Light weighs nothing. A single photon has zero mass. And yet, light can push a mirror, sweep dust between the planets, bend the tail of a comet, hold up a star against its own gravity, and even propel a spacecraft. How is that possible?

In this deep dive, we break down one of the most counterintuitive truths in all of physics — how something completely massless exerts a real, measurable, physical force on matter. We start with Newton's momentum equation and show why it breaks down for light. Then we follow Einstein's full energy-momentum relation to reveal where photon momentum actually comes from. We explore the electromagnetic mechanism behind radiation pressure — how oscillating electric and magnetic fields interact with charged particles inside matter to create a push that has nothing to do with physical contact.

Sources:

Maxwell, J.C. (1873). A Treatise on Electricity and Magnetism. Oxford: Clarendon Press.

Nichols, E.F. and Hull, G.F. (1903). "The Pressure Due to Radiation." Physical Review, 17(1), 26–50.

Compton, A.H. (1923). "A Quantum Theory of the Scattering of X-Rays by Light Elements." Physical Review, 21(5), 483–502.

Chu, S. (1998). "Nobel Lecture: The Manipulation of Neutral Particles." Reviews of Modern Physics, 70(3), 685–706.

Ferrarese, L. and Merritt, D. (2000). "A Fundamental Relation Between Supermassive Black Holes and Their Host Galaxies." The Astrophysical Journal Letters, 539(1), L9–L12.

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