Thermal photons, arising from thermally populated vacuum quantum fields, ubiquitously inhabit all physical systems within thermal environments. Interactions between vacuum fields and media have led to seminal quantum electrodynamics phenomena, and time-modulated optical media are known to convert zero-point fluctuations into real photon pairs. Although several schemes have been proposed for thermal control using time-modulated media, a clear and general law governing how such modulation affects thermally populated photons has remained elusive. Here, we uncover the general principle governing the action of time-modulated optical systems on thermal photons. By establishing a general pseudoconservation law for photon numbers, we demonstrate how thermal photons are rearranged in energy and direction, generating fluxes within the thermal bath. These fluxes exhibit diverse behaviors, including input-output disparities across frequencies and photon transport that is controllable via modulation phase. These findings provide fundamental insights into the dynamical processes related to thermal vacuum fields and offer a guiding principle for the design of thermal-photon manipulation schemes.

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• Near-field optics
• Non-reciprocal propagation
• Photonics
• Scattering theory