Theories of Newtonian gravity and empirical indistinguishability

In this essay, I examine the curved spacetime formulation of Newtonian gravity known as Newton-Cartan gravity and compare it with flat spacetime formulations. Two versions of Newton-Cartan gravity can be identified in the physics literature-a "weak" version and a "strong" version. The strong version has a constrained Hamiltonian formulation and consequently a well-defined gauge structure, whereas the weak version does not (with some qualifications). Moreover, the strong version is best compared with the structure of what Earman (World enough and spacetime. Cambridge: MIT Press) has dubbed Maxwellian spacetime. This suggests that there are also two versions of Newtonian gravity in flat spacetime-a "weak" version in Maxwellian spacetime, and a "strong" version in Neo-Newtonian spacetime. I conclude by indicating how these alternative formulations of Newtonian gravity impact the notion of empirical indistinguishability and the debate over scientific realism.