  We present a new method for inferring the gravitational potential of the
Galactic disk, using the time-varying structure of a phase-space spiral in the
$(z,w)$-plane (where $z$ and $w$ represent vertical position and vertical
velocity). Our method of inference extracts information from the shape of the
spiral and disregards the bulk density distribution that is usually used to
perform dynamical mass measurements. In this manner, it is complementary to
traditional methods that are based on the assumption of a steady state. Our
method consists of fitting an analytical model for the phase-space spiral to
data, where the spiral is seen as a perturbation of the stellar number density
in the $(z,w)$-plane. We tested our method on one-dimensional simulations,
which were initiated in a steady state and then perturbed by an external force
similar to that of a passing satellite. We were able to retrieve the true
gravitational potentials of the simulations with high accuracy. The
gravitational potential at 400-500 parsec distances from the disk mid-plane was
inferred with an error of only a few percent. This is the first paper of a
series in which we plan to test and refine our method on more complex
simulations, as well as apply our method to Gaia data.
