Light is its own messenger

Why this rule exists

Light needs no medium because it is a self-sustaining disturbance in the electromagnetic field: a changing electric field generates a magnetic field, and a changing magnetic field regenerates the electric field. Maxwell's equations show this pair propagates without any material to carry it, at a speed set purely by the electric and magnetic constants of the vacuum. The messenger and the message are the same field.

In practice

When light crosses empty space, do not look for a carrier — there is none, and c depends only on the vacuum. To find its speed, compute 1/sqrt(ε₀μ₀); to track energy flow, follow the field itself via the Poynting vector. Treat radiation as a field configuration that carries its own momentum and energy, independent of any source once emitted.

Example

Maxwell's equations give a wave speed:

c = 1 / sqrt(ε₀ · μ₀)

ε₀ ≈ 8.85e-12,  μ₀ ≈ 1.26e-6
c = 1 / sqrt(8.85e-12 · 1.26e-6)
c ≈ 3.0e8 m/s

No medium enters — only vacuum constants.

When it doesn't apply

In a material medium light slows to c/n and couples to charges, so the simple vacuum picture fails. The historical 'luminiferous ether' does not exist; special relativity replaces it, making c the same in every inertial frame.