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Cosmologists face a daunting task in selecting one out of the possible 10??? different flavours of string theory to apply to our universe (Joseph Silk's review of Parallel Worlds , Books, April 15).

Douglas Adams had the measure of the problem when he wrote in The Restaurant at the End of the Universe : "There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable...

"There is another theory which states that this has already happened."

William Barford
Sheffield