"Back in the 1970s there was a simple dream about how physics would end. A unified theory would be found that incorporated quantum theory, general relativity, and the various particles and forces known to us. This would not only be a theory of everything, it would be unique. We would discover that there was only one mathematically consistent quantum theory that unified elementary particle physics with gravity. ...Because it was unique, this theory would have no free parameters—there would be no adjustable masses or charges. ...There would be only one scale, against which everything would be measured... the Planck scale. The theory would allow us to calculate the results of any experiment to whatever accuracy we desired. ...Looking back, it is clear that the assumption that a unified theory would be unique was no more than that—an assumption. ....we know that there can be no such theory."