Overview: Physical Sciences 1900-1949

By the dawn of the twentieth century, more than two centuries had elapsed since the publication of Isaac Newton's (1642-1727) Principia, which set forth the foundations of classical physics. During those intervening centuries, scientists embraced empiricism and sought new and ingenious ways to understand the physical world. In addition to fueling industrial revolutions in Europe and the United States, that same persistence and technological inventiveness allowed scientists to make increasingly exquisite and delicate calculations regarding physical phenomena. Advances in mathematics, especially during the nineteenth century, allowed the development of sophisticated models of nature that became accepted as a common language of science.

More tantalizingly, many of these mathematical insights pointed toward a physical reality not necessarily limited to three dimensions and not necessarily absolute in time and space. On top of a steady tempo of refinement and discovery there emerged a new and uncharted harmony of mathematics, experimentation, and scientific insight. During the first half of the twentieth century, these themes found full expression in the intricacies of quantum and relativity theory. Scientists, mathematicians, and philosophers united to examine and explain the innermost workings of the universe—both on the scale of the very small subatomic world and on the grandest of cosmic scales.

Nineteenth-century experimentalism culminated in the unification of concepts regarding electricity, magnetism, and light, formulated by Scottish physicist James Clerk Maxwell (1831-1879) in his four famous equations describing electromagnetic waves. Moreover, at the start of the twentieth century, the speed of light was well known and precisely determined. The ingenious experiments of Albert Michelson (1852-1931) and Edward Morley (1838-1923), however, demonstrated an absence of a propagation medium or "ether" and, as a consequence, cast doubt on the existence of an absolute frame of reference for natural phenomena. In 1905, in one grand and sweeping theory of special relativity, Albert Einstein (1879-1955) provided an explanation for seemingly conflicting and counterintuitive experimental determinations of the constancy of the speed of light, length contraction, time dilation, and mass enlargements. A scant decade later, Einstein again revolutionized concepts of space, time, and gravity with his general theory of relativity.

Prior to Einstein's revelations, German physicist Maxwell Planck (1858-1947) proposed that atoms absorb or emit electromagnetic radiation in discrete units of energy termed quanta. Although Plank's quantum concept seemed counterintuitive to well-established Newtonian physics, quantum mechanics accurately described the relationships between energy and matter on an atomic and subatomic scale and provided a unifying basis to explain the properties of the elements.

Concepts regarding the stability of matter also seemed ripe for revolution. Far from indivisibility, advancements in the discovery and understanding of radioactivity culminated in a renewed quest to find the most elemental and fundamental particles of nature. In 1913 Danish physicist Niels Bohr (1885-1962) published a model of the hydrogen atom that, by incorporating quantum theory, dramatically improved existing classical Copernican-like atomic models. The quantum leaps of electrons between orbits proposed by the Bohr model accounted for Planck's observations and also explained many important properties of the photoelectric effect described by Einstein.

More mathematically complex atomic models were to follow based on the work of the French physicist Louis Victor de Broglie (1892-1987), Austrian physicist Erwin Schrödinger (1887-1961), German physicist Max Born (1882-1970) and English physicist P.A.M. Dirac (1902-1984). More than simple refinements of the Bohr model, however, these scientists made fundamental advances in defining the properties of matter—especially the wave nature of subatomic particles. Matter became to be understood as a synthesis of wave and particle properties. By 1950 the articulation of the elementary constituents of atoms grew dramatically in numbers and complexity.

Advancements in physics also spilled over into other scientific disciplines. In 1925 Austrian-born physicist Wolfgang Pauli (1900-1958) advanced the hypothesis that no two electrons in an atom can simultaneously occupy the same quantum state (i.e., energy state). Pauli's exclusion principle made completely understandable the structure of the periodic table and was a major advancement in chemistry.

Profound scientific advances were also made possible by the advent and growth of commercial research labs that provided additional outlets and facilities for research beyond traditional academia. In contrast, radio astronomy had more humble (literally backyard) beginnings as American engineer Karl Jansky (1905-1945) discovered the existence of radio waves emanating from beyond Earth. Subsequently, radio astronomy advanced to become one of the most important and productive means of astronomical observation.

A greater understanding of nature was not limited to the subatomic or astronomical worlds. Advances in geology and geophysics allowed scientists a deeper understanding of Earth processes. As details regarding the inner Earth yielded to seismic analysis, the enigmatic patterns of weather and climate yielded to meteorological analysis. Geologists and astronomers began to understand the dynamic interplay of gradual and cataclysmic geologic processes both on Earth and on extraterrestrial bodies.

Against a maddeningly complex backdrop of politics and fanaticism that resulted in two World Wars within the first half of the twentieth century, scientific knowledge and skill became more than a strategic advantage. The deliberate misuse of science scattered poisonous gases across World War I battlefields at the same time that advances in physical science (e.g., x-ray diagnostics) provided new ways to save lives.

Countries struggled to gain both offensive advantages and defensive capabilities by the development of radar and sonar. As the century progressed, the balance of scientific and technological power often swayed between adversaries. This balance eventually tipped toward the United States when large numbers of continental European scientists, particularly German Jews, emigrated to America during 1930s and 1940s to avoid Adolf Hitler's cruel and murderous Third Reich. Without such an exodus, these brilliant scientists may have perished in the Holocaust along with their great contributions to both wartime and postwar science. The dark abyss of World War II brought the dawn of the atomic age. In one blinding flash, the Manhattan Project created the most terrifying of weapons, which—in an instant—could forever change the course of human history and life on Earth.

Einstein's theories of relativity brought about a revolution in science and philosophy, rivaled only by the contributions of Newton. Although the development of relativity and quantum theory mooted the quest—spurred by prominent philosophical and religious thought—to find an absolute frame of reference for natural phenomena, the insights of relativity theory and quantum theory also stretched the methodology of science. No longer would science be mainly an exercise in inductively applying the results of experimental data. Experimentation, instead of being only a genesis for theory, became a testing ground to falsify the apparent truths unveiled by increasingly mathematical models of the universe. With the formulation of quantum mechanics, physical phenomena could no longer be explained in terms of deterministic causality, that is, as a result of at least a theoretically measurable chain causes and effects. Instead, physical phenomena were described as the result of fundamentally statistical, unreadable, indeterminist (unpredictable) processes.

The development of quantum theory, especially the delineation of Planck's constant and the articulation of the Heisenburg uncertainty principle, carried profound philosophical implications regarding the limits on knowledge. By mid-century, scientists were able to substantially advance the concept that space-time was a creation of the universe itself. This insight set the stage for the development of modern cosmological theory (i.e., theories regarding the nature and formation of the universe) and provided insight into the evolutionary stages of stars (e.g., neutron stars, pulsars, black holes, etc.) that carried with it an understanding of nucleosynthesis (the formation of elements), which linked mankind to the lives of the stars.

K. LEE LERNER