Physics

A plasma is any substance (usually a gas) whose atoms have one or more electrons detached and therefore become ionized. The detached electrons remain, however, in the gas volume that in an overall sense remains electrically neutral. The ionization can be effected by the introduction of large concentrations of energy, such as bombardment with fast external electrons, irradiation with laser light, or by heating the gas to very high temperatures (see Laser). The individually charged plasma particles respond to electric and magnetic fields and can therefore be manipulated and contained.

Plasmas are found in gas-filled light sources, such as a neon lamp, in interstellar space where residual hydrogen is ionized by radiation, and in stars whose great interior temperatures produce a high degree of ionization, a process closely connected with the nuclear fusion that supplies the energy of stars. For the hydrogen nuclei to fuse into heavier nuclei, they must be fast enough to overcome their mutual electric repulsion. This implies high temperature (millions of degrees) when the hydrogen ionizes into a plasma. In order to produce a controlled fusion, or thermonuclear reaction, it is necessary to generate and contain plasmas magnetically; this is an important but difficult problem that falls in the field of magnetohydrodynamics.

An important recent development is that of the laser, an acronym for light amplification by stimulated emission of radiation. In lasers, which may have gases, liquids, or solids as the working substance, a large number of atoms are raised to a high energy level and caused to release this energy simultaneously, producing coherent light where all waves are in phase. Similar techniques are used for producing microwave emissions by the use of masers. The coherence of the light allows for very high intensity, sharp wavelength light beams that remain narrow over tremendous distances; they are far more intense than light from any other source. Continuous lasers can deliver hundreds of watts of power, and pulsed lasers can produce millions of watts of power for very short periods. Developed during the 1950s and 1960s, largely by the American engineer and inventor Gordon Gould and the American physicists Charles Hard Townes, T. H. Maiman, Arthur Leonard Schawlow, and Ali Javan, the laser today has become an extremely powerful tool in research and technology, with applications in communications, medicine, navigation, metallurgy, fusion, and material cutting.

The construction of large and specially designed optical telescopes has led to the discovery of new stellar objects, including a number of quasars, which are billions of light-years away, and has led to a better understanding of the structure of the universe. Radio astronomy has yielded other important discoveries, such as pulsars and the cosmic background radiation, which probably dates from the origin of the universe. The evolutionary history of the stars is now well understood in terms of nuclear reactions. As a result of recent observations and theoretical calculations, the belief is now widely held that all matter was originally in one dense location and that about 14 billion years ago it exploded in one titanic event often called the big bang. The aftereffects of the explosion have led to a universe that appears to be still expanding. A puzzling aspect of this universe, recently revealed, is that the galaxies are not uniformly distributed. Instead, vast voids are bordered by galactic clusters shaped like filaments. The pattern of these voids and filaments lends itself to nonlinear mathematical analysis of the sort used in chaos theory. See also Inflationary Theory.