Stefan–Boltzmann law. Every object above absolute zero (0 K = −273 °C) emits EM radiation.

M_λ = σ · T⁴

• M_λ — total emitted radiation per unit area (W/m²).
• σ (sigma) — Stefan–Boltzmann constant, 5.6697 × 10⁻⁸ W·m⁻²·K⁻⁴.
• T — absolute temperature in Kelvin.

• The law assumes a blackbody — an ideal radiator that absorbs all incident energy and re-emits it. Real objects are described by M_λ = ε · σ · T⁴, where ε (emissivity, 0–1) is how close the object is to a true blackbody.

• Key point: emitted energy scales with the fourth power of temperature — small temperature changes produce big radiance changes (why thermal sensors are sensitive).