Steven Weinberg gives his view: arXiv:hep-th/0511037 v1 3 Nov 2005 UTTG-12-05

Living in the Multiverse: Opening Talk at the Symposium "Expectations of a Final Theory" at Trinity College, Cambridge, September 2, 2005; to be published in Universe or Multiverse?, ed. B. Carr (Cambridge University Press).

Steven Weinberg, Physics Department, University of Texas at Austin

Most advances in the history of science have been marked by discoveries about nature, but at certain turning points we have made discoveries about science itself. These discoveries lead to changes in how we score our work, in what we consider to be an acceptable theory.

For an example look back to a discovery made just one hundred years ago. As you recall, before 1905 there had been numerous unsuccessful efforts to detect changes in the speed of light due to the motion of the earth through the ether. Attempts were made by Fitzgerald, Lorentz, and others to construct a mathematical model of the electron (which was then conceived to be the chief constituent of all matter), that would explain how rulers contract when moving through the ether in just the right way to keep the apparent speed of light unchanged. Einstein instead offered a symmetry principle, which stated that not just the speed of light but all the laws of nature are unaffected by a transformation to a frame of reference in uniform motion. Lorentz grumbled that Einstein was simply assuming what he and others had been trying to prove. But history was on Einstein’s side. The 1905 Special Theory of Relativity was the beginning of a general acceptance of symmetry principles as a valid basis for physical theories.

This was how Special Relativity made a change in science itself. From one point of view, Special Relativity was no big thing — it just amounted to the replacement of one 10 parameter spacetime symmetry group, the Galileo group, with another 10 parameter group, the Lorentz group. But never before had a symmetry principle been taken as a legitimate hypothesis on which to base a physical theory...