The concept of the atom'the smallest physical building block of nature'has been around at least since ancient Greece. Leucippus and Democritus conceived of a mechanical or physical atom; in the Middle Ages the Islamic philosophers Ibn Rushd and Agostino Nifo added a chemical role to atomic theory. In the 17th century, Descartes' mechanical philosophy extended the idea of a corpuscle moving in a 'plenum'; and Robert Boyle suggested the possibility of subatomic particles. During the 18th and 19th centuries, atomic theory was involved in the growing understanding of chemistry and in debates about whether light is made of particles or waves. Atoms also were used to theoretically explain electricity, especially after J.C.Maxwell in 1873 showed that light, electricity, and magnetism are all forms of electromagnetic radiation. J.J. Thompson discovered the electron in 1897, stimulating interest in the internal structure of the atom. Prominent models of the atom included Kelvin's vortex model (1867), Thompson's plum pudding model (1904), and Rutherford's nuclear model (1911). Experimental and theoretical problems began to undermine classical physical theory, and in 1900 Max Panck postulated the 'quantum''a smallest possible unit of energy. Energy thus became 'atomized'; in 1905. Einstein's studies of the photoelectric effect suggested that light itself is atomized. In 1912, Neils Bohr created an atomic model that has 'rings' of orbiting electrons, thus accommodating quantum theory and the latest experimental results. Continued elaboration of this model produced what's called the 'Copenhagen Interpretation'; this conception of the atom explains experimental results yet it abandons precise definition of atomic behavior, abandons classical continuity in favor of quantum discontinuity, uses statistics rather than unambiguous definition, and abandons many traditional notions of determinism and causality.
In 1905, Albert Einstein published his Special Theory of Relativity, followed by the General Theory of Relativity in 1916. He firmly established (1) the idea that all judgement about motion is a matter of perspective; (2) that energy and mass are interrelated (E=mc2); and (3) that nothing can move faster than the speed of light (which does not vary). Einstein's theory of the space - time continuum was dramatically confirmed in a 1919 experiment during a solar eclipse. "Relativity" is a concept rooted in the tension between appearance and reality, and it reaches far back in history. Heraclitus argued that only change is real; Parmenides argued that change is impossible, and his follower Zeno invented paradoxes illustrating many of the problems in concepts like space, time, and infinity. Protagoras even argued that there is no single, correct view of reality, but that reality for any person is precisely as it seems to that person. In his words, "Man is the measure of all things." Plato used mathematical reasoning to discern reality from mere appearance, and modern natural science emerged from centuries of effort to acquire objective knowledge. The greatest scientists of the Renaissance and Enlightenment -- including Copernicus, Kepler, Galileo, and Newton -- believed that some real or absolute space and time are independent of the senses. But Immanuel Kant, J.C. Maxwell, Ernest Mach and Henri Poincare chipped away at this idea in the 18th and 19th centuries.
Science is a way of knowing that's characterized by the rules of logic and the methods of experiment. But the conflict between logic and experiment has created a long-standing tension in scientific understanding. The classical period and the Middle Ages favored rationalism; most thinkers began with general, self-evident truths, using deductive reasoning to draw more specific logical conclusions. Empiricism begins with experimental or sensory experience; a bottom-up process of inductive reasoning produces more general hypotheses and theories. David Hume emphasized the role of perception in knowing things, but he argued that past experience cannot justify firm conclusions about the future. Immanuel Kant emphasized the role of the mind in all observation, showing that everything we see is "theory laden;" Auguste Comte and the "positivists" insisted that what is real is observable. Karl Popper emphasized the role of refutation in weeding out bad scientific theories, and Thomas Kuhn suggested that scientific progress occurs in a succession of explanatory "paradigms." Kuhn, Feyerabend, and others have shown the difficulties of distinguishing between experiment and theory, or fact and interpretation. Science has shown that absolute truth is elusive. Yet when science is judged by its usefulness, it is one of the most spectacular achievements of human kind. Science is a never-ending cycle of questions and answers-a perpetual process of discovery.
