Blackbody Radiation
Blackbody radiation is the electromagnetic radiation emitted by a body in thermal equilibrium with its own radiation field — an ideal absorber and therefore an ideal emitter. Its spectrum depends on one thing only, the temperature, and on nothing about the material. That universality is what made it the problem on which classical physics broke.
The standard account
Kirchhoff established the universality in 1859–60 and set the determination of the spectral function as an open problem. Stefan found empirically in 1879, and Boltzmann derived thermodynamically in 1884, that the total emitted power goes as T4. Wien's displacement law of 1893 fixed the wavelength of peak emission as inversely proportional to temperature. Wien's 1896 distribution fitted the short-wavelength end; the Rayleigh–Jeans law, derived from equipartition over the modes of the cavity, fitted the long-wavelength end and diverged at short wavelengths — the ultraviolet catastrophe, a name Ehrenfest gave it in 1911.
Max Planck presented the correct formula to the German Physical Society in October 1900 and, in December, the derivation: the energy of the cavity oscillators is restricted to integer multiples of hf. The constant h entered physics here, and since the 2019 revision of the SI it has the exact defined value 6.62607015 × 10−34 J·s. It is worth being precise about what Planck did and did not do: he quantized the exchange of energy with the material oscillators, not the radiation field, and he regarded the step as a formal device. It was Albert Einstein in 1905, and more fully in 1917, who took the quantization to be a property of the light itself.
The most nearly perfect blackbody spectrum ever measured is not a laboratory cavity but the cosmic microwave background, measured by the FIRAS instrument on COBE and found to fit a blackbody at 2.725 K with deviations of less than a hundredth of a per cent.
On this wiki
Blackbody radiation appears here on two fronts, and they are largely independent of each other.
Classical derivations of the Planck law
If the spectrum can be obtained without quantizing anything, the founding argument for the photon weakens. Contributors attempting this include:
- Charles William Lucas, "A Physical Model for Atoms and Nuclei, Part 4: Blackbody Radiation and the Photoelectric Effect" — derives both the blackbody spectrum and the photoelectric relation from the extended-charge electrodynamics of Common Sense Science, in which particles are real toroidal rings rather than points. Treating the two effects as a single problem is the correct strategy, since they are two halves of the same inference.
- Philipp M Kanarev, "The Law of the Radiation of the Perfect Blackbody is the Law of Classical Physics" — argues the Planck law is recoverable within classical physics.
- Jian-Miin Liu, "An Electronic Radiation of Blackbody: Cosmic Electron Background" — connects laboratory blackbody radiation to a proposed cosmic electron background.
Related work on the thermodynamic side is catalogued at Entropy — see Pharis E Williams' "Mechanical Entropy and its Implications" and Ingvar Astrand's "A Unified Theory of Physics from a Newly Discovered Radiation Entropy Law".
The microwave background as a blackbody
The second front is cosmological, and here the blackbody character of the CMB is the point at issue rather than the Planck law. A thermalized blackbody spectrum requires many scatterings, and this is one of the strongest arguments for the Big Bang and against models in which the background is starlight reprocessed by intervening matter. Steady-state and eternal-universe advocates on this wiki have to account for how a non-cosmological background could be thermalized to the FIRAS precision; the usual proposal is thermalization by intergalactic dust or by whiskers of conducting material, following Hoyle, Burbidge and Narlikar. That argument is developed at Cosmic Microwave Background and Steady State Theory; the honest summary is that the spectrum is the hardest single datum for those models and the anisotropy pattern is the second hardest.