  We utilize generalized unitarity and recursion relations combined with
effective field theory(EFT) techniques to compute spin dependent interaction
terms for inspiralling binary systems in the post newtonian(PN) approximation.
Using these methods offers great computational advantage over traditional
techniques involving feynman diagrams, especially at higher orders in the PN
expansion. As a specific example, we reproduce the spin-orbit interaction up to
2.5 PN order as also the leading order $S^2$(3PN) hamiltonian for an arbitrary
massive object. We also obtain the unknown $S^3$(3.5PN) spin hamiltonian for an
arbitrary massive object in terms of its low frequency linear response to
gravitational perturbations, which was till now known only for a black hole.
Furthermore, we derive the missing $S^4$ Hamiltonian at leading order(4PN) for
an arbitrary massive object and establish that a minimal coupling of a massive
elementary particle to gravity leads to a black hole structure. Finally, the
Kerr metric is obtained as a series in $G_N$ by comparing the action of a test
particle in the vicinity of a spinning black hole to the derived potential.
