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**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).

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