Canonical energy is a valuable tool for analyzing the linear stability of black hole spacetimes; positivity of canonical energy for all perturbations implies mode stability, whereas the failure of positivity for any perturbation implies instability. Nevertheless, even in the case of 4-dimensional Schwarzschild spacetime -which is known to be stable -manifest positivity of the canonical energy is difficult to establish, due to the presence of constraints on the initial data as well as the gauge dependence of the canonical energy integrand. Consideration of perturbations generated by a Hertz potential would appear to be a promising way to improve this situation, since the constraints and gauge dependence are eliminated when the canonical energy is expressed in terms of the Hertz potential. We prove that the canonical energy of a metric perturbation of Schwarzschild that is generated by a Hertz potential is positive. We relate the energy quantity arising in the linear stability proof of Dafermos, Holzegel and Rodnianski (DHR) to the canonical energy of an associated metric perturbation generated by a Hertz potential. We also relate the Regge-Wheeler variable of DHR to the ordinary Regge-Wheeler and twist potential variables of the associated perturbation. Since the Hertz potential formalism can be generalized to a Kerr black hole, our results may be useful for the analysis of the linear stability of Kerr. * kartikprabhu@cornell.edu † rmwa@uchicago.edu not been able to directly establish the positivity of the potential energy U , Eq. 1.3, even for the Schwarzschild background. 3 The two main obstacles to showing positivity of U appear to be the following: (1) The variable q ab is not free data but is subject to the linearized Hamiltonian constraint Eq. 1.4b.(2) Even though U is gauge-invariant, the integrand in Eq. 1.3 is not. Thus, it would seem that to show positivity of U one would need to utilize the constraint equations effectively and make a suitable choice of gauge (as was done for the proof of positivity of K in [8]). However, we have not, as yet, found a way to do this. A promising strategy to prove positivity would be rewrite the canonical energy in terms of unconstrained variables. In 4-dimensions, for the case of algebraically special spacetimes such as Schwarzschild and Kerr, a possible choice of such variables are the Hertz-Bromowich-Debye-Whittaker-Penrose potentials (henceforth Hertz potentials) [11][12][13][14]. The Hertz potentials, which solve the Teukolsky equation [15], can be used to generate (complex) metric perturbations that solve the Einstein equation. The initial data corresponding to the Hertz potentials is unconstrained, and one could attempt to prove positivity of canonical energy for perturbations generated by a Hertz potential. 4 The main purpose of this paper is to show that for perturbations of Schwarzschild generated by a Hertz potential, the canonical energy is indeed positive. Since the Hertz potential formalism can be straightforwardly extended to Kerr spacetime, it is possible t...