light" the latter by contract would be "heavy".
In fact the holistic mathematical relationship as between electromagnetic and gravitational is as real to imaginary. Now it is postulated that both forces can phenomenally reveal themselves as waves or particles (which in turn are real and imaginary with respect to each other). In other words when one aspect - say wave - reveals itself as real the other remains imaginary (and vice versa). This would further suggest that when the real aspect of one force is manifest that the alternative aspect of the other force is likewise made phenomenally manifest. So if the wave aspect of the electromagnetic force is real (i.e. manifest) then the particle aspect of the gravitational is likewise made real. (And both are deeply interdependent in this manner!)
Now it is fascinating that the very words popularly used to describe the two new i.e. weak and strong also suggest a complementary pairing. Indeed women have long been described in the cultural stereotype of the feminine principle as the "weak" sex (relating to intuition and the unconscious); men then by contrast are viewed as "strong" (relating - apart from physical attributes - to reason and the conscious mind). So the Western cultural stereotype values conscious reason (as "real") above unconscious intuition (as "imaginary").
So in fact the relationship of the two new forces weak and strong is likewise as real and imaginary with respect to each other (with both in turn representing the negative expression of the electromagnetic and gravitational forces respectively).
In this way we can provide a wonderfully simple mapping of the holistic mathematical relationship of the four forces to each other through the 4 complex roots of the 8 roots of unity.
Once again in holistic mathematical terms it is easy to see the reasons for Einstein's difficulties in unifying the gravitational and electromagnetic forces and indeed Quantum Mechanics with General Relativity.
Due to the reduced philosophical nature of conventional scientific interpretation, the whole - in any suitable context - cannot be properly distinguished from the part. So effectively wholes are treated simply as "aggregate" parts (without any proper qualitative distinction being maintained).
So the relationship of Quantum Mechanics (dealing with minute parts) to General Relativity (dealing with the whole cosmos) is as real to imaginary.
However in relative independence from each other, both Quantum Mechanics and General Relativity can be given a real scientific interpretation (in the conventional sense).
However in relation to each other this breaks down. So if Quantum Mechanics is treated as "real" then General Relativity is "imaginary"; likewise if General Relativity is "real" then Quantum Mechanics is "imaginary".
What this actually implies that we cannot properly incorporate both aspects without including holistic (qualitative) as well as analytic (quantitative) understanding.
In other words, comprehensive scientific interpretation requires, in qualitative terms, a complex rational approach that can successfully combine both real (analytic) and imaginary (holistic) elements.