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 It begins with a scientist discovering that the tungsten on his desk has been replaced by plutonium-186, which cannot possibly exist in this universe, but is stable at first and then becomes gradually more radioactive.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.