Confessions of a Multiverse Skeptic

Okay, the title of my post is a little misleading. A more accurate, but less catchy, title for my post would be, “Confessions of a Skeptic of the Multiverse Objection to the Fine-Tuning Argument.” Whew! Just trying saying that five times fast!

On a serious note, I’ve mentioned before that I am not convinced by appeals to the multiverse hypothesis to probabilistic versions of the fine-tuning argument (FTA). In this post, I will try to explain why.


Informal Critique of the Multiverse Objection

According to the multiverse objection (M), a ‘fine-tuned’ universe is just as probable on naturalism as on theism since, for all we know, there could be multiple (or even infinite) universes. Since the physical laws in each of these universes are random, there is bound to be at least one, if not many, life-permitting universes; we just happen to live in a life-permitting universe.

The problem with M is that, in the absence of any independent evidence for a multiverse, the multiverse hypothesis is ad hoc. On the assumption that naturalism is true, we have little or no antecedent reason to expect a multiverse to exist. Therefore, unless or until physicists or cosmologists discover evidence that a multiverse actually exists, the multiverse is a weak objection to probabilistic versions of the FTA.

Formal Critique of the Multiverse Objection

Consider the following formulation of FTA.

Abbreviations

>!: much greater than
F: the universe is fine-tuned for life
T: theism
N: naturalism

Argument Formulated

(1) F is known to be true.
(2) Pr(F | T) >! Pr(F | N).
(3) N is not intrinsically much more probable than T.
(4) Other evidence held equal, Pr(T) > Pr(N).

The Multiverse Objection

According to the multiverse objection (M), (2) is false because, for all we know, there could be multiple (or even infinite) universes. Since the physical laws in each of these universes are random, there is bound to be at least one, if not many, life-permitting universe. We just happen to live in a life-permitting universe.

At first glance, M seems irrelevant to the above formulation of FTA, since (2) compares the antecedent probability of F on theism to the antecedent probability of F on naturalism, not naturalism conjoined with an auxiliary hypothesis about the multiverse. So how could M be relevant to (2)?

Those of you who have read my other recent postings can probably predict what I’m going to write next. Using the probability calculus, we can measure the effect that an auxiliary hypothesis like M has on Pr(F/N). In order to assess the evidential significance of an auxiliary hypothesis like M, we would simply need to consider a weighted average, as follows:

Pr(F/N) = Pr(M/N) x Pr(F/M&N;) + Pr(~M/N) x Pr(F/~M&N;)

This formula is an average because Pr(M/N) + Pr(~M/N) = 1. It is not a simple straight average, however, since those two values may not equal 1/2.

The weighted average formula above gives us some insight into what would need to be the case in order for M to be a good defeater for the FTA. I assume we all agree that the second half of the right-hand side of that equation, Pr(~M/N) x Pr(F/~M&N;), is not going to be useful for deriving a high value for Pr(F/N). (Otherwise, there would be no need to introduce M in the first place!)

So we’re stuck with the first half of the right-hand side: Pr(M/N) x Pr(F/M&N;). In order for M to be a good defeater of the FTA, then, Pr(M/N) x Pr(F/M&N;) needs to be high, the higher the better. The problem, however, is that we have little or no reason to believe that Pr(M/N) is high, i.e., we have little or no reason on naturalism (alone) to expect multiple universes. If Pr(M/N) is not high, then there is no reason to believe that Pr(F/N), as a weighted average of Pr(M/N) and Pr(~M/N), is high. So, unless there is independent evidence for M–i.e., evidence that is independent of the evidence for F–it appears that using M as a defeater against FTA fails and fails miserably.

About Jeffery Jay Lowder

Jeffery Jay Lowder is President Emeritus of Internet Infidels, Inc., which he co-founded in 1995. He is also co-editor of the book, The Empty Tomb: Jesus Beyond the Grave.

  • http://preparedfortheworst.wordpress.com/ preparedfortheworst

    Couldn't have said it better myself! Loving the articles posted on here, they're really helping me expand my thoughts.

  • http://www.blogger.com/profile/12045468316613818510 Blue Devil Knight

    Even if there are multiverses it isn't clear it would address the probabilistic arguments. Plus, you don't need multiverses to answer them this way: just say that there is one universe, selected from such-and-such distribution of constants or whatever (you can have probability densities even if you realize only one instance of the process: e.g., you do only one roll of a 20 sided die, that doesn't meant he probability wasn't 1/20).

    What does it matter if there is one universe with probability p, versus an infinite number of universes, with us living in one of them with the same probability calculated over the set?

    Discussion of multiverses seems largely a red herring.

    Not that I have any sympathy for fine-tuning arguments, anthropic principle gyrations, or any member of that in-bred family of pseudophysics.

  • http://www.blogger.com/profile/05034037930336299849 Mike Gage

    What should we be looking for as evidence of a multiverse? I think we're in a bit of muddy water here. As an amateur, I tend to take Brian Greene on his word that multiple widely held theories about the nature of physical reality imply a type of multiverse, including string theory (inluding variants, like M-theory) and the theory of inflation.

    Since a type of multiverse is implied by these theories, should evidence for them likewise count as evidence for the multiverse? If that is the case, then we're at least in a decent position. What people seem to be looking for instead is something like a lost graviton from a super collider experiment that we might attribute to "slipping" into another dimension (I apologize if I'm botching that). It's not clear to me why we should prefer that result, which is very subject to interpretation, to increased confidence in theories implying a multiverse.

    All that being said, I'm not sure how much confidence we should place in that hypothesis. But when I see discussions of evidence, I just wonder what we might even accept as evidence. It seems to me like the best we can do in this particular area is have our multiverse confidence correspond with, say, our M-theory confidence, etc.

  • http://www.blogger.com/profile/10289884295542007401 Jeffery Jay Lowder

    Mike — Great question! I don't consider myself qualified to answer; on the other hand, I do feel qualified to describe what would not be an acceptable answer, in the context of my post. What would not be an acceptable answer would be the same evidence that supports fine-tuning. So if the evidence for string theory (including variants like M-theory) is independent of the evidence for fine-tuning, then that could potentially work.

    You write:

    Since a type of multiverse is implied by these theories, should evidence for them likewise count as evidence for the multiverse?

    I would say "yes," if a multiverse is implied by these theories.

    All that being said, I'm not sure how much confidence we should place in that hypothesis.

    Exactly. I could easily be wrong, but I have the impression that the evidence for such theories is somewhat speculative at this point. If the evidence is speculative, then it's hard to see how we could have much confidence in the multiverse hypothesis. That would be a question for Taner, Bradley, Vic, Graham, and others with a physics background to address.

  • http://www.blogger.com/profile/05501109533475045969 Explicit Atheist

    Please read Max Tegemark's argument that there is a multiverse in Scientific American, July 19, 2011. I consider his argument to be very persuasive.http://www.scientificamerican.com/article.cfm?id=multiverse-the-case-for-parallel-universe

  • http://www.blogger.com/profile/10289884295542007401 Jeffery Jay Lowder

    Explicit Atheist: Thanks! I just created a new blog post to highlight both the link you provided and the article for the other point of view, both at Scientific American.


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