Alternate Physics in Stephenson's Baroque Cycle
A few years ago, I speculated that a minor change in physics would make nuclear bombs nearly impossible to make. If the neutron mass were a little bit higher, all the uranium 235 on Earth would have decayed to non-fissionable neptunium 235. (At least, I don't think it's fissionable. Part of the energy that makes uranium 235 nuclei fission comes from the fact that there's an odd number of neutrons in a U-235 nucleus and there's a larger energy yield when such nuclei absorb a neutron. Neptunium 235 has an even number of neutrons.)
More recently, I've been reading Neal Stephenson's Baroque Cycle. Among other differences between the Baroque Cycle universe and our universe is that gold has more than one stable isotope. In the hypothetical universe I mentioned above, gold 199 will be be stable as well as gold 197. The 20th-century discovery of the Theory of Relativity may have less spectacular effects there than here.