The moving explanation
Follow the momentum.
Four interactions. One essential question: where does the momentum go?
A moving explanation / 01—04
Light arrives. Momentum changes.
A reflected photon changes direction. The surface receives the corresponding momentum change. A sail exchanges momentum with light; it does not need to carry material propellant.
Normal incidence, perfect reflection and externally supplied light. The emitter belongs to the wider system.
Propellant-free describes the supply.
A solar sail can move without an onboard expendable propellant supply. An onboard light emitter recoils because photons leave it. A completely isolated device making only internal cyclic movements is a different claim: established physics does not give it sustained acceleration of the whole system.
Near-field forces have a system around them.
Thermal and nonreciprocal boundary interactions are an active theoretical research direction. A force on a plate or particle is not automatically thrust on all the hardware surrounding it. Model the bodies, supports, radiation, heat sources and electromagnetic fields together.
Research basis: David Gelbwaser-Klimovsky, 2021 · Laila Henkes, 2025 · M. Tajmar, 2022.