The current issue of Scientific American has an article, by George F. R. Ellis, expressing some skepticism about the multiverse. Sadly, it seems that only the beginning of the article is freely available online. However, replies to the article by Alexander Vilenkin and Max Tegmark are available online. And since Tegmark so perfectly summarizes my own views about multiverses, I’d like to take a look at his remarks.
After a brief introduction Tegmark gets down to business:
By our universe, I mean the spherical region of space from which light has had time to reach us during the 13.7 billion years since our big bang. When talking about parallel universes, I find it useful to distinguish between four different levels: Level I (other such regions far away in space where the apparent laws of physics are the same, but where history played out differently because things started out differently), Level II (regions of space where even the apparent laws of physics are different), Level III (parallel worlds elsewhere in the so-called Hilbert space where quantum reality plays out), and Level IV (totally disconnected realities governed by different mathematical equations).
In his critique, George classifies many of the arguments in favor of these multiverse levels and argues that they all have problems. Here’s my summary of his main anti-multiverse arguments:
- Inflation may be wrong (or not eternal)
- Quantum mechanics may be wrong (or not unitary)
- String theory may be wrong (or lack multiple solutions)
- Multiverses may be unfalsifiable
- Some claimed multiverse evidence is dubious
- Fine-tuning arguments may assume too much
- It’s a slippery slope to even bigger multiverses
(George didn’t actually mention (2) in the article, but I’m adding it here because I think he would have if the editor had allowed him more than six pages.)
Assuming that’s an accurate summary of Ellis’s argument, what strikes me is that none of those seven points is a reason for actively doubting the multiverse. They are merely arguments for showing that we lack strong reasons for accepting the multiverse, a far different thing. Tegmark now writes:
What’s my take on this critique? Interestingly, I agree with all of these seven statements and nonetheless, I’ll still happily bet my life savings on the existence of a multiverse!
I don’t know if I’d bet my life savings on it, but I think we do have a basis for thinking that a multiverse is pretty likely. Skipping ahead a bit we come to this:
Remember: Parallel universes are not a theory–they are predictions of certain theories.
To me, the key point is that if theories are scientific, then it’s legitimate science to work out and discuss all their consequences even if they involve unobservable entities. For a theory to be falsifiable, we need not be able to observe and test all its predictions, merely at least one of them. My answer to (4) is therefore that what’s scientifically testable are our mathematical theories, not necessarily their implications, and that this is quite OK. For example, because Einstein’s theory of general relativity has successfully predicted many things that we can observe, we also take seriously its predictions for things we cannot observe, e.g., what happens inside black holes.
Likewise, if we’re impressed by the successful predictions of inflation or quantum mechanics so far, then we need to take seriously also their other predictions, including the Level I and Level III multiverse. George even mentions the possibility that eternal inflation may one day be ruled out–to me, this is simply an argument that eternal inflation is a scientific theory.
This seems exactly right to me. Multiverse speculation arises naturally, even inevitably, from other prominent theories in physics. If we have strong reasons for accepting, say, eternal inflation or quantum mechanics (string theory is, admittedly, on shakier ground), and it seems that we do, then we also have strong reasons for accepting the multiverse.
Tegmark says quite a bit more, so I encourage you to go have a look.
Of course, my main interest in this topic arises from its relevance to certain theological questions. It has become very common not just for creationists, but even for high-brow theologians to argue that cosmological “fine-tuning” is strong evidence for God. The multiverse would put paid to that idea. If ours were just one universe among a very large, possibly infinite, ensemble, then fine-tuning would be no more noteworthy than the existence of one winning lottery ticket out of millions sold. That helps explain why theologians are often snidely dismissive of the idea.
For example, in his book God After Darwin theologian John Haught writes,
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 with 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.
Physicist turned theologian John Polkinghorne is even more blunt:
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.
This is precisely the sort of silly obscurantism that justifies a healthy contempt for theology. As we have seen, multiverse speculation is all but unavoidable once you have understood the major theories of modern physics. It may be wrong, but it has a solid basis and has a history that long predates the use of “fine-tuning” as a tool for Christian apologetics.
And let us not forget that the God hypothesis, which is, after all, the preferred alternative of Haught and Polkinghorne, also puts forth a speculative, unobservable entity without a trace of experimental support. The multiverse hypothesis at least arises as a natural consequence of certain theories that have a sound, evidential basis. The God hypothesis is just invented from whole cloth, and is supported solely by philosophical gobbledygook like the cosmological argument.
Another dubious theological attempt to dismiss the multiverse comes from Keith Ward. In his book Why There Almost Certainly is a God he writes:
It has to be admitted, however, that this is a very extravagant theory. It completely contradicts the principle of Occam’s Razor, which says that you should not multiply entities unnecessarily. … The hypothesis that every possible universe exists is the most extravagant hypothesis anyone could think of, an it breaks Occam’s rule of simplicity with a resounding smash. If the simple is good, then the fewer universes there are the better.
But this is just absurd. The entities we are to minimize, according to Occam, are not physical objects, but assumptions. Given two theories with equal explanatory power, we should prefer the one with fewer assumptions. (Prefer, incidentally, because the simpler theory is more likely to be useful, not because it is more likely to be correct.) Bertrand Russell formulated a version of Occam’s Razor which captures the spirit of Occam’s intent and is perfectly suited to our present discussion: “Whenever possible, substitute constructions out of known entities for inferences to unknown entities.”
The multiverse hypothesis basically just looks at our universe and says there’s lots more of the same. It is the God hypothesis that draws an inference to an unknown entity, indeed, one that has not the slightest analog to anything with which we have actual experience. Occam is clearly on the side of “more of the same” over “an awesomely powerful supernatural being for whose existence we have no direct evidence.”
Moving on, there’s an interesting flipside to this general theological pooh-poohing of the multiverse hypothesis. Michael Ruse has recently argued that the multiverse can actually salvage Christian theology from a serious problem posed to it by evolution:
The point I am making is that, as things stand at the moment, there is a flat-out contradiction between the claims of modern biological science and the theology of the Roman Catholic Church.
The contradiction is that the Cathollic Church (and Christianity generally) insists that humanity plays a central role in creation, while evolution says we are just an incidental byproduct of a chance, natural process. After dismissing certain popular theistic replies to this problem, Ruse provides his own solution:
My own thinking is that if you are going to get anywhere then you need to work on the theology. I have suggested that, since we have appeared, we could appear. Hence, God (being outside time and space) could simply go on creating universes until humans did appear. A bit of a waste admittedly but we have that already in our universe.
I think there are purely theological arguments against Ruse’s suggestion. Mostly, though, I find it amusing that after the theologians have snidely dismissed the multiverse, Ruse comes along to suggest they might need it after all.
Now, there is another angle to this that I find interesting. Return to the lottery analogy. If we know that millions of tickets were sold then we are not surprised when someone wins. But if all we know is that John down the street just won the lottery, can we reasonably assume that millions of tickets must have been sold? That doesn’t seem right. In a paper published in the late eighties, philosopher Ian Hacking dubbed this sort of thinking the inverse gambler’s fallacy. The gambler’s fallacy is when you are playing roulette, say, and decide that since the number ten has not come up for quite a while it is more likely to come up on the next turn of the wheel. The inverse gambler’s fallacy occurs when you see ten come up, and conclude that the wheel must have been spun many times before.
Hacking was specifically directing his attention to people who argue that the fine-tuning of the universe is actual evidence for the multiverse. Such people are arguing fallaciously, he argued. But his paper was strongly challenged by other philosophers, most notably John Leslie, who argued that his analogy was flawed and that the inference from fine-tuning to the multiverse does not commit the fallacy. Many more papers have appeared since then.
So, there you go. Multiverses may be speculative, and they may not be useful for generating testable hypotheses in cosmology (though see Vilenkin’s essay, linked above, for possible counterpoints), but they have a strong basis in modern physics and lead to many fascinating questions. All you need to find them plausible is to believe that whatever it was that created our universe, whether an omnipotent being or a purely natural process, also created other universes. And when you put it that way, what exactly is the reason for thinking that ours is the only universe there is.