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Introduction; Scope of Physics; Early History of Physics; Newton and Mechanics; Modern Physics; Developments in Physics Since 1930
By about 1880 physics was serene; most phenomena could be explained by Newtonian mechanics, Maxwell's electromagnetic theory, thermodynamics, and Boltzmann's statistical mechanics. Only a few problems, such as the determination of the properties of the ether and the explanation of the radiation spectra from solids and gases, appeared unsolved. These unexplained phenomena, however, formed the seeds of revolution, a revolution that was augmented by a series of remarkable discoveries within the last decade of the 19th century: the discovery of X rays by Wilhelm Conrad Roentgen of Germany in 1895; of the electron by Sir Joseph John Thomson of Great Britain in 1895; of radioactivity by Antoine Henri Becquerel of France in 1896; and of the photoelectric effect by Hertz, Wilhelm Hallwachs, and Philipp Eduard Anton Lenard of Germany during the period from 1887 to 1899 (see Photoelectric Cell). Coupled with the disturbing results of the Michelson-Morley experiments and the discovery of cathode rays, or electron stream, the experimental evidence in physics outstripped all available theories to explain it.
Two major new developments during the first third of the 20th century, the quantum theory and the theory of relativity, explained these findings, yielded new discoveries, and changed the understanding of physics as it is known today.
To extend the example of relative velocity introduced with the Michelson-Morley experiment, two situations can be compared. One consists of a person, A, walking forward with a velocity v in a train moving at velocity u. The velocity of A with regard to an observer B stationary on the ground is then simply V = u + v. If, however, the train were at rest in the station and A was moving forward with velocity v while observer B walked backward with velocity u, the relative speed between A and B would be exactly the same as in the first case. In more general terms, if two frames of reference are moving relative to each other at constant velocity, observations of any phenomena made by observers in either frame will be physically equivalent. As already mentioned, the Michelson-Morley experiment failed to confirm the concept of adding velocities, and two observers, one at rest and the other moving toward a light source with velocity u, both observe the same light velocity V, commonly denoted by the symbol c. Einstein incorporated the invariance of c into his theory of relativity. He also demanded a very careful rethinking of the concepts of space and time, showing the imperfection of intuitive notions about them. As a consequence of his theory, it is known that two clocks that keep identical time when at rest relative to each other must run at different speeds when they are in relative motion, and two rods that are identical in length (at rest) will become different in length when they are in relative motion. Space and time must be closely linked in a four-dimensional continuum where the normal three-space dimensions must be augmented by an interrelated time dimension. Two important consequences of Einstein's relativity theory are the equivalence of mass and energy and the limiting velocity of the speed of light for material objects. Relativistic mechanics describes the motion of objects with velocities that are appreciable fractions of the speed of light, while Newtonian mechanics remains useful for velocities typical of the macroscopic motion of objects on earth. No material object, however, can have a speed equal to or greater than the speed of light. Even more important is the relation between the mass m and energy E. They are coupled by the relation E = mc2, and because c is very large, the energy equivalence of a given mass is enormous. The change of mass giving an energy change is significant in nuclear reactions, as in reactors or nuclear weapons, and in the stars, where a significant loss of mass accompanies the huge energy release. Einstein's original theory, formulated in 1905 and known as the special theory of relativity, was limited to frames of reference moving at constant velocity relative to each other. In 1915, he generalized his hypothesis to formulate the general theory of relativity that applied to systems that accelerate with reference to each other. This extension showed gravitation to be a consequence of the geometry of space-time and predicted the bending of light in its passage close to a massive body like a star, an effect first observed in 1919. General relativity, although less firmly established than the special theory, has deep significance for an understanding of the structure of the universe and its evolution. See also Cosmology.
The quandary posed by the observed spectra emitted by solid bodies was first explained by the German physicist Max Planck. According to classical physics, all molecules in a solid can vibrate with the amplitude of the vibrations directly related to the temperature. All vibration frequencies should be possible and the thermal energy of the solid should be continuously convertible into electromagnetic radiation as long as energy is supplied. Planck made a radical assumption by postulating that the molecular oscillator could emit electromagnetic waves only in discrete bundles, now called quanta, or photons. See Photon; Quantum Theory. Each photon has a characteristic wavelength in the spectrum and an energy E given by E = hf, where f is the frequency of the wave. The wavelength λ related to the frequency by λf = c, where c is the speed of light. With the frequency specified in hertz (Hz), or cycles per second, h, now known as Planck's constant, is extremely small (6.626 × 10-27 erg-sec). With his theory, Planck again introduced a partial duality into the theory of light, which for nearly a century had been considered to be wavelike only.
If electromagnetic radiation of appropriate wavelength falls upon suitable metals, negative electric charges, later identified as electrons, are ejected from the metal surface. The important aspects of this phenomenon are the following: (1) the energy of each photoelectron depends only on the frequency of the illumination and not on its intensity; (2) the rate of electron emission depends only on the illuminating intensity and not on the frequency (provided that the minimum frequency to cause emission is exceeded); and (3) the photoelectrons emerge as soon as the illumination hits the surface. These observations, which could not be explained by Maxwell's electromagnetic theory of light, led Einstein to assume in 1905 that light can be absorbed only in quanta or photons, and that the photon completely vanishes in the absorption process, with all of its energy E (=hf) going to one electron in the metal. With this simple assumption Einstein extended Planck's quantum theory to the absorption of electromagnetic radiation, giving additional importance to the wave-particle duality of light. It was for this work that Einstein was awarded the 1921 Nobel Prize in physics.
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