Where Does a Photon Get Its Momentum From?

Light has no mass. Absolutely zero. And yet, it pushes mirrors, traps living cells, shapes the surfaces of stars, and could one day propel spacecraft across the solar system. If momentum equals mass times velocity, how does something massless carry momentum at all? And where does that momentum come from? In this deep dive, we unravel one of the most elegant puzzles in physics. We start with Newton's formula and show why it's incomplete. We follow Einstein's energy-momentum relation to discover that energy itself, not mass, is the true source of momentum. We trace the life of a photon from the instant it's born inside an atom, already moving at full speed, already carrying its momentum, while the atom recoils in the opposite direction to keep the universe's books balanced. Sources: Griffiths, D.J. (2017). Introduction to Electrodynamics (4th Edition). Cambridge University Press. Compton, A.H. (1923). "A Quantum Theory of the Scattering of X-Rays by Light Elements." Physical Review, 21(5), 483–502. https://doi.org/10.1103/PhysRev.21.483 Ashkin, A. (1970). "Acceleration and Trapping of Particles by Radiation Pressure." Physical Review Letters, 24(4), 156–159. https://doi.org/10.1103/PhysRevLett.2... Phillips, W.D. (1998). "Laser Cooling and Trapping of Neutral Atoms." Reviews of Modern Physics, 70(3), 721–741. https://doi.org/10.1103/RevModPhys.70... Einstein, A. (1905). "Zur Elektrodynamik bewegter Körper." Annalen der Physik, 322(10), 891–921. https://doi.org/10.1002/andp.19053221004 #PhotonMomentum #QuantumPhysics #LightHasNoMass #RadiationPressure #EinsteinRelativity #ComptonScattering #SpaceScience