The most cherished goal in physics, as in bad romance novels, is unification. To bring together two things previously understood as different and recognize them as aspects of a single entity - when we can do it - is the biggest thrill in science.
The only sane response to a proposed unification is surprise. The sun is just another star - and the stars are just suns that happen to be very far away! Imagine the reaction of a late-sixteenth century blacksmith or actor on hearing this wild idea of Giordano Bruno’s. What could be more absurd than to unify the sun with the stars? People had been taught that the sun was a great fire created by God to warm the earth, while the stars were pinholes in the celestial sphere that let in the light of heaven. Unification instantly turns your world upside down. What you used to believe becomes impossible. If the stars are suns, the universe is vastly bigger than we thought! Heaven cannot be just overhead!
Even more important, a new proposal for unification brings with it previously unimagined hypotheses. If the stars are other suns, there must be planets around them, on which other people live! The implications often extend beyond science. If there are other planets with other people on them, then either Jesus came to all of them, in which case his coming to Man was not a unique event, or all those people lose the possibility of salvation! No wonder the Catholic Church burned Bruno alive.
Great unifications become the founding ideas on which whole new sciences are erected. Sometimes the consequences so threaten our worldview that surprise is quickly followed by disbelief. Before Darwin, each species was in its own eternal category. Each had been made, individually, by God. But evolution by natural selection means that all species have a conmion ancestor. They are unified into one great family. Biology before Darwin and biology afterward are hardly the same science.
Such powerful new insights lead quickly to new discoveries. If all living things have a common ancestor, they must be similarly made! Indeed, we are made of the same stuff, because all life turns out to be composed of cells. Plants, animals, fungi, bacteria seem very different from one another, but they are all just groups of cells arranged in different ways. The chemical processes that construct and power these cells are the same, across the whole empire of life.
If proposals for unification are so shocking to our previous ways of thinking, how is it that people come to believe them? This is in many ways the crux of our story, for it is a story of several proposed unifications, some of which have come to be strongly believed by some scientists. But none of them have achieved consensus among all scientists. As a consequence, we have lively controversy and, at times, emotional debate, the result of the attempted radical alteration of worldviews. So how do we tell, when someone proposes a new unification, whether it is true or not?