The discovery of radioactivity by the French physicist Henri Becquerel in 1896 is generally taken to mark the beginning of 20th-century physics. The successful isolation of radium and other intensely radioactive substances by Marie and Pierre Curie focused the attention of scientists and the public on this remarkable phenomenon and promoted a wide range of experiments.

Ernest Rutherford soon took the lead in studying the nature of radioactivity. He found that there are two distinct kinds of radiation emitted in radioactivity called alpha and beta rays. The alpha rays proved to be positively charged particles identical to ionized helium atoms. Beta rays are much less massive negatively charged particles; they were shown to be the same as the electrons discovered by J.J. Thomson in cathode rays in 1897. A third kind of ray, designated gamma, consists of high-frequency electromagnetic radiation.

Rutherford proposed that radioactivity involves a transmutation of one element into another. This proposal called into question one of the basic assumptions of 19th-century chemistry: that the elements consist of qualitatively different substances—92 of them by the end of the century. It implied a return to the ideas of Prout and the ancient atomists—namely, that everything in the world is composed of only one or a few basic substances.

Transmutation, according to Rutherford and his colleagues, was governed by certain empirical rules. For example, in alpha decay the atomic number of the “daughter” element is two less than that of the “mother” element, and its atomic weight is four less; this seems consistent with the fact that the alpha ray, identified as helium, has atomic number 2 and atomic weight 4, so that total atomic number and total atomic weight are conserved in the decay reaction.

Using these rules, Rutherford and his colleagues could determine the atomic numbers and atomic weights of many substances formed by radioactive decay, even though the substances decayed so quickly into others that these properties could not be measured directly. The atomic number of an element determines its place in Mendeleyev’s periodic table (and thus its chemical properties; see above). It was found that substances of different atomic weight could have the same atomic number; such substances were called isotopes of an element.

Although the products of radioactive decay are determined by simple rules, the decay process itself seems to occur at random. All one can say is that there is a certain probability that an atom of a radioactive substance will decay during a certain time interval, or, equivalently, that half of the atoms of the sample will have decayed after a certain time—i.e., the half-life of the material.

The nucleus

At the University of Manchester (England), Rutherford led a group that rapidly developed new ideas about atomic structure. On the basis of an experiment conducted by Hans Geiger and Ernest Marsden in which alpha particles were scattered by a thin film of metal, Rutherford proposed a nuclear model of the atom (1911). In this model, the atom consists mostly of empty space, with a tiny, positively charged nucleus that contains most of the mass, surrounded by one or more negatively charged electrons. Henry G.J. Moseley, an English physicist, showed by an analysis of X-ray spectra that the electric charge on the nucleus is simply proportional to the atomic number of the element.

During the 1920s physicists thought that the nucleus was composed of two particles: the proton (the positively charged nucleus of hydrogen) and the electron. In 1932 English physicist James Chadwick discovered the neutron, a particle with about the same mass as the proton but no electric charge. Since there were technical difficulties with the proton–electron model of the nucleus, physicists were willing to accept Heisenberg’s hypothesis that it consists instead of protons and neutrons. The atomic number is then simply the number of protons in the nucleus, while the mass number, the integer closest to the atomic weight, is equal to the total number of neutrons and protons. As mentioned above, this simple model of nuclear structure provided the basis for Hans Bethe’s theory of the formation of elements from hydrogen in stars.

In 1938 German physicists Otto Hahn and Fritz Strassmann found that, when uranium is bombarded by neutrons, lighter elements such as barium and krypton are produced. This phenomenon was interpreted by Lise Meitner and her nephew Otto Frisch as a breakup, or fission, of the uranium nucleus into smaller nuclei. Other physicists soon realized that since fission produces more neutrons, a chain reaction could result in a powerful explosion. World War II was about to begin, and physicists who had emigrated from Germany, Italy, and Hungary to the United States and Great Britain feared that Germany might develop an atomic bomb that could determine the outcome of the war. They persuaded the U.S. and British governments to undertake a major project to develop such a weapon first. The U.S. Manhattan Project did eventually produce atomic bombs based on the fission of uranium or of plutonium, a new artificially created element, and these were used against Japan in August 1945. Later, an even more powerful bomb based on the fusion of hydrogen atoms was developed and tested by both the United States and the Soviet Union. Thus, nuclear physics began to play a major role in world history.

Einstein’s 1905 trilogy

In a few months during the years 1665–66, Newton discovered the composite nature of light, analyzed the action of gravity, and invented the mathematical technique now known as calculus—or so he recalled in his old age. The only person who has ever matched Newton’s amazing burst of scientific creativity—three revolutionary discoveries within a year—was Albert Einstein, who in 1905 published the special theory of relativity, the quantum theory of radiation, and a theory of Brownian movement that led directly to the final acceptance of the atomic structure of matter.

Relativity theory has already been mentioned several times in this article, an indication of its close connection with several areas of physical science. There is no room here to discuss the subtle line of reasoning that Einstein followed in arriving at his amazing conclusions; a brief summary of his starting point and some of the consequences will have to suffice.

In his 1905 paper on the electrodynamics of moving bodies, Einstein called attention to an apparent inconsistency in the usual presentation of Maxwell’s electromagnetic theory as applied to the reciprocal action of a magnet and a conductor. The equations are different depending on which is “at rest” and which is “moving,” yet the results must be the same. Einstein located the difficulty in the assumption that absolute space exists; he postulated instead that the laws of nature are the same for observers in any inertial frame of reference and that the speed of light is the same for all such observers.

From these postulates Einstein inferred: (1) an observer in one frame would find from his own measurements that lengths of objects in another frame are contracted by an amount given by the Lorentz–FitzGerald formula; (2) each observer would find that clocks in the other frame run more slowly; (3) there is no absolute time—events that are simultaneous in one frame of reference may not be so in another; and (4) the observable mass of any object increases as it goes faster.

Closely connected with the mass-increase effect is Einstein’s famous formula E = mc²: mass and energy are no longer conserved but can be interconverted. The explosive power of the atomic and hydrogen bombs derives from the conversion of mass to energy.

In a paper on the creation and conversion of light (usually called the “photoelectric effect paper”), published earlier in 1905, Einstein proposed the hypothesis that electromagnetic radiation consists of discrete energy quanta that can be absorbed or emitted only as a whole. Although this hypothesis would not replace the wave theory of light, which gives a perfectly satisfactory description of the phenomena of diffraction, reflection, refraction, and dispersion, it would supplement it by also ascribing particle properties to light.

Until recently the invention of the quantum theory of radiation was generally credited to another German physicist, Max Planck, who in 1900 discussed the statistical distribution of radiation energy in connection with the theory of blackbody radiation. Although Planck did propose the basic hypothesis that the energy of a quantum of radiation is proportional to its frequency of vibration, it is not clear whether he used this hypothesis merely for mathematical convenience or intended it to have a broader physical significance. In any case, he did not explicitly advocate a particle theory of light before 1905. Historians of physics still disagree on whether Planck or Einstein should be considered the originator of the quantum theory.

Einstein’s paper on Brownian movement seems less revolutionary than the other 1905 papers because most modern readers assume that the atomic structure of matter was well established at that time. Such was not the case, however. In spite of the development of the chemical atomic theory and of the kinetic theory of gases in the 19th century, which allowed quantitative estimates of such atomic properties as mass and diameter, it was still fashionable in 1900 to question the reality of atoms. This skepticism, which does not seem to have been particularly helpful to the progress of science, was promoted by the empiricist, or “positivist,” philosophy advocated by Auguste Comte, Ernst Mach, Wilhelm Ostwald, Pierre Duhem, Henri Poincaré, and others. It was the French physicist Jean Perrin who, using Einstein’s theory of Brownian movement, finally convinced the scientific community to accept the atom as a valid scientific concept.