Introduction

There is an interpretation of quantum physics that is known as the many worlds interpretation. The interpretation is not, by any means, a majority view, but it is much discussed, and is much beloved by authors of fiction, especially the authors of science fiction. In this chapter we will look at the many worlds interpretation from the perspective of possible worlds.

Our considerations will be much assisted if we begin by looking at time from a possible worlds perspective. There are logics for time that have been developed on the basis of standard modal logics with possible world semantics. These logics provide an interesting background for the discussion of the many worlds interpretation of quantum physics, as we shall see.

Possible worlds and time

States of the universe at any point in history can be seen as possible worlds. Such states are sometimes called “time-slices”, but we will call them “instant states”. The state of the universe at one particular instant state, say right now, could be seen as possible world n. World n is the possible world in which it is true that you are reading this book, the Earth is orbiting the Sun, and the Sun is in the Milky Way galaxy. The state at the next instant can be seen as possible world k, the next after that as possible world l. Earlier instant states are before the later instant states; later instant states are after the earlier ones.