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The first crack in the edifice of classical physics came with attempts to explain the colour of hot objects using classical physics and electromagnetism. The light from these objects is a mixture of different frequencies (colours). Observations reveal that such objects have a distinctive spectrum (pattern of energy distribution at different frequencies). However attempts to explain this in classical terms failed abjectly - they predicted instead that the amount of energy would tend towards infinity at the high-energy (violet) end of the spectrum - an ultraviolet catastrophe.

The dominant frequency of the radiation emitted from a Black Body depends on its temperature.

Enter Max Planck . In 1900 he suggested that physics should abandon the assumption that electromagnetic energy is continuous and wavelike. If, instead, energy can only be absorbed and emitted in discrete packets (or quanta), theory can be made to fit observations exactly. However, while his suggestion certainly gave the right answer, its abandonment of a cherished assumption of classical physics gave it an air of contrivance that led to its relative neglect for several years.

German physicist whose explanation of blackbody radiation in the context of quantized energy emissions initiated quantum theory (1858-1947)Max Planck's solution of this problem led to one of the early portions of Quick Facts about: quantum mechanics

The branch of quantum physics that accounts for matter at the atomic level; an extension of statistical mechanics based on quantum theory (especially the Pauli exclusion principle)quantum mechanics.

It was called the "ultraviolet" catastrophe because ultraviolet radiation had the highest frequencies of any radiation known at the time (X-rays and gamma rays had not been discovered yet). It was also sometimes called the "violet catastrophe" for short. Since the first appearance of the term, it has also been used for other predictions of a similar nature, e.g. in quantum electrodynamics (also used in those cases: ultraviolet divergence).

The ultraviolet catastrophe results from the equipartition theorem of classical statistical mechanics which states that all modes (degrees of freedom) of a system at equilibrium have an average energy of . According to classical electromagnetism, the number of electromagnetic modes in a 3-dimensional cavity, per unit frequency, is proportional to the square of the frequency. This therefore implies that the radiated power per unit frequency should follow the Rayleigh-Jeans law, and be proportional to frequency squared. Thus, both the power at a given frequency and the total radiated power go to infinity as higher and higher frequencies are considered: this is clearly an impossibility.

Max Planck resolved this issue by postulating that electromagnetic energy did not follow the classical description, but could only oscillate or be emitted in discrete packets of energy proportional to the frequency (as given by Quick Facts about: Plancks law

This has the effect of reducing the number of possible modes with a given energy at high frequencies in the cavity described above, and thus the average energy at those frequencies by application of the equipartition theorem. The radiated power eventually goes to zero at infinite frequencies, and the total predicted power is finite.

The formula for the radiated power for the idealized system (black body) was in line with known experiments, and came to be called Planck's law of black body radiation. Based on past experiments, Planck was also able to determine the value of its parameter, now called Planck's constant. The packets of energy later came to be called photons, and played a key role in the quantum description of electromagnetism.

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