In some fields, not only is it impossible to directly control variables but also the phenomena themselves may not be directly observable. A research design known as indirect experimentation is often used in such fields. Explanations can be tested even if the phenomena being studied are too far away, too small, or too far back in time to be observed directly. For example, giant planets recently have been discovered orbiting distant stars—though we cannot directly observe them. Their presence is indicated by the gravitational effects they have on the suns around which they revolve: because of what we know about how the theory of gravitation works, we can infer that the passage of a big planet around a sun will make the sun wobble. Through the application of principles and laws in which we have confidence, it is possible to infer that these planetary giants do exist and to make estimates of their size and speed of revolution.
Similarly, the subatomic particles that physicists study are too small to be observed directly, but particle physicists certainly are able to test their explanations. By applying knowledge about how particles behave, they are able to create indirect experiments to test claims about the nature of particles. Let's say that a physicist wants to ascertain properties of a particle—its mass, charge, or speed. On the basis of observations of similar particles, he makes an informed estimate of the speed. To test the estimate, he might bombard it with another particle of known mass, because if the unknown particle has a mass of m, it will cause the known particle to ricochet at velocity v. If the known particle does ricochet as predicted, this would support the hypothesis about the mass of the unknown particle. Thus, theory is built piece by piece, through inference based on accepted principles.
In truth, most scientific problems are of this if-then type, whether or not the phenomena investigated are directly observable. If male guppy color is related to predation, then we should see duller males in high-predation environments. If a new drug stimulates the immune system, then individuals taking it should have fewer colds than the controls do. If human hunters were involved in the destruction of large Australian land mammals, we should see extinction events that correlate with the appearance of the first Aborigines. We test by consequence in science all the time. Of course—because scientific problems are never solved so simply—if we get the consequence we predict, this does not mean we have proved our explanation. If you found that guppy color does vary in environments where predation differs, this does not mean you've proved yourself right about the relationship between color and predation. To understand why, we need to consider what we mean by proof and disproof in science.
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