Quantum Mechanics
The atomic model proposed by Niels Bohr in 1913 was an important milestone in the history of science for two quite different reasons. First, the Bohr theory answered some fundamental questions about the location and behavior of electrons in an atom. In a sense it provided the final step in the development of the modern theory of the atom that had begun with Joseph J. Thomson's discovery of the electron.
For all its success, however, the Bohr model confronted physicists with a somewhat disturbing situation. Bohr had not so much devised a new way of looking at the atom as he had pieced together ideas from classical physics with new, ad hoc assumptions that "explained" empirical observations. A number of physicists felt that some new--perhaps revolutionary--theory of matter was needed to provide a more sound basis for the Bohr model.
Interestingly enough, a number of ideas needed to develop such a theory were already in place by the time Bohr announced his atomic model. Yet, it took more than a decade for various researchers to bring together these ideas and use them as the basis for a new way of viewing nature. The development and application of these theories is known today as quantum mechanics.
Quantum mechanics differs from classical physics in three fundamental ways: the integration of particle and wave phenomena, with the relative equivalence of mass and energy; the quantization of both wave and particle phenomena; and the uncertainty involved in making physical measurements. In his explanation of the photoelectric effect in 1905, Albert Einstein had proposed that light (a form of energy) be thought of as consisting of tiny particle-like packages of energy, later called photons .
Einstein's use of the photon concept was itself based on the earlier work of Max Planck who, in 1900, had suggested the concept of "quantized" energy to explain the nature of black-body radiation. In his explanation, Planck had proposed that heat energy travels in tiny, discrete packages which he called quanta. Einstein's use of the photon to describe waves represented an important break with the traditional duality of matter and energy. Prior to 1905, scientists had accepted the proposition that matter and energy are two distinct phenomena with their own constituent parts, particles on the one hand, and waves on the other.
Yet another body of Einstein's work had also demonstrated that the traditional matter-energy duality could not be sustained. In a second 1905 paper, he had developed the well-known E = mc2 formula that demonstrated the interconvertability of matter and energy.
The particle-like nature of energy having been shown, it still took another two decades for the now-commonly accepted complimentary idea of the wave-like nature of particles to be proposed. In 1924, Louis de Broglie devised a series of equations that describe the wave properties of an electron. De Broglie's ideas were so revolutionary that many physicists rejected them out of hand. The discovery in 1927 by Clinton Davisson and Germer of just such properties, however, convinced physicists that de Broglie's formulations had been correct.
The third key to the development of quantum mechanics was the notion of uncertainty. Prior to the twentieth century, scientists held a deterministic view of the universe. If one could measure the properties (for example, the mass and momentum) of any one particle at any one time and place, they believed, it should be possible to predict precisely the movement of that particle for all future time.
By 1927, however, Werner Heisenberg had demonstrated that nature is characterized not by determinism, but by uncertainty. For example, the very act of measuring an object alters that object. What we find out from the measurement is not what the object is like, but what the object is like as a result of the measurement . It was soon recognized that the correct way to consider nature was in terms of the probability that various events will occur.
The work of Max Planck, Einstein, Heisenberg, and others meant that scientists had to find a whole new mathematical system to deal with physical phenomena, a system that would take into account probabilities, quantization, and the integration of matter and energy. During the late 1920s, two such systems were developed. One was matrix mechanics, invented primarily by Heisenberg, Max Born, and Pascal Jordan. The other was wave mechanics, which evolved out of the work of Erwin Schrödinger.
Matrix mechanics was a satisfactory method for dealing with phenomena from a quantum standpoint, but it was a difficult and cumbersome technique with which most physicists were uncomfortable. Schrödinger's wave mechanics, on the other hand, incorporated physical and mathematical techniques with which physicists were familiar and comfortable. As a result, it, rather than matrix mechanics, became the tool with which the concepts of quantum mechanics were rapidly developed. Nearly two decades later, John von Neumann was able to show that the two approaches to quantum mechanics--matrix mechanics and wave mechanics--are, in any case, mathematically equivalent to each other.
One of the many consequences of quantum mechanics has been a rather dramatic shift in scientists' view of the world. Today, it is less likely that scientists have in mind concrete particles or specific waves when they think about matter or energy. Instead, the description of phenomena tends to be couched in terms of mathematical equations that bring together fundamental properties such as quantization, uncertainty, and particle-wave integration.
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