To her amazement, Nora finds herself in a library where every book tells the story of her life differently. In one she is an Olympic medallist; in another she is a rock star. She can live any of these lives simply by opening the book in question.

This is the scenarioAn imaginary situation. The word was originally Italian and referred to the plot of a stage drama. of Matt Haig's novel The Midnight Library. Its inspiration is the "many worlds" theory: the idea that every decision and choice we make leads to a different version of the universe.

It has its origins in a theologicalThe study of religious belief. debate about free will and why evil exists.

A 16th Century Spanish priest, Luis de Molina, suggested that God knows how we will behave under particular circumstances - so he places us in a world which will bring out the best in the greatest number of people, while leaving us free to make our own choices.

In the 18th Century, philosopher Gottfried Wilhelm Leibniz argued that we live in "the best of all possible worlds": any other conceivable world would be worse.

The development of quantum physicsThe study of matter and energy on the atomic and subatomic level. has taken the idea of multiple worlds further.

Experiments show that photonsThe elementary particle of electromagnetic radiation and the basic unit of light. The LHC has two-photon beams travelling in opposite directions.  can exist both as particles and waves at the same time. From this, the American physicist Hugh Everett argued that the world we live in co-exists with many possible variations. As Timothy Andersen writes in Aeon:

"According to one form of this belief, somewhere out there is an exact duplicate of you, your house, your family, but one small detail is different... you have red hair instead of brown."

But another view is that these worlds are not actually separate. They are like parts of a jigsaw puzzle, in which we can only see one piece:

"This suggests that our lives too might be a jigsaw puzzle," Andersen explains. "Perhaps they make sense only when we look at them across a multiverse of possible lives."

This theory, and the idea that someone's life could be completely different as a result of a single decision, have intrigued many authors and film-makers. They have inspired works from Robert Frost's great poem The Road Not Taken to the recent movie Everything Everywhere All at Once.

In his recent book The Grand Biocentric Design, scientist Robert Lanza suggests that the universe results from our conscious observation of it. In that case, a conscious being lives for ever, and death means simply moving into other worlds.

Yes: Thinking about what might have been if we had only made a better decision is a recipe for misery. To imagine that there are alternative realities we cannot get into is hugely frustrating.

No: It is wonderful to know that the things we have dreamed of could actually be true, even if they are not in the world we perceive. If Robert Lanza is right we may yet be able to experience them.

Or... We already inhabit many different worlds, because everybody has a different perception of reality, and our own experience of it shifts according to what mood we happen to be in at a particular time.