Physics is an experimental science. Experiments are designed to ask questions of Nature, and the better they are designed, the clearer is Nature's answer. As the answers obtained through experiments become systematized, the theory of a subject develops. A theory is based on generalizations or idealizations from experiments and then permits us to make what are loosely called “predictions”. Of course, used in this way, the word means no more than saying: If my assumptions (the generalizations or idealizations) are right, then my predictions follow. A new experiment, if it confirms these predictions, then strengthens our belief in the theory, and if it does not, it disproves it. But note, an experiment cannot by itself prove a theory. The same experimental results might have been obtained on the basis of other assumptions or idealizations. The more often a theoretical prediction is confirmed, the more we trust the assumptions of the theory, and finally we use it with as great confidence as if it were itself a fact of nature. Of course, this confidence is considerably greater if the theory is based on very general principles and if we use it only for interpolation rather than for extrapolation to regions of new experience.

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