Then came Schrodinger, who pointed out that quantum uncertainty could apply to a whole cat. If the cat's life depended on the precise position of a bit of a decaying particle, which the physicists claimed was in two different places at the same time, then the cat had to be dead and alive at the same time.
Some physicists were unworried by this. As soon as someone actually looked at the cat, it stopped being both dead and alive and became one or the other. So what's all the fuss about?
Other physicists thought that the undead cat needed some sort of justification, so they came up with the Many Worlds (or Parallel Universes) Hypothesis. Every time a potentially fatal-to-felines quantum event occurs, the universe splits into two, with a dead cat in one and a purring pussy in the other. So we're all floating around in an infinity of possible universes surrounded by cats, alive and dead.
All these universes, of course, according to another trendy theory, are made out of superstrings which exist in goodness knows how many dimensions, though we get to peek at only three of them, or four if we're prepared to wait.
None of this is mentioned in Douglas Adams's latest work, which bills itself as 'The fifth book in the increasingly inaccurately named Hitch Hiker's Guide to the Galaxy trilogy'.
However, the theme of multi-dimensional parallel universes is what drove him to resurrect Ford Prefect and Arthur Dent for another final adventure set in the Whole Sort of General Mish-Mash, as the space-time-superstring continuum has become known. The heroes do not, in fact, bump into each other again until page 160, and only later still return to an earth parallel to the one destroyed by the Vogons at the start of the trilogy.
Meanwhile, Tricia McMillan (or Trillion in her parallel state), has given birth (or not) to Arthur's daughter without his knowledge, thanks to an inter-galactic sperm bank, and is then abducted by the Grebulons and taken to Rupert, the newly discovered and furthest planet of the solar system, where the inhabitants thrive on reheated McDonald's hamburgers, watch American television shows and dabble very seriously in astrology. Elvis Presley sings 'Heartbreak Hotel' before Ford Prefect buys the spacecraft in which Elvis escaped from earth; Arthur lands himself a neat job making sandwiches on a dull planet; Prostetnik Vogon Jeltz prepares to demolish earth again; and Zaphod Beeblebrox doesn't even make an appearance.
Mostly Harmless has all the wit and inventiveness of vintage Douglas Adams, though its loose ends are not tied together as comprehensively as in previous Hitch-Hiker books. Not in this universe, anyway.