This article explores the quantum structure of electromagnetic fields in space-time varying materials, focusing on the implications of time and space dependence on photon behavior. The authors propose a Hamiltonian that describes how quantum field operators evolve according to classical equations in the Heisenberg picture, and discuss the quantum phenomenon of photon-pair creation. In time-varying systems, frequency is not conserved, and negative frequencies can appear. However, the energy of photons remains positive, as defined by the photon creation and annihilation operators. The paper addresses the challenge of quantizing the electromagnetic field in such materials, showing that the Hamiltonian is not diagonal in the creation and annihilation operators, and that off-diagonal elements represent transitions between different wave frequencies. The authors also compare quantum and classical descriptions of electromagnetic waves in such media, justifying a general Hamiltonian for use in time-dependent systems. The paper highlights the importance of waves that cross the positive or negative frequency divide in the emission of radiation. The authors also discuss the implications of negative frequencies in quantum processes, noting that they are central to understanding quantum phenomena in space-time varying media. The paper concludes that spontaneous emission is generated when annihilation operators evolve into a mixture of annihilation and creation operators, and that this process is responsible for the radiation produced by these systems operating on the ground state. The authors also note that in the impedance matched case, spontaneous black hole radiation is associated with transitions between positive and negative frequencies.This article explores the quantum structure of electromagnetic fields in space-time varying materials, focusing on the implications of time and space dependence on photon behavior. The authors propose a Hamiltonian that describes how quantum field operators evolve according to classical equations in the Heisenberg picture, and discuss the quantum phenomenon of photon-pair creation. In time-varying systems, frequency is not conserved, and negative frequencies can appear. However, the energy of photons remains positive, as defined by the photon creation and annihilation operators. The paper addresses the challenge of quantizing the electromagnetic field in such materials, showing that the Hamiltonian is not diagonal in the creation and annihilation operators, and that off-diagonal elements represent transitions between different wave frequencies. The authors also compare quantum and classical descriptions of electromagnetic waves in such media, justifying a general Hamiltonian for use in time-dependent systems. The paper highlights the importance of waves that cross the positive or negative frequency divide in the emission of radiation. The authors also discuss the implications of negative frequencies in quantum processes, noting that they are central to understanding quantum phenomena in space-time varying media. The paper concludes that spontaneous emission is generated when annihilation operators evolve into a mixture of annihilation and creation operators, and that this process is responsible for the radiation produced by these systems operating on the ground state. The authors also note that in the impedance matched case, spontaneous black hole radiation is associated with transitions between positive and negative frequencies.