What is a possible world?

Bakor asked:

What is a possible world?

Answer by Craig Skinner

It’s a way the world might have been eg I might have been a pop star, so we can say that there is some possible world in which I (or my counterpart in that world) is a pop star.

Possible worlds (PW) talk is a way of expressing modal talk: the way (mode) in which a proposition can be true or false, namely necessarily, contingently, impossibly.
Thus,

* necessity = occurs at all PW (eg 2+2=4)

* impossibility = occurs at no PW (eg 2=2=5)

* contingently = occurs at some but not all PW (eg Obama is USA president).

Talking about modality in this way started with Duns Scotus, continued with Leibniz, and really got going in the 20th Century.

In addition, PW talk elucidates counterfactuals (= occurs at close PW), supervenience and causality.

Most of us think these PW are just a convenient way of thinking — they are abstract or fictional entities in the actual world, and can be variously construed as sets of propositions, or states of affairs, or uninstantiated properties, or stories, or different sets of occupied/unoccupied spacetime points.

A few people think PW really exist, notably the late David Lewis. He argues that all PW are real, the actual world (for us) being one of them. Actuality is indexical like ‘here’ and ‘now’ — every world is actual to its inhabitants (if any). PW are spatiotemporally and causally unrelated. An individual is world-bound. Thus, you exist only in the actual world, but your counterparts exist in other PW. What makes ‘John McCain might have been USA president’ true, is that PW exist in which McCain (our McCain’s counterpart) is USA president.

These PW are the primitives reductively explaining modal talk non-circularly, and dissolving difficulties in modal logic. But the cost of ideological economy is commitment to the reality of all possibilities — talking donkeys, unicorns, ghosts, myriad counterparts of each of us doing all possible things. Few buy into it, but Lewis’s ‘On the Plurality of Worlds’ (1986) is compelling, and after reading it, I felt rather sorry at not quite being able to believe it.

Separately, the term is sometimes used for the many universes posited by modern cosmology (the inflationary multiverse; the quilted multiverse — our visible universe is just one patch in an infinite quilt; the braneworlds of M-theory; and, less convincingly, the constantly splitting worlds posited by the many-worlds interpretation of quantum mechanics). But ‘parallel’ or ‘alternate’ ‘universes’ tend to be the preferred terms here.