"It appears likely that the universe that we can observe is just one of an infinity of 'pocket universes,' which are continually being created by a process called eternal inflation. These pocket universes are believed to split off from a region of 'false vacuum', which expands so quickly that its volume increases forever, despite the loss of volume to the formation of pocket universes. The problem is to find a reliable way to extract predictions from this picture. The properties of the pocket universes can vary, and with an infinity of trials essentially anything will happen an infinite number of times. We need to learn how to distinguish the probable from the improbable, but so far such a probability calculation has never been given a precise definition."