That title is somewhat facetious, of course, but I do think the multiverse is far more than an idle speculation. I think it is an idea that is sufficiently well-supported that it is those who deny it who should be on the defensive.
I would make an elaborate argument in defense of that claim, but Coel Hellier, a physics professor at Keele University in the UK, has saved me the trouble. He recently posted a two-part article on the subject over at Scientia Salon: Part One, Part Two. Hellier writes:
The multiverse concept is often derided as “unscientific” and an example of physicists indulging in metaphysical speculation of the sort they would usually deplore. For example, commenters here at Scientia Salon have said that the multiverse is “by definition not verifiable and thus outside the bounds of empirical science,” and that “advocates of multiverses seem to be in need of serious philosophical help” .
Critics thus claim that the multiverse amounts to a leap of faith akin to a religious belief. Indeed, the religious often accuse atheistic scientists of inventing the multiverse purely to rebut the “fine-tuning” argument that they say points to a creator god, though the fine-tuning argument is readily refuted in several other ways, and anyhow physicists really don’t care enough about theology these days to let that worry them; further, the concepts leading to a multiverse were developed well before theologians started taking note of the issue .
Quite right, especially that part about physicists not caring about theology. I would only cavil that I am not interested in whether the multiverse meets some arbitrary definition of what is science and what is not. What I care about is whether or not the idea is reasonable and well-supported.
The “demarcation problem,” as philosophers of science refer to the problem of distinguishing science from nonscience, is mostly just an asinine distraction from more serious questions. You cannot avoid it entirely, since it does become relevant when questions of science education are being litigated. Outside of that context, however, trying to draw clear lines around science is not a worthwhile use of time.
End of digression. How does Hellier support his claim?
The purpose of this article is to argue that the multiverse is an entirely scientific hypothesis, arrived at for good scientific reasons and arising out of testable and tested cosmological models. To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds.
Several different concepts could be labelled a “multiverse”, but I am advocating one particular multiverse concept, that arising from what cosmologists call the “eternal inflation” version of Big Bang cosmology , which was developed to explain observations of our universe and predictions from this model have since been verified, putting it on a sound footing.
I am arguing that if a scientific theory predicts consequences A, B, C and D, and if we then verify that A, B and C are indeed the case, thus giving us confidence in the theory, then we have sound reasons for accepting D even if D cannot be directly verified. Indeed, we would be obliged to accept D unless we can construct another equally good explanation of A, B and C.
Quite right again, though “obliged to accept” seems a bit strong. I would want an extra assumption that A, B and C are really good evidence for the theory. That’s just a quibble, though, and the principle here is correct. If well-supported cosmological theories have the multiverse as a consequence, then it is perfectly reasonable to accept that the multiverse exists. Max Tegmark has been especially eloquent on this point, emphasizing that the multiverse should not be seen as a theory in itself, but is rather a consequence of other theories.
I was also cheering when Hellier wrote this:
The multiverse model is often criticized as “unscientific” for invoking universes that can never be seen and thus making claims that can never be verified. But this applies just as much to all cosmological models, which are usually presumed to extend to infinity. All of them are thus postulating that the universe stretches well beyond the observable horizon, from where (owing to the finite speed of light) we can never obtain information to verify any hypotheses. This feature does not make the model unscientific. If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.
Hat trick! One of the more annoying tropes of this topic is the idea that Occam’s Razor militates against the multiverse. This seems entirely wrong to me. It is the people who claim there is only one universe who have some explaining to do. Multiverse proponents are simply saying that whatever created our universe, a quantum fluctuation or whatever, created other universes as well. Given that there is some sort of mechanism that created our universe, why is it parsimonious to assume that the mechanism only operated once?
The Occam’s Razor argument is especially obnoxious when it comes from theists. They will deride the multiverse as just a crude and desperate attempt to avoid the conclusion that God exists. Here’s theologian John Haught, from his book God After Darwin:
Of course, if you are truly addicted to the idea that our life-bearing universe is a purely random, undirected, and unintelligible occurrence, and that life within it must in no sense be the product of divine intelligence and wisdom, you may then imaginatively conjure up an endless series or proliferation of other “universes,” so as to increase the probability that randomness rules.
And here’s John Polkinghorne expressing similar thoughts in an interview:
The multiverse theory in its more extreme forms is the idea that there are these vast portfolios of different universes, disconnected from ours, unobservable by us. It’s a metaphysical guess. It has mostly been popular and mostly been invented in order to explain away the fine-tuning of our particular universe.
What nerve! Among the many things wrong with these quotes is that Haught and Polkinghorne’s preferred explanation is that an omnipotent superbeing created the universe. That is what they put forward as the natural, parsimonious explanation for the universe. I demur from that conclusion. Given the choice between more of the same on the one hand, and hypothesizing into existence a God who can simply bring universes into being with acts of His will on the other, it seems clear which one Occam would choose.
Anyway, go have a look at Hellier’s essays. There is far more to them than what I have quoted here.