Quantum theory was developed by physicists in the 20th century as a means to describe the behavior of light, atoms, and subatomic particles. This theory evolved from attempts to explain certain physical phenomena that could not be understood using the prevailing theories of classical physics. Classical physics describes a world in which, given enough information regarding the velocity, position, and mass of objects, it would be possible to exactly predict their future behavior. Quantum physics, however, refutes this claim and states that the behavior of matter at an atomic and subatomic level can be described only in terms of probability. In addition, quantum theory unites three of the four fundamental forces of nature: the strong force, the weak force, and the electromagnetic force. The force that is not included is gravity. Finding a unification theory capable of explaining the relationship of all four forces is an important area of research in theoretical physics.
The nature of light
In the 17th century, the Dutch physicist Christiaan Huygens proposed that light was a kind of wave. Around the same time, however, the British physicist Sir Isaac Newton proposed that light was made up of particles called corpuscles, and it was this theory that prevailed until the 19th century, when physicists once more began to investigate the wavelike properties of light. One experimental observation that caused physicists to reassess their understanding of light was devised by the British physicist Thomas Young and is called Young’s double-slit experiment.
In water, if a series of wave fronts passes through two narrow openings in a wall, the waves emerge from the openings as two series of concentric waves. When the waves from each hole merge with one another, the peaks of the waves join together forming constructive interference while the troughs merge forming destructive interference. Where a trough meets a peak, however, the two cancel each other out. Young’s experiment uses a single source of light shining through two slits onto a screen. The light emerging from the two slits interferes, leaving a series of light and dark bands on the screen. This phenomenon is explainable only if light is a wave.
The limitations of classical physics
In 1900, problems with the wave theory of light led the German physicist Max Planck to present a theory proposing that atoms produce packets of energy called quanta. Planck arrived at this theory by studying the behavior of blackbody radiation—an ideal blackbody is one that absorbs all radiation. In practice, an ideal blackbody is approximated by a cavity with a tiny opening through which radiation can pass. Conversely, a blackbody is also an ideal emitter of radiation, and it is in understanding the intensity of wavelengths emitted from a blackbody that physicists began to notice the limitations of classical physics. Classical physics, relying on a theory of light as a wave, predicts a much larger amount of radiation at high frequencies than actually occurs, a phenomenon that perplexed and worried physicists and consequently became known as the ultraviolet catastrophe. This discovery led Planck to his conclusion that "oscillators" in atoms could produce packets of energy only in certain quantities that he called quanta (singular, quantum). The energy carried by each quantum is proportional to the frequency of the radiation, so that
Energy = Frequency × Constant
The constant, written h, soon became known as the Planck constant. Planck assumed that quanta of radiation spread out like waves on the surface of a pond after leaving their source, allowing the long-established wavelike behavior of radiation to be explained. Planck did not suggest, however, that light itself was quantized, merely that something inside the atom permitted the absorption or emission of only certain quantities of energy.
