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. It explores the idea that every choice we make leads to a different version of the universe.

This began with a debate among Christians about the world God put us in. The 18th Century thinker Gottfried Wilhelm Leibniz said that we live in "the best of all possible worlds."

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. As a result some scientists argue that our world has 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."

Another view is that our world is like part of a jigsaw puzzle. The problem is that 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."

Many writers and film-makers have been intrigued by this theory - and the idea that one decision could change your life completely.

Robert Frost's great poem The Road Not Taken is one example. The recent movies Everything Everywhere All at Once and Spider-Man: Across the Spider-Verse have the same theme.

In a recent book, scientist Robert Lanza says that the universe only exists because we are conscious of it. He believes that 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 know. 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.