“According to general theory of relativity, gravitation is not a force but a property of spacetime geometry. A test particle and light move in response to the geometry of the spacetime.”[1] Einstein's interpretation of gravity is purely geometrical, where even a free point particle without any properties and any interactions, moves in a curved spacetime along geodesics, but which are generated by the energy tensor Tµν [2]. Why isn't gravity generated directly by Tµν, but must take a circuitous route and be generated by the geometry of spacetime Gµν‡?
Gµν=G*Tµν
This is Einstein's field equation, and the Einstein tensor Gµν describes the Space-Time Curvature. We know that in classical mechanics and quantum field theory, it is the Hamiltonian, Lagrangian quantities that determine motion. Motion is essentially generated by energy-momentum interactions. Why is it irrelevant to energy-momentum in GR? Einstein had always expected the unification of electromagnetic and gravitational forces to be geometrically realized [3]*. Is such an expectation an exclusion of energy-momentum interactions in motion? Can the ultimate unification of forces be independent of energy-momentum and manifest itself only in motion in pure spacetime? If not, one of these must be wrong.
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Notes
* "After his tremendous success in finding an explanation of gravitation in the geometry of space and time, it was natural that he should try to bring other forces along with gravitation into a “unified field theory” based on geometrical principles."
‡ If one thinks that it holds only at Tµν = 0, see the next question NO.37: Is there a contradiction in the Schwarzschild spacetime metric solution?
https://www.researchgate.net/post/NO37Doubts_about_General_Relativity_2-Does_the_Energy_Tensor_Tmn_in_the_Field_Equations_Contain_the_Energy-momentum_of_the_Spacetime_Field
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References
[1] Grøn, Ø., & Hervik, S. (2007). Einstein's Field Equations. In Einstein's General Theory of Relativity: With Modern Applications in Cosmology (pp. 179-194). Springer New York. https://doi.org/10.1007/978-0-387-69200-5_8
[2] Earman, J., & Glymour, C. (1978). Einstein and Hilbert: Two months in the history of general relativity. Archive for history of exact sciences, 291-308.
[3] Weinberg, S. (2005). Einstein’s Mistakes. Physics Today, 58(11).