Order the reasoning by which Maxwell’s equations predict electromagnetic waves.

1. In empty space there are no charges or currents
2. A changing magnetic field induces an electric field (Faraday)
3. A changing electric field induces a magnetic field (Maxwell’s term)
4. Each field regenerates the other in a self-sustaining cycle
5. The cycle propagates at speed $c = 1/\sqrt{\mu_0\varepsilon_0}$

With sources removed, the two circulation laws chase each other’s tails: $\partial B/\partial t \to E$, $\partial E/\partial t \to B$. The propagation speed computed from the constants matched the measured speed of light, one of the great unifications in science.