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we call it Occam’s razor. The idea is that if we have two alternate explanations for the same phenomenon, we should prefer the simpler explanation. This belief is also reflected in other ways. When we come up with a single, simple explanation that explains a wide variety of apparently disparate phenomena, we’re inclined to think that it’s true. We shout “Eureka,” we’ve found it, when something that seemed complex turns out to have a simple explanation. Think of Newton’s amazing discovery that his laws of gravitation explain both how objects fall on Earth, and also the motion of the planets around the sun. Before Newton’s discovery, those phenomena seemed completely separate from one another: how remarkable that the same laws explain both!

Our confidence in the truth of simple explanations is so great that when we discover apparent violations of such an explanation, we may go to great lengths to save it. In the 1970s the astronomer Vera Rubin discovered that stars toward the outer reaches of our Milky Way galaxy are rotating around the center of the galaxy much faster than we’d expect on the basis of our best theory of gravity, the general theory of relativity. But rather than give up on general relativity, most astronomers instead prefer to postulate the existence of invisible dark matter permeating the galaxy. If the distribution of dark matter is just right, then general relativity can correctly account for the speed of stars on the outer edges of the galaxy. By comparison to the popularity of dark matter, new theories of gravity have been pursued by relatively few astronomers.

So far I’ve made little distinction between conventional explanations and complex models. This blithe conflation of the two has perhaps bothered some readers. Many people believe there is a hard and fast distinction between an explanation and a model: explanations contain some element of the truth, while models are merely convenient crutches, useful for illuminating some phenomenon, but ultimately not expressing the truth. This point of view has an intuitive appeal, but in the history of science the distinction between models and explanations is blurred to the point of nonexistence. Ideas that start out as “mere” models often contain the seed of truths that surprise even their originators. In 1900 the physicist Max Planck was trying to understand how the color and intensity of light emitted by an object depend upon its temperature. For example, burning coals at first glow red, but as the coals heat up, they change color and will eventually glow blue. Figuring out the relationship between temperature and color was a puzzle because the best physical theories of the day gave two different answers, both of which were contradicted by experiment! Planck tried many ideas to solve the problem, eventually settling on a model in which he made the ad hoc assumption that the energy associated with light must ity in quantized packets, that is, must be a multiple of some basic unit. This was an arbitrary assumption, and Planck himself later said, “I really did not give it much thought”—it was just a trick that led him to the result he wanted. In fact, it turned out that the idea in Planck’s model was ultimately the seed for one of the great discoveries of science, the theory of quantum mechanics. So should we regard Planck’s ideas as merely a model, or as an explanation? At the time, it looked like a model, but that model contained a truth deeper than any of the theories of the day. In any reasonable accounting, Planck’s ideas are both a model and an explanation: models and explanations are both part of the same continuum. And so, as online tools enhance our ability to construct and extract meaning from complex models, they will also change the nature of scientific explanation.

CHAPTER 7

Democratizing Science

On August 7, 2007, a 25-year-old Dutch schoolteacher named Hanny van Arkel was surfing the web when she came across the Galaxy Zoo website. As you may recall from the opening chapter, Galaxy Zoo recruits volunteers to help classify galaxy