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Lorentz contraction (lôr`ĕnts), in physics, contraction or foreshortening of a moving body in the direction of its motion, proposed by H. A. Lorentz on theoretical grounds and based on an earlier suggestion by G. F. Fitzgerald; it is sometimes called the Fitzgerald, or Lorentz-Fitzgerald, contraction. The Lorentz contraction hypothesis was put forward in an attempt to explain the negative result of the Michelson-Morley experiment of 1887 designed to demonstrate the earth's absolute motion through space (see  ether ;  relativity ). The hypothesis held that any material body is contracted in the direction of its motion by a factor 1− v 2 / c 2 , where  v  is the velocity of the body and  c  is the velocity of light. Although the Lorentz contraction did not succeed entirely in reconciling the results of the Michelson-Morley experiment with classical theory, it did serve as the basis for the mathematics of Einstein's theory of relativity. The equations used in ...

Atomic clock, electric or electronic timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks. Much greater accuracy can be attained by using the oscillations of atoms or molecules. Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, the clock is controlled by a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices. The first atomic clock, invented in 1948, utilized the vibration...

Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition (see stellar structure and stellar evolution ). The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory . Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars (see Hertzsprung-Russell diagram ). With the construction of ever more powerful telescopes (see observatory) , the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding (see Hubble's law ). Usin...

The Copernican Revolution After the fall of Rome, European astronomy was largely dormant, but significant work was carried out by the Muslims and the Hindus. It was by way of Arabic translations that Greek astronomy reached medieval Europe. One of the great landmarks of the revival of learning in Europe was the publication (1543) by Nicolaus Copernicus (1473–1543) of his De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). According to the Copernican system , the earth rotates on its axis and, with all the other planets, revolves around the sun. The assertion that the earth is not the center of the universe was to have profound philosophical and religious consequences. Copernicus's principal claim for his new system was that it made calculations easier. He retained the uniform circular motion of the Ptolemaic system, but by placing the sun at the center, he was able to reduce the number of epicycles. Copernicus also determined the sidereal periods (t...

Astronomy is the oldest of the physical sciences. In many early civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict future events. The first practical function of astronomy was to provide a basis for the calendar , the units of month and year being determined by astronomical observations. Later, astronomy served in navigation and timekeeping. The Chinese had a working calendar as early as the 13th cent. B.C. About 350 B.C., Shih Shen prepared the earliest known star catalog, containing 800 entries. Ancient Chinese astronomy is best known today for its observations of comets and supernovas . The Babylonians, Assyrians, and Egyptians were also active in astronomy. The earliest astronomers were priests, and no attempt was made to separate astronomy from astrology . In fact, an early motivation for the detailed study of planetary positions was the preparation of horoscopes. Greek Innovations The highest development of astr...

Space shuttle, reusable U.S. space vehicle. Developed by the National Aeronautics and Space Administration (NASA), it consists of a winged orbiter, two solid-rocket boosters, and an external tank. As with previous spacecraft, the shuttle is launched from a vertical position. Liftoff thrust is derived from the orbiter's three main liquid-propellant engines and the boosters. After 2 min the boosters use up their fuel, separate from the spacecraft, and—after deployment of parachutes—are recovered following splashdown. After about 8 min of flight, the orbiter's main engines shut down; the external tank is then jettisoned and burns up as it reenters the atmosphere. The orbiter meanwhile enters orbit after a short burn of its two small Orbiting Maneuvering System (OMS) engines. To return to earth, the orbiter turns around, fires its OMS engines to reduce speed, and, after descending through the atmosphere, lands like a glider. Five different orbiters—Columbia, Challenger, Atlantis, D...

The invention of the rocket is generally ascribed to the Chinese, who as early as A.D. 1000 stuffed gunpowder into sections of bamboo tubing to make military weapons of considerable effectiveness. The 13th-century English monk Roger Bacon introduced to Europe an improved form of gunpowder, which enabled rockets to become incendiary projectiles with a relatively long range. Rockets subsequently became a common if unreliable weapon. Major progress in design resulted from the work of William Congreve, an English artillery expert, who built a 20-lb (9-kg) rocket capable of traveling up to 2 mi (3 km). In the late 19th cent., the Austrian physicist Ernst Mach gave serious theoretical consideration to supersonic speeds and predicted the shock wave that causes sonic boom. The astronautical use of rockets was cogently argued in the beginning of the 20th cent. by the Russian Konstantin E. Tsiolkovsky , who is sometimes called the "father of astronautics." He pointed out that a rock...