But now there has been a backlash. The total failure to find any trace of life in this solar system, or to pick up any of the interstellar radio signals that our great antennae should be easily able to detect, has prompted some scientists to argue ‘Perhaps we are alone in the Universe…’ Dr Frank Tipler, the best-known exponent of this view, has (doubtless deliberately) outraged the Saganites by giving one of his papers the provocative title ‘There Are No Intelligent Extra-Terrestrials’. Carl Sagan et al argue (and I agree with them) that it is much too early to jump to such far-reaching conclusions.

Meanwhile, the controversy rages; as has been well said, either answer will be awe-inspiring. The question can only be settled by “evidence, not by any amount of logic, however plausible. I would like to see the whole debate given a decade or two of benign neglect, while the radioastronomers, like gold-miners panning for dust, quietly sieve through the torrents of noise pouring down from the sky.

This novel is, among other things, my attempt to create a wholly realistic piece of fiction on the interstellar theme — just as, in Prelude to Space (1951), I used known or foreseeable technology to depict mankind’s first voyage beyond the Earth. There is nothing in this book which defies or denies known principles; the only really wild extrapolation is the ‘quantum drive’, and even this has a highly respectable paternity. (See Acknowledgements.) Should it turn out to be a pipe-dream, there are several possible alternatives; and if we twentieth-century primitives can imagine them, future science will undoubtedly discover something much better.