If there are very large numbers of multiple universes, we are certain to exist in one in which we CAN exist, according to the anthropic principle. In an awkward adaptation of Descartes, “We exist, therefore at least one such universe exists,” that is one in which prevailing conditions permit our existence. But does that mean that we can never cease to exist, that we can always find a path to a human-friendly universe? Not necessarily.

A pre-condition of continuing existence through time would seem to be the ability to draw a continuous time-line through the branching structure of the multiple universes. If at the next tick of the cosmic computer clock there is no human-friendly universe connected to this one by a direct line of descent, then we cease to exist. There may be a human-friendly universe elsewhere in the branching structure of multiple universes, but we have no way to access it. “We can’t get there from here” – unless we are allowed to go back in time to a previous point of juncture. A diagram will make this clear.

Human-friendly universes are marked in black. If we are at b, we can get to d, but not to h, o, or q. Those at h cannot get to another friendly universe. Those at o can get to q, but not to b or h.