Einstein's vision Einstein spent the last forty years of his life trying to unify gravity and electromagnetism in a way that also lead to a new subtle understand of quantum mechanics. Just like for special relativity and general relativity, he was looking for a new logical foundation that would not change any experimental tests much at all.
Earlier versions are still accessible.
Most of today's research on gravity involves quantum gravity theory. I have two ideas in this direction. If that form of the force equation is correct, then if hbar goes to zero, then the unified force disappears. This is a basic characteristic of any quantum theory. What puzzled Einstein for decades was the why of quantum mechanics.