There are several versions of the Multiple Universes theory. One is that we live in an oscillating universe that goes through an infinite number of cycles, in which big bangs are followed by big crunches. If the constants of nature vary in each cycle, at some point a particular combination will be realized by chance, like a winning number in a casino slot machine. A second version is that the Big Bang spawned multiple universes, each with its own set of laws. These universes are like separately expanding balloons that cannot relate to one another. Each has its own set of laws. A third version, sometimes called the parallel worlds theory, holds that at each act of quantum measurement the world splits into a series of parallel universes. In fact, universes are springing up even as you read this sentence. Don’t ask where; the universes are disconnected from each other and we have access only to our own universe. Another idea, presented by Smolin, is that our universe emerged from a black hole in a previous universe, and indeed the black holes in our universe are even now generating other universes. Smolin has even raised the possibility of the Darwinian principle of natural selection applying to universes, in which some universes adapt and survive and others don’t. (Not surprisingly, Darwinian hounds like Dawkins and Dennett have rushed to praise this theory.) In all these versions an infinite horizon of possibilities is invoked in order to allow for the possibility that our universe could have arisen purely by chance.

What is one to make of all this? As with all scientific theories, we begin by asking for the evidence. So what is the empirical evidence for oscillating and parallel and multiple universes? Actually, there isn’t any. As Weinberg admits, “These are very speculative ideas … without any experimental support.” Smolin is even more candid. He calls his ideas “a fantasy…. It is possible that all I have done here is cobble together a set of false clues that only seem to have something to do with each other…. There is every chance that these ideas will not succeed.” I appreciate this candor, and I am reminded of that old Ptolemaic remedy for problematic data: “just add epicycles.” Now we are in the realm of “just add universes.”