Quantum Theory
Toward the end of the nineteenth century, a famous scientist is reputed to have expressed sympathy for his younger colleagues in physics. He said that all the great discoveries had already been made and that all physicists had to look forward to was calculating known answers to a precision of one or two more decimal points. He could not have been more wrong. Within a period of three decades, physics was to undergo one of the most dramatic revolutions in its history. The apparently solid, dependable classical physics--based on the mechanics of Isaac Newton and the electromagnetism of James Clerk Maxwell--were overthrown and replaced by an entirely new view of the world. One of the two cornerstones of this new world view was quantum theory.
The architect of this new view was the German physicist, Max Planck. The problem that led Planck to the quantum theory was one that had puzzled his predecessors for a number of years: black-body radiation. The term black body refers to any object that absorbs and radiates all frequencies of light. As early as 1860, Gustav Kirchhoff, one of Planck's teachers, had devised methods for studying the radiation emitted by a black body. Although collecting data for this phenomenon was relatively straightforward, no one had been able to find a formula that correctly described these data.
For a long time, Planck too was unsuccessful in producing a theory of black-body radiation based on principles of classical physics. Finally, and somewhat reluctantly, he adopted a radically new approach. He assumed that energy was absorbed and emitted by a black body not in a continuous spectrum, but in tiny discrete packages to which he gave the name quanta. The word quantum in Latin means "how much?". Furthermore, he found that the size of a quantum depended on only one factor, the frequency () of the emitted radiation, according to the formula E = . In this formula, h is a constant of proportionality equal to 6.625 x 10-27 erg second and is now known as Planck's constant.
Planck's quantum theory was such a departure from classical physics that many scientists refused to consider it seriously. Planck himself thought that the quantum concept might be nothing other than a mathematical trick that happened to solve a particularly difficult physical problem. He hoped that he might eventually find a way that the quantum concept might be absorbed into the laws of classical physics, but he was aware of the possible consequences of his work. He is reported to have said to his son shortly after his discovery, "Today I have made a discovery which is as important as Newton's discovery. "
The true significance of the quantum theory did not become obvious for more than a decade. Two events were crucial to its widespread acceptance. The first was Albert Einstein's analysis of the photoelectric effect in 1906. Einstein showed that the release of electrons from a metal exposed to light can only be explained if one assumes that the light exists in the form of tiny, discrete particles, which he called lightquanta and are now known as photon s.
The second event was the use by Niels Bohr of the quantum concept in the development of his theory of atomic structure in 1913. Bohr suggested that the electrons in an atom can travel only in specific, discrete orbits around the nucleus and that they can move from one orbit to another only with the loss and gain of discrete quanta of energy.
By the 1920s, most scientists had accepted the quantum theory of nature and, with it, Planck's constant as one of the fundamental constants of nature, similar to the velocity of light and the gravitational constant. They turned their attention next to the development of mathematical systems on which they could build a new analysis of matter and energy. Out of that effort grew first matrix mechanics, then wave mechanics, and finally, the overarching new approach to the study of nature known as quantum mechanics.
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