Ross Douthat and John Schwenkler have recently pointed to an article on the “multiverse.” There was similar attention devoted to an earlier (and more interesting, in my view) article in the popular science magazine Discover last fall. The basic issue at play in multiverse theory is what is often called “fine tuning” or the Anthropic Principle: the assertion that if any of a very large number of constants in fundamental physical laws were even slightly different, then intelligent life would be impossible in our universe. One philosophical solution to this is that “God made the universe.” Another is the idea of the multiverse: more or less that are there are many universes beyond the one apparent to us, and that we happen to see this universe because it’s the one we’re in, and it’s the one we’re in because it’s the one that happens to have the values for these constants that allow intelligent life to evolve. It’s “monkeys on typewriters” on a (super-)cosmic scale. This sounds like a scientific theory; but if we take “universe” to equal “all physical reality that is observable in principle by human beings,” then I’ll argue that it is not scientific at all, but pure metaphysics.
One starting point is the observation that this basic concept is not a new idea. In 1934, Sir Karl Popper published The Logic of Scientific Discovery, generally considered to be one of the two most significant works on the philosophy of science in the past hundred years. Here he is in Part 67 of that book:
[S]ince probability estimates are not falsifiable; it must always be possible in this way to ‘explain’ by probability estimates, any regularity we please. Take, for example, the law of gravity. We may contrive hypothetical probability estimates to ‘explain’ this law in the following way. We select events of some kind to serve as elementary or atomic events; for instance the movement of a small particle. We then assume that these events show a chance-like distribution. Finally we calculate the probability that all the particles within a certain finite spatial region, and during a certain finite period of time – a certain ‘cosmic period’ – will with a specified accuracy move, accidentally, in the way required by the law of gravity. The probability calculated will, of course, be very small; negligibly small, in fact, but still not equal to zero. Thus we can raise the question of how long an n-segment of the sequence would have to be, or in other words, how long a duration must be assumed for the whole process, in order that we may expect with a probability close to 1 (or deviating from 1 by not more than an arbitrarily small value) the occurrence of one such cosmic period in which, as the result of an accumulation of accidents, our observations will agree with the law of gravity. For any value as close to 1 as we chose, we obtain a definite, though extremely large, finite number. We can then say: if we assume that the segment of this sequence has this very great length – or in other words that the ‘world’ lasts long enough – then our assumption of randomness entitles us to expect that the occurrence of a cosmic period in which the laws of gravity will seem to hold good, though ‘in reality’ nothing ever occurs but random scattering. This type of ‘explanation’ by means of the assumption of randomness is applicable to any regularity we choose. In fact we can in this way ‘explain’ our whole world, with all its observed regularities, as a phase in a random chaos – as an accumulation of purely accidental coincidences. [Italics in original]
As he says in the final sentence of this paragraph, we could theoretically explain the whole ‘world’ (i.e., all that can be observed, even in principle; the universe) this way. Modern cosmologists have to speak of ‘alternative unobservable universes,’ because it is so widely accepted that our observable universe was created recently enough (about 14 billion years ago) that we don’t have nearly enough time for the simpler (and more obviously non-scientific) statement that we are just in a long enough random sequence in an almost infinitely long-lived universe. Multiverse theory is simply a modern incarnation of the same argument that Popper described 70 years ago.
So what? This doesn’t mean it’s not true. Lots of things that seem really weird to us have been shown to be scientifically true.
Here’s Popper’s next paragraph:
It seems to me that speculations of this kind are ‘metaphysical’, and that they are without any significance for science. And it seems equally clear that this fact is connected with their nonfalsifiability – with the fact that we can always and in all circumstances indulge in them.
In a much later edition, published decades later, Popper begins a wry footnote to these paragraphs with the following lines:
When writing this, I thought that speculations of the kind described would be easily recognized as useless, just because of their unlimited applicability. But they seem to more tempting than I imagined.
They sure have. But even if the idea of non-observable universes is, well, non-observable, there is of course the argument that we can theorize that some phenomenon X should be apparent in our observable universe if there really is a multiverse, and that if X is apparent then this can be used to show that the multiverse exists. Here is a section from the Discover article:
When I ask Linde whether physicists will ever be able to prove that the multiverse is real, he has a simple answer. “Nothing else fits the data,” he tells me. “We don’t have any alternative explanation for the dark energy; we don’t have any alternative explanation for the smallness of the mass of the electron; we don’t have any alternative explanation for many properties of particles.
“What I am saying is, look at it with open eyes. These are experimental facts, and these facts fit one theory: the multiverse theory. They do not fit any other theory so far. I’m not saying these properties necessarily imply the multiverse theory is right, but you asked me if there is any experimental evidence, and the answer is yes. It was Arthur Conan Doyle who said, ‘When you have eliminated the impossible, whatever remains, however improbable, must be the truth.’?”
We can’t explain it any other way, so my hypothesized metaphysical agent, which by definition escapes human observation, must be the cause. After all, this is the only potential cause that has not been disproved.
But, of course, this is precisely the form of the argument for Intelligent Design (ID). Maybe we should call it a “multiverse of the gaps” argument.
Now the counter argument is that we actually do have good scientific explanations for many of the phenomena that were claimed to be unexplainable without an intelligent designer. But scientific knowledge is never absolute, so there are always gaps, and therefore always space for such an argument. The problem with both ID and multiverse theory is the same: Neither is true and neither is false in a scientific sense; they are metaphysical frameworks with the scientific task of inspiring testable hypotheses, but are not themselves scientific theories capable of testing through scientific means.
It’s tempting to see ID and multiverse theory as mirror images — one looking desperately to prove scientifically that humans are special, and the other desperately seeking to avoid this conclusion. This is almost, but not quite, appropriate in my view. The proper question to ask about both multiverse theory and ID is whether they are fruitful. Ultimately, either each framework will help scientists develop physical theories in the form of predictive rules that can be tested through observation, or it will not. It’s very hard to see how ID can do this, but I guess that anything’s possible. Multiverse theory is more likely to do so, if only because it is a point of view that embeds a metaphysic that is far more congenial to so many more smart scientists.
But to look to science to answer a metaphysical questions like “Did God create us?” or “Are there completely unobservable aspects of reality?” is a category error of the first order.