Book set in two universes [duplicate]

Question

I am trying to remember the name of a book I read when I was a teenager about 20 odd years ago. I think it was written in the 70's or 80's.

The premise of the book as I remember it is as follows.

There are two universes.

The first is Earth but set in the future relative to the date the book was written. A means of creating free energy has been discovered; I think it is being removed from an alternate universe, but a scientist has discovered that this has the potential to cause problems for the planet.

The second story line runs parallel and relates to a being in an alternate universe travelling through that species' version of puberty. At the start, this being (I believe it is referred to as male) is young and is romantically attached to a female of the species. They are described as being a type of energy that changes colour. She develops and changes into a different type of being and he resists changing. There is a third male that he is jealous of. At some point, however, I believe he then accepts that he must grow and he changes and then melds with the female and other male to form a combined consciousness.

I never got to the end of the book; at the time I found it a really hard read, but I found myself thinking about it the other day and wanted to try and find it so I can give it another go.

Answer 1

"The Gods Themselves" by Isaac Asimov.

Quoting Wikipedia's summary:

The main plot-line is a project by aliens who inhabit a parallel universe (the para-Universe) with different physical laws from this one. By exchanging matter with Earth, they seek to exploit these differences in physical laws. The exchange of matter provides an alternative source of energy in their dying Universe. However, the exchange of physical laws will have the ultimate result of turning the Earth's Sun into a supernova, and possibly even turning a large part of the Milky Way into a quasar. This is the alien's ultimate goal, as it would provide more energy for the para-Universe.

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The first part takes place on Earth, almost a century after the "Great Crisis", where ecological and economic collapse reduced the world's population from six billion to two billion. Radiochemist Frederick Hallam discovers that a container's contents have been altered. He finds out that the sample, originally tungsten, has been transformed into plutonium 186—an isotope that cannot occur naturally in our universe. As this is investigated, Hallam gets the credit for suggesting that the matter has been exchanged by beings in a parallel universe; this leads to the development of a cheap, clean, and apparently endless source of energy: the "Pump", which transfers matter between our universe (where plutonium 186 decays into tungsten 186) and a parallel one governed by different physical laws (where tungsten 186 turns into plutonium 186), yielding a nuclear reaction in the process.

The second part is set in the parallel universe where, because the nuclear force is stronger, stars are smaller and burn out faster than in our universe. It takes place on a world orbiting a sun that is dying. Because atoms behave differently in this universe, substances can move through each other and appear to occupy the same space. This gives the intelligent beings unique abilities.