Developments and trends of the 20th and 21st centuries
Astronomy
Some of the most spectacular advances in modern astronomy have come from research on the large-scale structure and development of the universe. This research goes back to William Herschel’s observations of nebulas at the end of the 18th century. Some astronomers considered them to be “island universes”—huge stellar systems outside of and comparable to the Milky Way Galaxy, to which the solar system belongs. Others, following Herschel’s own speculations, thought of them simply as gaseous clouds—relatively small patches of diffuse matter within the Milky Way Galaxy, which might be in the process of developing into stars and planetary systems, as described in Laplace’s nebular hypothesis.
In 1912 Vesto Slipher began at the Lowell Observatory in Arizona an extensive program to measure the velocities of nebulas, using the Doppler shift of their spectral lines. (Doppler shift is the observed change in wavelength of the radiation from a source that results from the relative motion of the latter along the line of sight.) By 1925 he had studied about 40 nebulas, most of which were found to be moving away from Earth according to the redshift (displacement toward longer wavelengths) of their spectra.
Although the nebulas were apparently so far away that their distances could not be measured directly by the stellar parallax method, an indirect approach was developed on the basis of a discovery made in 1908 by Henrietta Swan Leavitt at the Harvard College Observatory. Leavitt studied the magnitudes (apparent brightnesses) of a large number of variable stars, including the type known as Cepheid variables. Some of them were close enough to have measurable parallaxes so that their distances and thus their intrinsic brightnesses could be determined. She found a correlation between brightness and period of variation. Assuming that the same correlation holds for all stars of this kind, their observed magnitudes and periods could be used to estimate their distances.
In 1923 American astronomer Edwin Hubble identified a Cepheid variable in the so-called Andromeda Nebula. Using Leavitt’s period–brightness correlation, Hubble estimated its distance to be approximately 900,000 light-years. Since this was much greater than the size of the Milky Way system, it appeared that the Andromeda Nebula must be another galaxy (island universe) outside of our own.
In 1929 Hubble combined Slipher’s measurements of the velocities of nebulas with further estimates of their distances and found that on the average such objects are moving away from Earth with a velocity proportional to their distance. Hubble’s velocity–distance relation suggested that the universe of galactic nebulas is expanding, starting from an initial state about two billion years ago in which all matter was contained in a fairly small volume. Revisions of the distance scale in the 1950s and later increased the “Hubble age” of the universe to more than 10 billion years.
Calculations by Aleksandr A. Friedmann in the Soviet Union, Willem de Sitter in the Netherlands, and Georges Lemaître in Belgium, based on Einstein’s general theory of relativity, showed that the expanding universe could be explained in terms of the evolution of space itself. According to Einstein’s theory, space is described by the non-Euclidean geometry proposed in 1854 by the German mathematician G.F. Bernhard Riemann. Its departure from Euclidean space is measured by a “curvature” that depends on the density of matter. The universe may be finite, though unbounded, like the surface of a sphere. Thus, the expansion of the universe refers not merely to the motion of extragalactic stellar systems within space but also to the expansion of the space itself.
The beginning of the expanding universe was linked to the formation of the chemical elements in a theory developed in the 1940s by the physicist George Gamow, a former student of Friedmann who had emigrated to the United States. Gamow proposed that the universe began in a state of extremely high temperature and density and exploded outward—the so-called big bang. Matter was originally in the form of neutrons, which quickly decayed into protons and electrons; these then combined to form hydrogen and heavier elements.
Gamow’s students Ralph Alpher and Robert Herman estimated in 1948 that the radiation left over from the big bang should by now have cooled down to a temperature just a few degrees above absolute zero (0 K, or −459 °F). In 1965 the predicted cosmic background radiation was discovered by Arno Penzias and Robert Woodrow Wilson of the Bell Telephone Laboratories as part of an effort to build sensitive microwave-receiving stations for satellite communication. Their finding provided unexpected evidence for the idea that the universe was in a state of very high temperature and density 13.8 billion years ago.
The study of distant galaxies also revealed that ordinary visible matter is a tiny fraction of the matter-energy of the universe. In 1933 Fritz Zwicky found that the Coma cluster of galaxies did not contain enough mass in its stars to keep the cluster together. American astronomers Vera Rubin and W. Kent Ford confirmed this finding in the 1970s when they discovered that the stellar mass of a galaxy is only about 10 percent of that needed to keep the stars bound to the galaxy. This “missing mass” came to be called dark matter and makes up 26.5 percent of the matter-energy of the universe.
The dominant component of the universe is dark energy, a repulsive force that accelerates the universe’s expansion. Despite being 73 percent of the universe’s matter-energy, its nature is not well understood. Dark energy was discovered only by observations of distant supernovas in the 1990s made by two international teams of astronomers that included American astronomers Adam Riess and Saul Perlmutter and Australian astronomer Brian Schmidt.
Evolution of stars and formation of chemical elements
Just as the development of cosmology relied heavily on ideas from physics, especially Einstein’s general theory of relativity, so did theories of stellar structure and evolution depend on discoveries in atomic physics. These theories also offered a fundamental basis for chemistry by showing how the elements could have been synthesized in stars.
The idea that stars are formed by the condensation of gaseous clouds was part of the 19th-century nebular hypothesis (see above). The gravitational energy released by this condensation could be transformed into heat, but calculations by Hermann von Helmholtz and Lord Kelvin indicated that this process would provide energy to keep the Sun shining for only about 20 million years. Evidence from radiometric dating, starting with the work of the British physicist Ernest Rutherford in 1905, showed that Earth is several billion years old. Astrophysicists were perplexed: what source of energy has kept the Sun shining for such a long time?
In 1925 Cecilia Payne, a graduate student from Britain at Harvard College Observatory, analyzed the spectra of stars using statistical atomic theories that related them to temperature, density, and composition. She found that hydrogen and helium are the most abundant elements in stars, though this conclusion was not generally accepted until it was confirmed four years later by the noted American astronomer Henry Norris Russell. By this time Prout’s hypothesis that all the elements are compounds of hydrogen had been revived by physicists in a somewhat more elaborate form. The deviation of atomic weights from exact integer values (expressed as multiples of hydrogen) could be explained partly by the fact that some elements are mixtures of isotopes with different atomic weights and partly by Einstein’s relation between mass and energy, E = mc2 (taking account of the binding energy of the forces that hold together the atomic nucleus). German physicist Werner Heisenberg proposed in 1932 that, whereas the hydrogen nucleus consists of just one proton, all heavier nuclei contain protons and neutrons. Since a proton can be changed into a neutron by fusing it with an electron, this meant that all the elements could be built up from protons and electrons—i.e., from hydrogen atoms.
In 1938 German-born physicist Hans Bethe proposed the first satisfactory theory of stellar energy generation based on the fusion of protons to form helium and heavier elements. He showed that once elements as heavy as carbon had been formed, a cycle of nuclear reactions could produce even heavier elements. Fusion of hydrogen into heavier elements would also provide enough energy to account for the Sun’s energy generation over a period of billions of years. Bethe’s theory was extended by Fred Hoyle, Edwin E. Salpeter, and William A. Fowler.
According to the theory of stellar evolution developed by Indian-born American astrophysicist Subrahmanyan Chandrasekhar and others, a star will become unstable after it has converted most of its hydrogen to helium and may go through stages of rapid expansion and contraction. If the star is much more massive than the Sun, it will explode violently, giving rise to a supernova. The explosion will synthesize heavier elements and spread them throughout the surrounding interstellar medium, where they provide the raw material for the formation of new stars and eventually of planets and living organisms.
After a supernova explosion, the remaining core of the star may collapse further under its own gravitational attraction to form a dense star composed mainly of neutrons. This so-called neutron star, predicted theoretically in the 1930s by astronomers Walter Baade and Fritz Zwicky, was first observed as pulsars (sources of rapid, very regular pulses of radio waves), discovered in 1967 by Jocelyn Bell.
More massive stars may undergo a further stage of evolution beyond the neutron star: they may collapse to a black hole, in which the gravitational force is so strong that even light cannot escape. The black hole as a singularity in an idealized space-time universe was predicted from general relativity theory by German astronomer Karl Schwarzschild in 1916. Its role in stellar evolution was later described by American physicists J. Robert Oppenheimer and John Wheeler. Beginning in the 1970s, black holes were observed in X-ray sources and at the centre of some galaxies, particularly quasars.
Solar-system astronomy and extrasolar planets
This area of investigation, which lay relatively dormant through the first half of the 20th century, was revived by the stimulus of the Soviet and American space programs. In 1959 Luna 3 took the first picture of the Moon’s far side. Mariner 2 made the first planetary flyby when it passed Venus in 1962, and Mariner 4 was the first flyby to send back images when it flew by Mars in 1965. Since then, space probes have visited all the planets as well as some dwarf planets, asteroids, and comets, and 12 astronauts landed on the Moon as part of the Apollo program.
These solar-system missions yielded a wealth of complex information. A single example of the resulting change in ideas about the history of the solar system will have to suffice here. Before the first manned lunar landing in 1969, there were three competing hypotheses about the origin of the Moon: (1) formation in its present orbit simultaneously with Earth, as described in the nebular hypothesis; (2) formation elsewhere and subsequent capture by Earth; and (3) ejection from Earth by fission (popularly known theory that the Moon emanated from what is now the Pacific Ocean basin). Following the analysis of lunar samples and theoretical criticism of these hypotheses, lunar scientists came to the conclusion that none of them was satisfactory. Photographs of the surface of Mercury taken by the Mariner 10 spacecraft in 1974, however, showed that it is heavily cratered like the Moon’s surface. This finding, together with theoretical calculations by V.S. Safronov of the Soviet Union and George W. Wetherill of the United States on the formation of planets by accumulation (accretion or aggregation) of smaller solid bodies, suggested that Earth was also probably subject to heavy bombardment soon after its formation. In line with this, a theory proposed by the American astronomers William K. Hartmann and A.G.W. Cameron has become the most popular. According to their theory, Earth was struck by a Mars-sized object, and the force of the impact vaporized the outer parts of both bodies. The vapour thus produced remained in orbit around Earth and eventually condensed to form the Moon. Like the hypothesis proposed by Luis Alvarez that attributes the extinction of the dinosaurs to an asteroid impact, the Hartmann–Cameron theory seemed so bizarre that it could not have been taken seriously until compelling evidence became available.
In 1992 the first extrasolar planets were discovered around a pulsar. More than 4,000 planets have been discovered, many by the Kepler space telescope, which observes the slight dimming of a star when a planet passes in front of it. Many of these planets are unlike those seen in the solar system, and a few orbit within their star’s habitable zones, the orbital space where liquid water (and thus possibly life) could survive on a planet’s surface.
Physics
During the years 1896–1932 the foundations of physics changed so radically that many observers describe this period as a scientific revolution comparable in depth, if not in scope, to the one that took place during the 16th and 17th centuries. The 20th-century revolution changed many of the ideas about space, time, mass, energy, atoms, light, force, determinism, and causality that had apparently been firmly established by Newtonian physics during the 18th and 19th centuries. Moreover, according to some interpretations, the new theories demolished the basic metaphysical assumption of earlier science that the entire physical world has a real existence and objective properties independent of human observation.
Closer examination of 19th-century physics shows that Newtonian ideas were already being undermined in many areas and that the program of mechanical explanation was openly challenged by several influential physicists toward the end of the century. Yet there was no agreement as to what the foundations of a new physics might be. Modern textbook writers and popularizers often try to identify specific paradoxes or puzzling experimental results—e.g., the failure to detect Earth’s absolute motion in the Michelson–Morley experiment—as anomalies that led physicists to propose new fundamental theories such as relativity. Historians of science have shown, however, that most of these anomalies did not directly cause the introduction of the theories that later resolved them. As with Copernicus’s introduction of heliocentric astronomy, the motivation seems to have been a desire to satisfy aesthetic principles of theory structure rooted in earlier views of the world rather than a need to account for the latest experiment or calculation.
