Physics

VI

Developments in Physics Since 1930

The rapid expansion of physics in the last few decades was made possible by the fundamental developments during the first third of the century, coupled with recent technological advances, particularly in computer technology, electronics, nuclear-energy applications, and high-energy particle accelerators.

A

Accelerators

Rutherford and other early investigators of nuclear properties were limited to the use of high-energy emissions from naturally radioactive substances to probe the atom. The first artificial high-energy emissions were produced in 1932 by the British physicist Sir John Douglas Cockcroft and the Irish physicist Ernest Thomas Sinton Walton, who used high-voltage generators to accelerate protons to about 700,000 eV and to bombard lithium with them, transmuting it into helium. One electron volt is the energy gained by an electron when the accelerating voltage is 1 V; it is equivalent to about 1.6 × 10^-19 joule (J). Modern accelerators produce energies measured in million electron volts (usually written mega-electron volts, or MeV), billion electron volts (giga-electron volts, or GeV), or trillion electron volts (tera-electron volts, or TeV). Higher-voltage sources were first made possible by the invention, also in 1932, of the Van de Graaff generator by the American physicist Robert van de Graaff.

This was followed almost immediately by the invention of the cyclotron by the American physicists Ernest Orlando Lawrence and Milton Stanley Livingston. The cyclotron uses a magnetic field to bend the trajectories of charged particles into circles, and during each half-revolution the particles are given a small electric “kick” until they accumulate the high energy level desired. Protons could be accelerated to about 10 MeV by a cyclotron, but higher energies had to await the development of the synchrotron after the end of World War II (1939-1945), based on the ideas of the American physicist Edwin Mattison McMillan and the Soviet physicist Vladimir I. Veksler. After World War II, accelerator design made rapid progress, and accelerators of many types were built, producing high-energy beams of electrons, protons, deuterons, heavier ions, and X rays. For example, the accelerator at the Stanford Linear Accelerator Center (SLAC) in Stanford, California, accelerates electrons down a straight “runway,” 3.2 km (2 mi) long, at the end of which they attain an energy of more than 20 GeV.

While lower-energy accelerators are used in various applications in industry and laboratories, the most powerful ones are used in studying the structure of elementary particles, the fundamental building blocks of nature. In such studies elementary particles are broken up by hitting them with beams of projectiles that are usually protons or electrons. The distribution of the fragments yields information on the structure of the elementary particles.

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To obtain more detailed information in this manner, the use of more energetic projectiles is necessary. Since the acceleration of a projectile is achieved by “pushing” it from behind, to obtain more energetic projectiles it is necessary to keep pushing for a longer time. Thus, high-energy accelerators are generally larger in size. The highest beam energy reached at the end of World War II was less than 100 MeV. A bigger accelerator, reaching 3 GeV, was built in the early 1950s at the Brookhaven National Laboratory at Upton, New York. A breakthrough in accelerator design occurred with the introduction of the strong focusing principle in 1952 by the American physicists Ernest D. Courant, Livingston, and Hartland S. Snyder. Today the world's largest accelerators have been or are being built to produce beams of protons beyond 1 TeV. Two are located at the Fermi National Accelerator Laboratory, near Batavia, Illinois, and at the European Organization for Nuclear Research, known as CERN, in Geneva, Switzerland. See Particle Accelerators.

B

Particle Detectors

Detection and analysis of elementary particles were first accomplished through the ability of these particles to affect photographic emulsions and to energize fluorescent materials. The actual paths of ionized particles were first observed by the British physicist Charles Thomson Rees Wilson in a cloud chamber, where water droplets condensed on the ions produced by the particles during their passage. Electric or magnetic fields can be used to bend the particle paths, yielding information about their momentum and electric charges. A significant advance on the cloud chamber was the construction of the bubble chamber by the American physicist Donald Arthur Glaser in 1952. It uses a liquid, usually hydrogen, instead of air, and the ions produced by a fast particle become centers of boiling, leaving an observable bubble track. Because the density of the liquid is much higher than that of air, more interactions take place in a bubble chamber than in a cloud chamber. Furthermore, the bubbles clear out faster than water droplets, allowing more frequent cycling of the bubble chamber. A third development, the spark chamber, evolved in the 1950s. In this device, many parallel plates are kept at a high voltage in a suitable gas atmosphere. An ionizing particle passing between the plates breaks down the gas, forming sparks that delineate its path.

A different type of detector, the discharge counter, was developed early during the 20th century, largely by the German physicist Hans Wilhelm Geiger, and later improved by the German American physicist Walther Müller. It is now commonly known as the Geiger-Müller counter, and although small and convenient, it has been largely replaced by faster and more convenient solid-state counting devices, such as the scintillation counter, developed about 1947 by the German American physicist Hartmut Paul Kallmann and others. It uses the ability of ionized particles to produce a flash of light as they pass through certain organic crystals and liquids. See Particle Detectors.

C

Cosmic Rays

About 1911 the Austrian-American physicist Victor Franz Hess discovered that cosmic radiation, consisting of rays originating outside the earth's atmosphere, arrived in a pattern determined by the earth's magnetic field (see Cosmic Rays). The rays were found to be positively charged and to consist mostly of protons with energies ranging from about 1 GeV to 10¹¹ GeV (compared to about 30 GeV for the fastest particles produced by artificial accelerators). Cosmic rays trapped into orbits around the earth account for the Van Allen radiation belts discovered during an artificial-satellite flight in 1959 (see Radiation Belts).

When a very energetic primary proton smashes into the atmosphere and collides with the nitrogen and oxygen nuclei present, it produces large numbers of different secondary particles that spread toward the earth as a cosmic-ray shower. The origin of the cosmic-ray protons is not yet fully understood; some undoubtedly come from the sun and the other stars. Except for the slowest rays, however, no mechanism can be found to account for their high energies and the likelihood is that weak galactic fields operate over very long periods to accelerate interstellar protons (see Galaxy; Milky Way).

D

Elementary Particles

To the electron, proton, neutron, and photon have been added a number of fundamental particles. In 1932 the American physicist Carl David Anderson discovered the antielectron, or positron, predicted in 1928 by Dirac. Anderson found that the stopping of an energetic cosmic gamma ray near a heavy nucleus yielded an electron-positron pair out of pure energy. When a positron subsequently meets an electron, they annihilate each other with a burst of photons of energy.

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