The multiverse is a hypothetical realm comprising not only our universe but an indeterminate number of other universes too. It is closely allied to the notion of parallel universes, a conceit of particular theoretical significance within the field of quantum mechanics (see here).
Credit for first expressing the idea of the multiverse is generally given to Erwin Schrödinger, who warned his audience at a lecture in 1952 that what he was about to postulate might ‘seem lunatic’. The famous ‘Schrödinger’s Cat’ thought experiment involved a hypothetical cat which was simultaneously both alive and dead (due to poison released – or not – by quantum activity), until its box was opened and one state or other was observed. At the quantum level, Schrödinger was illustrating the possibility of a multitude of different histories occurring not as alternatives, but happening simultaneously, though his thought experiment was intended to illustrate the absurdity of this concept.
Concepts of the multiverse have split scientists and philosophers ever since. Many dismiss the idea as essentially unscientific; others, though, are more sympathetic. Professor Brian Cox, for instance, has observed: ‘That there’s an infinite number of universes sounds more complicated than there being one. But actually, it’s a simpler version of quantum mechanics. It’s quantum mechanics without wave function collapse . . . the idea that by observing something you force a system to make a choice.’
The infinite ‘you’
Why, it may be reasonably asked, is there a need to imagine a multiverse given that the universe seemingly contains all that there is? Part of the answer comes down to the question of history. A single universe allows only for the application of a single history. As you are where you are, doing what you’re doing (reading this book), our single universe does not allow you to be somewhere else, doing something else at the same time. But what if there is a ‘you’ experiencing an alternative history that has led you down an entirely different path? Conceivably, the multiverse allows for a potentially infinite number of ‘yous’ living out an infinite number of histories.
While this may not seem to have particular relevance to us on a personal, day-to-day level, it has more immediate theoretical implications for the quantum physicist.