The term "paradigm" has become common parlance over recent years, and almost everywhere you go these days people are talking of paradigm shifts -- in science, economics, business, world affairs. As often happens, the popularization of the notion has resulted in a diluting of its original meaning, and since the concept plays an important part in what follows, it is worth briefly recapping the principal features of a paradigm shift.
Kuhn used the word paradigm to refer to the set of beliefs and assumptions that constitute a particular worldview. A paradigm can be thought of as a super-theory, the mental lens through which scientists look at the world. Quantum theory, Darwins Theory of evolution and the psychoanalytic theory of the unconscious mind are all examples of paradigms in particular scientific disciplines.
Kuhn was particular interested in the processes by which paradigms changed. In his seminal work, The Structure of Scientific Revolutions, he showed that science does not always progress through a series of steady steps, but from time to time it makes relatively sudden shifts to radically new paradigms.
For as long as the prevailing model accounts for all the data within a given field, it is accepted as the truth. Over time, however, anomalies may be discovered that cannot be easily explained by the current model. (The anomaly for the geocentric model of the universe was the uneven motion of the planets.) At first the anomalies are ignored or rejected as erroneous. Later, when they can no longer be so easily dismissed, attempts are made to incorporate them in some way, often clumsily, into the existing paradigm -- witness the epicycles of mediaeval astronomers.
From time to time, some brave soul may challenge the assumptions behind the existing worldview and propose a radical new model of reality that can account for the anomalies. More often than not, however, the new models are rejected, and often ridiculed, by the establishment. It is easy for us today to pour scorn on the way the establishment refused to consider the Copernican theory. It is easy because we have been born into a world in which the heliocentric view of reality is the accepted truth. But four hundred years ago you would have been brought up to believe in a very different model of reality. Not only was this worldview an article of faith, it was confirmed by experience. One had only to look up to see that the sun was moving across the sky each day, while the earth remained as still as could be. To suggest otherwise would have seemed ludicrous.
The new paradigm gradually gains acceptance as it shows it is able to explain the anomalies in a straightforward way (the retrograde motion of the planets is just a consequence of the relative motion of the earth to the planets), and successfully predict and account for future observations. It then takes over as the dominating belief; until, that is, anomalies arise that this new model cannot account for, and another shift is triggered -- as happened when Newtons Laws were unable to explain the apparent constancy of the speed of light.