VI DEVELOPMENTS IN PHYSICS SINCE 1930

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

Van de Graaff Generator

Van de Graaff generators can build up very high charges of static electricity. As this girl touches a generator, her body becomes charged along with the generator. The charged strands of her hair repell each other, causing her hair to stand on end.

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.

Particle Accelerator

The big circle marks the location of the Large Hadron Collider (LHC) at the European particle physics laboratory in CERN. The tunnel where the particles are accelerated is located 100 m (320 ft) underground and is 27 km (16.7 mi) in circumference. The smaller circle is the site of the smaller proton-antiproton collider. The border of France and Switzerland bisects the CERN site and the two accelerator rings.

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.

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

Geiger Counter

A Geiger counter is a device used by scientists and surveyors for detecting the presence and intensity of radiation. The tube is filled with low-pressure gas and acts as an ionizing chamber. An electronic circuit maintains a strong electric field between a fine wire in the center of the tube and its walls. When ionizing radioactive particles enter the tube and collide with gas atoms, they ionize the gas, producing free electrons. These electrons flow along the center wire and create an electrical pulse, which is amplified and counted electronically. When radiation is detected, the Geiger counter produces a clicking, staticlike sound.

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.

Charles T. R. Wilson

English physicist Charles T. R. Wilson won the 1927 Nobel Prize in physics. Inventor of the cloud chamber, a type of particle detector, he made visible the paths of sub-atomic particles, eventually enabling scientists to determine their mass and charge.

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

Cosmic Rays

Cosmic rays are extremely energetic subatomic particles that travel through outer space at nearly the speed of light. Scientists learn about deep space by studying galactic cosmic rays, which originate many light-years away (a light-year represents the distance light travels in one year). This photograph, taken in the late 1940s with a special photographic emulsion called the Kodak NT4, records a collision of a cosmic-ray particle with a particle in the film. A cosmic-ray particle produced the track that starts at the top left corner of the photograph; this particle collided with a nucleus in the center of the photograph to create a spray of subatomic particles.

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.

D1

Discovery of the Muon

In 1935 the Japanese physicist Yukawa Hideki developed a theory explaining how a nucleus is held together, despite the mutual repulsion of its protons, by postulating the existence of a particle intermediate in mass between the electron and the proton. In 1936 Anderson and his coworkers discovered a new particle of 207 electron masses in secondary cosmic radiation; now called the mu-meson or muon, it was first thought to be Yukawa's nuclear “glue.” Subsequent experiments by the British physicist Cecil Frank Powell and others led to the discovery of a somewhat heavier particle of 270 electron masses, the pi-meson or pion (also obtained from secondary cosmic radiation), which was eventually identified as the missing link in Yukawa's theory.

Many additional particles have since been found in secondary cosmic radiation and through the use of large accelerators. They include numerous massive particles, classed as hadrons (particles that take part in the “strong” interaction, which binds atomic nuclei together), including hyperons and various heavy mesons with masses ranging from about one to three proton masses; and intermediate vector bosons such as the W and Z⁰ particles, the carriers of the “weak” nuclear force. They may be electrically neutral, positive, or negative, but never have more than one elementary electric charge e. Enduring from 10^-8 to 10^-14 sec, they decay into a variety of lighter particles. Each particle has its antiparticle and carries some angular momentum. They all obey certain conservation laws involving quantum numbers, such as baryon number, strangeness, and isotopic spin.

In 1931 Pauli, in order to explain the apparent failure of some conservation laws in certain radioactive processes, postulated the existence of electrically neutral particles of zero-rest mass that nevertheless could carry energy and momentum. This idea was further developed by the Italian-born American physicist Enrico Fermi, who named the missing particle the neutrino. Uncharged and tiny, it is elusive, easily able to penetrate the entire earth with only a small likelihood of capture. Nevertheless, it was eventually discovered in a difficult experiment performed by the Americans Frederick Reines and Clyde Lorrain Cowan, Jr. Understanding of the internal structure of protons and neutrons has also been derived from the experiments of the American physicist Robert Hofstadter, using fast electrons from linear accelerators.

Yukawa Hideki

Japanese physicist Yukawa Hideki won the 1949 Nobel Prize in physics. Based on his research into quantum mechanics and the fields of force affecting elementary particles, he theoretically deduced the existence of mesons, a family of subatomic particles composed of quarks and antiquarks and having intermediate mass.

In the late 1940s a number of experiments with cosmic rays revealed new types of particles, the existence of which had not been anticipated. They were called strange particles, and their properties were studied intensively in the 1950s. Then, in the 1960s, many new particles were found in experiments with the large accelerators. The electron, proton, neutron, photon, and all the particles discovered since 1932 are collectively called elementary particles. But the term is actually a misnomer, for most of the particles, such as the proton, have been found to have very complicated internal structure.

Elementary particle physics is concerned with (1) the internal structure of these building blocks and (2) how they interact with one another to form nuclei. The physical principles that explain how atoms and molecules are built from nuclei and electrons are already known. At present, vigorous research is being conducted on both fronts in order to learn the physical principles upon which all matter is built.

One popular theory about the internal structure of elementary particles is that they are made of so-called quarks (see Quark), which are subparticles of fractional charge; a proton, for example, is made up of three quarks. This theory was first proposed in 1964 by the American physicists Murray Gell-Mann and George Zweig. The theory explains a number of phenomena, and physicists have collected a great deal of evidence of quarks in combinations with each other. No individual quarks have been observed, however, and current theory suggests that quarks may never be released as separate entities except under such extreme conditions as those found during the very creation of the universe. The theory postulated three kinds of quarks, but later experiments, especially the discovery of the J/psi particle in 1974 by the American physicists Samuel C. C. Ting and Burton Richter, called for the introduction of three additional kinds.