As a compliment to Jim's informative series on the anthropic coincidences I would like to draw attention to Max Tegmark’s excellent page on multiverse theories here. The various multiverses fall into four distinct categories:
Level I: (Open multiverse) A generic prediction of cosmic inflation is an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
Level II: (Andrei Linde's bubble theory) In chaotic inflation, other thermalized regions may have different effective physical constants, dimensionality and particle content.
Level III: Each quantum possibility correspondes to a universe. Suppose your throw a die that contains 6 sides. When the die fall, one could ask why the outcome is the way it is. The answer is: All 6 possible ways the die can fall is actually actualized in 6 different universes.
Level IV: (The Ultimate ensemble theory of Tegmark) Other mathematical structures give different fundamental equations of physics. This level considers "real" any hypothetical universe based on one of these structures. Since this subsumes all other possible ensembles, it brings closure to the hierarchy of multiverses: there cannot be a Level V.
I am delighted to inform you that all of these theories have the effect of turning reality into a sick joke, as evidenced by Tegmark’s agony aunt section in which his baffled readership struggle to come to terms with the implications.
Question : The personally troubling aspect of the multiverse theory, which, fortunately and unfortunately, seems quite plausible, is that---if every conceivable universe exists---that means that your similar being, and mine, somewhere out there is (....to be gentle as I can be) an axe-murder and (not so gentle) worse! That is hard to accept for me in this universe, even if true. I guess the good news is that it isn't really me...but pretty darnn close to me since if every iteration is plausible then somewhere our "clone" has seen typed this email and then went out and robbed the local gas station, 7-11, next closest gas station, etc. etc.!
Tegmark : Things inconsistent with the laws of physics will never happen - everything else will. However, to cheer you up: even if some of your twins hold up gas stations, most of your twins certainly don't, given what I already know about your personality; it's important to keep track of the statistics, since even if everything conceivable happens somewhere, really freak events happen only exponentially rarely.
Quodlibeta Comment : Of course, the flip side of all that is, if you happen to be an axe murderer in this universe and this arises from a key flaw in your personality, the chances are that the vast majority of your other clones in the set of all possible universes will be axe murderers too, although statistically some will be peace loving hippies.
Question : Within the context of the multiverse, doesn't every conceivable physical possibility occur? If I'm driving my car and stop abruptly to keep from hitting a squirrel, don't I purposely run over that same squirrel in an alternate universe. And if so, isn't the number of universes that follow each outcome approximately the same?
Tegmark : No - and that's the crux. The laws of physics and your behavior evolved through natural selection create much regularity across the multiverse, so you'll try to spare that squirrel in the vast majority of all parallel universes where "you" are pretty similar to the copy reading this email (just as regards the above-mentioned gas station robbery). The fractions only split close to 50-50 for decisions that you perceive as a very close call.
Quodlibeta Comment : Why must these other versions of the questioner in the set of possible universes be driving cars?. Might they not be riding giant squirrels and swerving to avoid marsupials?.
Question : Doesn't the multiverse theory completely trivialize existence? It puts the burden for individual responsibility on the shoulders of the universe. Why do anything? If you decide to be a lazy slug, that just means that your particle clone elsewhere will be the one who wins the Nobel prize.
Tegmark : I'm not convinced that the existence of parallel universes implies that I should dramatically alter my behavior. Yes, some near-clones of me indeed win the Nobel prize, but only a very small fraction of them! As in the gas station question above, it's important to keep track of the statistics, since even if everything conceivable happens somewhere, really freak events happen rarely, in an exponentially small fraction of all parallel universes. It's these statistics that make existence complex and interesting rather than trivial.
Quodlibeta Comment : Great, now I don't just have to cope with the fact that most of my contempories are more successful than me, a large proportion of my clones in the multiverse are as well!. At least I have the satisfaction of knowing that statistically, a large percentage of them will be bums.
Having wiped the tears of laughter from my face, the question remains, should those of a monotheistic disposition believe in the multiverse?. I was delighted to discover that this topic came up during the Middle Ages when a number of clerics asked whether god could create more than one universe or whether he/she/it would be happy sticking with one?. In the event the Bishop of Paris was forced to intervene and said that yes, God could create as many universes as he wanted and there the matter was settled. Later John of Vassals said that God could create an infinite universe provided that it was not too infinite as this would be tantamount to creating another god. Thomas Bradwardine on the other hand (who later became archbishop of Canterbury) insisted that the universe was has to be infinite because god is infinite and exists in the universe. Moreover, present day commentators such as Peter Bussey of Glasgow University warn of the risk of ‘dumbing down God’. A multiverse is, after all, far harder for a deity to create than a single universe. On the basis of that it would be prudent to plump for a multiverse category, although you might want to 'fine tune' it to make it less silly.
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