How did Einstein plan to Unify Electromagnetism and Gravity?

Question

I'm asking the question on this site because I'm more concerned with the physics and philosophy of unification than historical details

I know that Einsteins description of gravity includes curvature of spacetime, how it is caused by energy density and how it affects the trajectory of particles whereas electromagnetism was treated as a field that exists on spacetime and was known to exist as quanta.

Considering these two very different description of forces, how did he plan to describe them both on an equal footing? Were there any interesting ideas or concepts worth noting that he used? (By "plan" I mean some sort of physical or philosophical arguments which he felt he could exploit to achieve it. If he didn't have such arguments, I don't think he would have chased it.)

I also know that both these forces can be described on an equal footing in terms of something called "gauge fields" each of which is based on a "symmetry" like our modern description of forces in the Standard Model (If i'm wrong please correct me). But unifying forces in terms of symmetries makes use of a concept called "spontaneous symmetry breaking" which wasn't widely known about until much later.

I also want to know whether quantum mechanics is essential to unify forces since all forces can be described classically, (I believe they were voluntarily "quantized" by physicists to accurately describe nature) and if not can we unify forces classically and then "quantize" the resulting field?

By "plan" I mean some sort of physical or philosophical arguments which he felt he could exploit to achieve it. If he didn't have philosophical or physical arguments, I don't think he would have chased it.

Answer 1

I don't think Einstein had a particular plan on how to unify EM and gravity. He did however collaborate with many researchers including Kaluza, who came up with the important theory of gravity in five dimensions (Kaluza-Klein theory), which gave rise to 4D gravity and electrodynamics. This is important because later (quantum) string theory turned out to require many extra dimensions (26 for bosonic strings, 10 for superstrings), which can be compactified in the Kaluza-Klein manner.

About Eintein's gravity as a gauge theory see answers to this post, for example.

Quantum mechanics (or rather, quantum field theory) is essential for any theory that is supposed to work at small scales. Furthermore, not all forces have valid classical description. Only gravity and electromagnetism do, since they are long-range forces. Other two act on very small distance scales (thus, nuclear forces).

Next, gravity is very different from the other forces, so unification of electromagnetism, weak, and strong forces is one story (Grand Unification), but unification of ALL forces is another story (String theory), which is supposed to include both Grand Unification, and quantization of gravity.