  It is known that a geometric measurement of the light cones of supernovae
determines the conformal class of the visible part of the spacetime. The
conformal factor is physically meaningful but cannot be determined
geometrically by anything with zero mass, such as the photon. We show that
measuring the neutrino cones in addition to light cones completely removes this
gauge freedom. We describe the physical model in great detail, including why
ultrarelativistic neutrinos are the only option.
