Every object above absolute zero emits electromagnetic radiation because of its temperature. This thermal radiation can transfer energy through vacuum and does not require a material medium.

Blackbody emission

An ideal blackbody absorbs all incident radiation and emits the maximum possible thermal radiation at a given temperature.

Its total emissive power is given by the Stefan–Boltzmann law:

$$E_b=\sigma T^4,$$

where $T$ is absolute temperature and $\sigma$ is the Stefan–Boltzmann constant.

Real surfaces

A real surface is described by an emissivity $\varepsilon$ between zero and one. For an idealized gray surface,

$$E=\varepsilon\sigma T^4.$$

Interaction with incoming radiation

Incident radiation can be absorbed, reflected or transmitted. The corresponding fractions satisfy

$$\alpha+\rho+\tau=1.$$

For an opaque surface, transmissivity is zero, so absorption and reflection account for all incident energy.

Thermal radiation connects macroscopic temperature with electromagnetic emission and becomes especially important at high temperatures because of the fourth-power dependence on $T$.