The conventional understanding at the time was that heat radiation traveled, like other forms of energy, through space as a wave. The problem was that the wave theory could not explain the way certain hot objects radiate energy. So Planck proposed the radical idea that the matter in the walls of these hot bodies vibrated at certain discrete frequencies, which had the consequence that the heat energy was only radiated in tiny discrete lumps, or “quanta,” that could not be subdivided. His simple theory was remarkably successful, but was a radical departure from the classical theory of radiation,
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