Which Revolution was more Important?

The Reformation (1500)
The Scientific Revolution (1600)
The Industrial Revolution (1850)
The Green Revolution (1950)

What is a Scientific Theory?

We examine what a scientific theory is and how it differs from the usual English meaning of the word.

There are Theories and there are Theories

We need to establish right away that the way the word theory is used in English is quite different from the way it used in science (I once read a long series of vituperative and futile letters to the editor that was based on failing to realize this distinction). In common speech, theory is taken to mean something hypothetical or unproven, as in, That's only a theory. In science, a theory is a systematic description of previous observations. The failure to realize this distinction is often made in the Intelligent Design controversy, where adherents of ID claim that evolution is just a theory. Yes, it is just a theory, but it is a scientific theory, supported by thousands upon thousands of observations and experiments, and biologists are justly proud of it.

Science is not Democratic

Mixing up theory and scientific theory can lead to sadly mistaken reasoning. Thus, for example, if one thinks of all scientific theories as equally plausible hypotheses about nature, there is no reason to prefer one over another. Thus the politicization of science is facilitated. And what perfect fodder for wishy-washy politicians! In August of 2005, George Bush said that he felt that both sides of the evolution debate should be taught in schools. John McCain repeated the same inanity in an interview in the Arizona Daily Star in September of 2005.

All scientific theories are not equal -- there is no democracy in science. You may adhere to the idea that matter is made of atoms and I may insist that it is made of tiny twinkies held together by vanilla cream. Should these theories be ascribed equal weight? No. The atomist view of nature is supported by thousand of experiments while the twinky theory is only supported by one, somewhat twisted, mind.

How can a non-expert tell the difference between proffered theories? Its like declaring a winner before the horses have crossed the finish line. Unfortunately, its not easy. Typically one attempts to judge the preponderance of expert opinion. But this also requires a certain expertise; thus it is important for scientists and journalists to step to the plate and shoulder some of this responsibility. Here we have another problem: scientists are not trained as popular communicators of science and journalists are trained to examine both sides of an issue (even if one side is the world community of, say, climate experts and the other is a wild-eyed doctor with a twitch and an obvious agenda).

Scientific Theories are Predictive

A theory is generally constructed to describe (or explain) a set of observations (for example, the orbits of the planets). But to be useful it needs to be predictive, namely, it should be able to accurately describe things that we do not know already (for example, the orbits of planets around distant stars, or of recently discovered comets). A good theory also needs to be consistent with the Laws of Physics, since these encapsulate fundamental aspects of nature.

Because theories are predictive, they are also testable (provided their predictions are practically realizable -- predicting what would happen if George Bush plummeted into the sun is not very testable). The idea of testing theories, also called falsifiability, is often touted as their most important property, but I think that this feature has been interpreted rather naively (for a more in-depth discussion see Advanced Theory). Typically one does not throw away a theory simply because one of its predictions is wrong or inaccurate. Why do that if it is useful in all other cases? We simply make do with our flawed theory and note where it has failed. In this sense theories are tentative, and scientists are always striving to overthrow the current paradigm. In fact, the history of science is essentially a continuous chain of theory modification.

The Evolution of Scientific Theory

Many modern theories originate in antiquity; tracing their evolution provides visceral realization of the continuity of science and the sense in which we stand on the shoulders of giants.

Electricity and Magnetism

Rudimentary ideas of electricity date back to the ancient Greeks, like Thales of Miletus (624-547 BC), who noticed the (static) electric properties of materials like amber and fur and the peculiar magnetic properties of lodestone. Knowledge did not advance much until the invention of a reliable source of electric current by Volta (1745 - 1827), called the voltaic pile, which allowed serious experimentation to begin. The early 1800s saw a flowering of experimental knowledge, led by Coulomb (1736 - 1806), Orsted (1777 - 1851), the incomparable Faraday (1791 - 1867), and many others. With their work came the realization that electricity and magnetism are different faces of the same electromagnetic phenomena -- an idea that was mathematically entrenched by Maxwell (1831-1879). Maxwell's theory of electromagnetism brilliantly encompassed known experimental properties of electricity and magnetism and, more importantly, made astonishing predictions for the speed of light and the properties of radio waves. (You can find Maxwell's desk at the Cavendish Lab in Cambridge. Oddly, a little sign sits on his desk that reads, Please do not put cups of tea on Maxwell's desk.)

Maxwell's work led to a second great flowering of knowledge that started in the late 1800s and continues to this day. Experimentalists such as Hertz (1857 - 1894) and Helmholtz (1821 - 1894) established that electricity could be used to create and manipulate electromagnetic (radio) waves. This led directly to the development of radio, television, radar, microwaves, and related technology that are such a familiar part of everyday life.

Maxwell's theory that so effectively supplanted earlier patch-work laws was soon challenged by the emerging quantum theory of the microscopic world. At issue was how these these different and fundamental theories could be combined. The solution was finally obtained through the work of Dirac (1902 - 1984), Feynman (1918 - 1988), Schwinger (1918 - 1994), and a great many others. The result was that, until 1968, Maxwell's electromagnetic theory had been superseded by Quantum Electrodynamics. This was in turn superseded by the Standard Model, which combines electromagnetism with the weak force. It comes as no surprise that thousands of people are currently working around the world to knock the Standard Model off of its pedestal.

Every improvement in this 2500 year history of electricity has been one that expanded previous knowledge, making the theory applicable to a wider range of phenomena, sometimes unifying disparate phenomena, but never completely displacing the older theory. In fact laws from the early 1800s are still taught in university physics and engineering classes because they remain useful. In turn, Maxwell's theory remains valid (by which I mean accurate) except at atomic scales, and we expect the current quantum theory to be accurate in all cases except those with extremely high energies (such as present immediately after the Big Bang). Continuous improvement, yes. Revolution, sometimes. But a good theory never truly dies.

Gravity

It is difficult to look up at night and not wonder what is going on up there. The ancient Babylonians made remarkably good progress in measuring basic properties of the solar system, right down to details in the wobbles of the moon's orbit. By 300 BC Aristotle was able to argue that the Milky Way could not be made up of stars that were in the Earth's shade (as claimed by Democritus) based on statements that the sun is much larger than the Earth and that stars are much farther from the Earth than the sun.

The greatest astronomer of antiquity was Hipparchus (c. 190 - c. 120 BC). He was the first European to make accurate quantitative models of lunar and solar motion (predicting, for example, solar eclipses). These models were based on epicycles, which place the moon and planets on circular orbits that follow larger circular orbits. He also made decent estimates of the sizes of the moon and sun and their distances to the Earth.

It would be more than 200 years before the theories of Hipparchus were surpassed by Ptolemy (83 - 161). Ptolemy expanded the idea of planetary epicycles to model the universe as nested spheres. His model and computational methods were sufficiently accurate that navigators and astrologers could make practical use of them.

The influence of Ptolemy and Aristotle was great enough that it would be 1300 years before the next major advance occurred. The occasion was the death-bed publication of Copernicus's De Revolutionibis, which removed the Earth from the center of the universe. Although Copernicus was proceeded by a long line of heliocentrists (beginning with Aristarchus of Samos around 300 BC) he was the first to place it on firm mathematical ground, showing heliocentrism to be the equal of Ptolemaic epicycles in terms of predictive power.

With the correct basic structure in place, the pace of advances picked up. Kepler (1571 - 1630) built on the unprecedented measurements of his (noseless) mentor Tycho Brahe to postulate three laws of planetary motion. Kepler's Laws proved to be simpler and more accurate than Ptolemaic epicycles and, more importantly, led to the mechanistic revolution of Newton. It is worth mentioning that Kepler (and everyone else mentioned here) was not immune to contemporary prejudices. Kepler devoted much effort to reconciling heliocentrism with the bible, arguing that the universe was an image of God, the Sun corresponded to the Father, the stellar sphere to the Son, and the interstitial void to the Holy Spirit. Kepler's Laws proved to be simpler and more accurate than Ptolemaic epicycles and, more importantly, led to the mechanistic revolution of Newton.

The revolutionary aspect of Newton's great work, Principia (1687), was the claim that gravity controlled the mundane (such as apples falling on noggins) and the celestial. Specifically, Newton was able to derive Kepler's Laws with some fancy mathematics and some simple postulates about a universal force of gravity. Newton's achievement was so overwhelming that many scientists thought that scientific knowledge was essentially complete. This attitude was only firmly overturned with the advent of quantum theory at the turn of the 20th century.

Eventually Newton's majestic achievement was to succumb to Einstein's Theory of General Relativity (1916) which gave a geometrical interpretation to gravity and predicted many new and surprising effects. The story ends here, but researchers are not happy because General Relativity is not compatible with quantum mechanics, and it is widely believed that GR must be supplanted by something fundamentally different.

Our theme emerges again: scientific theory is built by a long series of incremental improvements, disturbed by occasional great dashes forward. Old theories may fall by the wayside, but generally retain validity in the regime that they were first designed to explain. It is remarkable that the process is still not complete for gravity after 3000 years of sustained effort.