The Gods Themselves is a book by Isaac Asimov. It begins with a scientist discovering that the tungsten on his desk has been replaced by plutonium-186, which cannot possibly exist in this universe, but is stable at first and then becomes gradually more radioactive. It turns out that aliens from a parallel universe are doing this. We start communicating with them and they tell us how to set up an "electron pump" to generate infinite amounts of energy. However, the electron pump is also gradually changing the laws of our universe, and could cause our sun to explode.

A wonderful book; very much worth reading, particularly the middle section. I believe that this book could have been quite successful as a short story encompassing only that section. While most aliens are portrayed as impervious, disgusting or frightening, Asimov's are simply fascinating. There are two distinct types -- the "Soft Ones" and the "Hard Ones". Members of the former are divided into three sub-groups: "Parentals", "Emotionals" and "Rationals". These beings join together to form "triads", composed of one member of each sub-group. Only as a threesome can they "melt" together, combining their atoms together into one, thicker version of themselves.

The book is strangely disjunctive -- the other two sections are more typically Asimov, about human scientists, devices and politics. But the surreal, beautiful portrayal of the aliens and the parallel universe is something I haven't forgotten...