It's 30 years since Douglas Adams' The Hitchhiker's Guide to the Galaxy made its debut on BBC radio, but its most famous mystery is still waiting to be resolved.
The radio series - which subsequently became both bestselling book, television series and film - traces the travels around the galaxy of Arthur Dent, after the earth is destroyed to make way for a "hyperspatial express route".
Possibly the most famous line in the whole book is the "answer to life, the universe, and everything" given by the supercomputer, Deep Thought.
For seven and a half million years, this stupendously powerful, office-block of a machine had whirred. When it came to announcing what it had discovered, crowds had quite understandably gathered. "You aren't going to like it," Deep Thought warned. "Forty-two," it said, with infinite majesty and calm.
Ever since, speculation has been rife as to what Adams meant. There is the "paperback line theory" - 42 apparently being the average number of lines on the page of a paperback book. Was Adams paying homage to the medium of his success?
Then there is the "Lewis Carroll theory" - Adams celebrating Carroll's use of the number in Alice in Wonderland.
Numerical base 13
In the book, there is Rule 42 which says that anyone taller than a mile must leave the court immediately. That becomes a problem for Alice when she eats some mushrooms.
There is another theory that rests on a complex allusion to 42 in numerical base 13. It sparked Adams' retort: "I don't write jokes in base 13."
Douglas Adams never revealed the secret of number 42
Tragically, Douglas Adams died in 2001. So what does Stephen Fry, a close friend, voice of the audiobook, and possibly one of the most intelligent admirers of The Hitchhiker's Guide think?
"Of course, it would be unfair for me to comment," he confides. "Douglas told me in the strictest confidence exactly why 42. The answer is fascinating, extraordinary and, when you think hard about it, completely obvious. Nonetheless amazing for that.
"Remarkable really. But sadly I cannot share it with anyone and the secret must go with me to the grave. Pity, because it explains so much beyond the books. It really does explain the secret of life, the universe, and everything."
But the notion that a computer could provide an answer to the meaning of life is ridiculous, explains Michael Hanlon, author of The Science of The Hitchhiker's Guide to the Galaxy.
Even if every existing atom were co-opted into a mind-bogglingly vast computational matrix, it still wouldn't be able to calculate every possible permutation on a chess board, let alone anything truly complex.
There is still hope that science might come up with answers to the big questions. "Of all the ways of looking for meaning, science has answered the most questions so far," Hanlon continues.
"It has triumphed at explaining many things. However, it hasn't provided answers to the most fundamental questions like why we are here, what is the universe for. But just because it hasn't yet, doesn't mean it can't or won't."
Having said that, it is possible that questions of meaning are simply of a different sort to questions of matter, the physical world in which science has proven so powerful. If so, asking why there is something rather than nothing with mathematics might make no more sense than asking whether a triangle is happy or whether the rocks in the asteroid belt are friends.
Similarly, cosmologists like Stephen Hawking once thought physics would come to know the mind of God in a "theory of everything". He now doubts that is possible.
"Though we haven't run up against a class of questions that we couldn't answer yet," adds Hanlon. With fiendishly difficult phenomena, like consciousness, scientists have yet to exhaust all their theories.
But could there be a serious side to the answer 42? Might there be method in Deep Thought's madness?
The answer can be interpreted in two ways. One is that it is a bad joke, implying that there simply is no answer, no meaning, no sense in the universe, and you would be no worse off if you jumped into the nearest black hole.
But the other interpretation is that the joke was wise. It shows that seeking numerical answers to questions of meaning is itself the problem. Digits, like a four and a two, can no more do it than a string of digits could represent the poetry of Shakespeare.
Fans have come up with numerous theories since
Shakespeare's work was the product of a life, and a life lived to the full. Meaning too might only emerge from such fulsome engagement.
To put it another way, life is a gift. It is good. It flourishes in experiences like love, explains John Cottingham, professor of philosophy at the University of Reading, and author of On the Meaning of Life. He believes that philosophy can no more provide meaning than science can.
This is because life's giftedness, its goodness and its loveliness are essentially spiritual qualities. They can be assessed by rational enquiry. But they cannot be accessed by the cool calculations of reason. They must be experienced.
To put it another way, when the poet William Blake urged us "to see a World in a Grain of Sand", he was not suggesting anything literal. Rather, his words captured something of the world transfigured through beauty and meaning.
For Prof Cottingham, this is what it means to have faith. "For in acting as if life has meaning, we will find, thank God, that it does."
Mark Vernon is the author of 42: Deep Thought on Life, the Universe, and Everything.
Send us your comments using the form below.
Douglas Adams was a 'devout' Atheist, if I could be so bold as to use the word devout without his permission. So to use religious connotations to explain any of his jokes would fundamentally be flawed from the outset. I am also an atheist and therefore am not conditioned to automatically look to religion for answers, the same as Adams.
I believe that some of the answers given in the article are close, that a question of such magnitude cannot be singularly answered for each and every individual on the planet. That the answer '42' is simply a catalyst for thought, deep thought you might say, about everyone's own meaning whether they use religion as a hook to humanity or not. Adams would not be so crass as to believe that he could answer the question, so why not give an answer that makes no sense at all but will instigate discussion on the subject?
Lee Lawson, Edinburgh
This piece is just so off target; the religious overtones especially so. Mark Vernon and the equally bewildered Prof Cottingham need some external spiritualism to explain life and its meaning. Adams didn't. Adams was an atheist and proud of it. He rejected a God, metaphysics and fairies at the bottom of the garden, yet still enjoyed life to the full.
Alex McDonald, Edinburgh UK
"Was Adams paying homage to the medium of his success?" - No, because he hadn't written a book when he wrote the Hitchhikers radio series. He chose "42" because it was funny.
Rich Johnston, London, UK
I think this article rather misses the point of the joke, which is that nobody actually knew what was the question to which 42 was the answer. "What is the meaning of life, the universe and everything?" is far too vague a question to have a meaningful answer. After the answer "42" was revealed, a new project had to be started to find the question to which it was the answer - but the question was never discovered. Douglas Adams was pointing out the foolishness of demanding answers to non-questions.
Stephen Lowe Watson, Lewes
If I remember rightly, the answer to Life, The Universe and Everything was obvious - you just needed to know what the question was.....and isn't the Earth a gigantic organic supercomputer designed to find that out? Hang on, I'll just check with my friend Benji the mouse...
Simon Taylor, Maidstone, UK
The answer is actually 52. Deep Thought forgot to carry the '1'.
Martin Willoughby, Stevenage, UK
Douglas Adams was asked many times during his career why he chose the number 42. Many theories were proposed but he rejected them all. On 3 November 1993 he gave an answer on alt.fan.douglas-adams:
"The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do'. I typed it out. End of story."
Peter Stilliard, Cardiff, Wales
42. Is the answer to the question "What do you get if you multiply 6 by 9". Once you come to accept that everything is based on a simple error, all the bigger errors are so much easier to live with.
Brendan Johnson, Kenilworth, Warks, UK
I also have a proof that the question regarding the meaning of life can be answered by the number 42. Unfortunately the proof is too long to be conveniently fitted into this comment box.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.