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verifiable predictions. That is, we must be able to make observations about the real world that either support it or disprove it. Atomic theory was initially speculative, but gained more and more credibility as data from chemistry piled up supporting the existence of atoms. Although we couldn’t actually see atoms until scanning-probe microscopy was invented in 1981 (and under the microscope they do look like the little balls we envision), scientists were already convinced long before that atoms were real. Similarly, a good theory makes predictions about what we should find if we look more closely at nature. And if those predictions are met, it gives us more confidence that the theory is true. Einstein’s general theory of relativity, proposed in 1916, predicted that light would be bent as it passed by a large celestial body. (To be technical, the gravity of such a body distorts space-time, which distorts the path of nearby photons.) Sure enough, Arthur Eddington verified this prediction in 1919 by showing, during a solar eclipse, that light coming from distant stars was bent as it went by the sun, shifting the stars’ apparent positions. It was only when this prediction was verified that Einstein’s theory began to be widely accepted.
    Because a theory is accepted as “true” only when its assertions and predictions are tested over and over again, and confirmed repeatedly, there is no one moment when a scientific theory suddenly becomes a scientific fact. A theory becomes a fact (or a “truth”) when so much evidence has accumulated in its favor—and there is no decisive evidence against it—that virtually all reasonable people will accept it. This does not mean that a “true” theory will never be falsified. All scientific truth is provisional, subject to modification in light of new evidence. There is no alarm bell that goes off to tell scientists that they’ve finally hit on the ultimate, unchangeable truths about nature. As we’ll see, it is possible that despite thousands of observations that support Darwinism, new data might show it to be wrong. I think this is unlikely, but scientists, unlike zealots, can’t afford to become arrogant about what they accept as true.
    In the process of becoming truths, or facts, scientific theories are usually tested against alternative theories. After all, there are usually several explanations for a given phenomenon. Scientists try to make key observations, or conduct decisive experiments, that will test one rival explanation against another. For many years, the position of the earth’s landmasses was thought to have been the same throughout the history of life. But in 1912, the German geophysicist Alfred Wegener came up with the rival theory of “continental drift,” proposing that continents had moved about. Initially, his theory was inspired by the observation that the shapes of continents like South America and Africa could be fitted together like pieces of a jigsaw puzzle. Continental drift then became more certain as fossils accumulated and paleontologists found that the distribution of ancient species suggested that the continents were once joined. Later, “plate tectonics” was suggested as a mechanism for continental movement, just as natural selection was suggested as the mechanism for evolution: the plates of the earth’s crust and mantle floated about on more liquid material in the earth’s interior. And although plate tectonics was also greeted with skepticism by geologists, it was subject to rigorous testing on many fronts, yielding convincing evidence that it is true. Now, thanks to global positioning satellite technology, we can even see the continents moving apart, at a speed of two to four inches per year, about the same rate that your fingernails grow. (This, by the way, combined with the unassailable evidence that the continents were once connected, is evidence against the claim of “young-earth” creationists that the earth is only six to