Quantum theory is the theory that radiant energy is given off and absorbed in units, sort of like bullets from a gun, rather than a steady stream, like a water hose. It was originated by Max Planck, a German physicist, in 1900 through studies he made on radiant energy. Before his research, scientists thought that energy, like all other physical processes, was emitted and absorbed continuously. But when Planck introduced the term quantum theory, he gave scientists a new look at electromagnetic light and radiation. For ages, light was thought to travel in waves, and it does appear to -- sometimes. Quantum theory starts with the principle that light can act not just like waves but as little packets, or quanta. In 1905, Albert Einstein took it a step further and established quantum theory by showing that the energy of light is concentrated in particle-like quanta, rather than in a continuous wave (much to his own consternation, as the uncertainty underlying quantum theory never sat well with the great physicist, who spent much of the latter part of his life trying to unify his notions with those of the less certain, more probabilistic quantum physics).
Just don't ask life down in the quantum realm to be predictable, however! A key component of quantum theory -- as hinted at above with Einstein -- was the "uncertainty principle," established by Werner Heisenberg in 1927. Heisenberg showed that uncertainty was an essential part of quantum theory, and that it was impossible to measure a particle's position and momentum without, in so doing, distorting either measurement. In short, scientists could agree that electrons are particles but that we also can't know with utter certainty where, precisely, they are. The principle ultimately underscored the complimentary nature of waves and particles [source: Univ. of Nebraska Lincoln].
Today, physicists generally treat light as a wave in processes that involve its transmission, and as quanta, called photons, in processes involving emission or absorption. Quantum theory completely changed the study of physics and paved the way for discoveries about photoelectricity, photochemistry and the structure and activities of atoms. Modern lasers and Magnetic Resonance Imaging (MRI) systems all depend on applying ideas in quantum theory.
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