The word physics comes from a Greek word meaning "knowledge of nature." Physics attempts to describe the fundamental nature of the universe and how it works, always striving for the simplest explanations common to the most diverse behavior. For example, physics explains why rainbows have colors, what keeps a satellite in orbit, and what atoms and nuclei are made of. The goal of physics is to explain as many things as possible using as few laws as possible, revealing nature's underlying simplicity and beauty.

In achieving their goal, physicists construct models to represent the world around us. To the physicist, a model is an idealized description of a physical system or natural phenomenon. Such a model forms a conceptual framework that permits us to reduce complex situations into simpler, more understandable forms. For example, although we cannot see atoms, we can construct useful models of them that enable us to understand their behavior. Usually, such models of physical systems take a mathematical form. It should be understood that these models are by nature incomplete and, therefore, imperfect. For instance, we can describe the main features of the motion of a baseball if we use a model that ignores air resistance. Such a model has limitations, however, because it does not accurately describe the path of a curve ball thrown by a major league pitcher. We can obtain better agreement with observations by using a model that includes air resistance.

Physics is an experimental science. By this we mean that the acceptance of any physical theory depends on its success in predicting and explaining reproducible observations. To understand physics we must be able to connect our theoritical description of nature with our experimental observations of nature. This connection is made through quantitative measurements. In part, a thorough understanding of physical theories rests on knowing how measurements are made and how reliable the measured information is.

Until this century, scientists assumed that a sufficiently clever observer, given enough time and money, could, in principle, measure any thing or set of things as accurately as necessary. Our understanding of the measurement process is now more refined. We know that we cannot make a measurement that does not in some way affect the system being measured, thereby limiting the precision of our measurement. This limitation is of little or no importance in the everyday measurements with which we are most familiar: the length of a board or the speed of an automobile. However, we will find that when we come to submolecular processes, the interaction of the observer with the measured quantity cannot be ignored.