Consider a fine-tuning argument like this:
On theism, it is moderately likely that there would be a fine-tuned universe.
On naturalism, it is extremely unlikely that there would be a fine-tuned universe.
So, the existence of a fine-tuned universe is very significant evidence for theism over naturalism.
These days, the main response to this is to invoke a rich multiverse, and to note:
- On multiverse naturalism, it is nearly certain that there would be a fine-tuned universe.
It follows from (1) and (4) that the existence of a fine-tuned evidence is moderate evidence for multiverse naturalism over theism.
If (4) undercuts anything in the argument (1)–(3), it is (2). How could (4) undercut (2)? It would have to be roughly as follows:
- On naturalism, prior to the evidence of a fine-tuned universe, it is not very unlikely that there is a multiverse.
When we combine (4) with (5), we do indeed get that it’s not extremely unlikely that there would be a fine-tuned universe.
But (5) is dubious. For prior to the evidence of a fine-tuned universe, the rational credence in a naturalistic multiverse should be extremely small. This is because one of the prior ratioanl constraints on credences is that they should make skeptical hypotheses extremely unlikely. And a naturalistic multiverse is a kind of skeptical hypothesis, for multiple reasons. First, it denies the uniformity of nature (at least if it’s the kind of multiverse relevant to fine-tuning, where the laws of nature vary between universes). Second, it implies intuitively absurd claims, such as that probably there are fairies and Greek gods out of sight of our observation (namely in other universes). Third, on many versions it threatens most of our common-sense knowledge by making Boltzmann brains at least as likely as ordinary brains. Fourth, at least the infinite versions of the multiverse hypotheses endanger probabilistic reasoning, since crazy things happen infinitely many times and non-crazy things happen infinitely many times in a multiverse, and it’s hard to say that the crazy things are less likely.
I suppose it is possible that (a) the rational credence in a naturalistic multiverse is extremely small, but (5) is still true. But the only way that could be is if the prior probability of naturalism is quite low. And while I am happy to say that, I think few naturalists will be. Thus a typical naturalist should, I think, deny (5), and should hold that prior to the evidence of a fine-tuned universe, even on naturalism, a multiverse would be very and maybe even extremely unlikely. The evidence of fine-tuning will greatly raise the probability of a naturalistic multiverse, but given that it started extremely small relative to theism, it is going to stay small.
12 comments:
Alex
If the laws of nature do not vary between universes, the fine-tuning argument does not make any sense at all for it would mean that the laws of nature automatically lead to a fine-tuned universe.
Second, I don't know why it would be absurd that there are probably fairies and Greek Gods in other universes. Suppose that there is only one univresewith an infinite number of planets. In that case, it may be probable that there are Klingons and Ferengi out there.
Whether Boltzmann brains are even possible is very controversial, but if they are possible, there is no way of telling whether they are likely or not.
Your final remark is about infinities. If actual infinities are impossible, then there may be lots and lots of universes, but not an infinite number of them, so probabilistic reasoning is not endangered.
Anyway, the real question should be whether on naturalism it is extremely unlikely that there is a multiverse regardless of evidence for or against fine-tuning. And I see no reason to think it is because to me, it would be very unlikely that the Big Bang or something similar to it, only happened one time. On naturalism, it's hard to see what would prevent other Big Bangs from happening.
Finally, the multivrese hypothesis is not speculation to avoid the fine-tuning problem, it is part of some serious fundemental theories
Walter, unless I insert the word "possible" before "universes" in your first sentence, I cannot understand your point about fine-tuning arguments against naturalism. The laws of nature lead to a fine-tuned universe, according to these arguments, precisely because the laws of nature themselves are chosen by an intelligent agent out of an infinite set of possible equations.
Professor Pruss, whether naturalism more naturally predicts a multiverse or universe is impossible to say, in my opinion, unless one knows something of the mathematical "shape" of the TOE it posits. Naturalism amounts to a claim that the material cosmos is all there is and has no cause in the proper sense, being a "brute fact" that happens to have (non-brute) rational structure in all its processes. Insofar as it rests all its explanatory power in the TOE at base, it cannot make any a priori predictions about the issue at hand, but must wait till something of the TOE is known.
However, the problem with the multiverse explanation for fine-tuning is (4) above. It is not in fact likely a priori that any TOE leading to (or describing) multiverses would lead to at least one fine-tuned universe, let alone certain. It is much easier, mathematically, to have a TOE that is either too simple or too complex (or has the wrong "shape") ever to allow for the peculiar combination of long-term stability but potentiality for intricate structural and functional combinations that ours clearly does. We see this by looking at how easy it is to change our own laws in tiny ways that leave a complete mess or abyss of tedium (e.g., a universe of mere radiation or one that lasts a fraction of a second, etc.), and in how boring simplified model universes physicists play with on computers are. There may be no meaningful way to calculate the fraction of possible equations that are bio-compatible out of the inifnite set of possible fundamental equations/TOEs, but I can see no reason to assume a bio-compatible multiverse TOE is likely, let alone certain. In other words, even if the multiverse is true, it still requires fine-tuning, just as Paul Davies has pointed out.
Fr Kirby
If nature is uniform, the laws of nature do not vaart between universes. So, yes, it follows that there is only one possible way a universe can be. In that case it is not possible to 'change our own laws in tiny ways.'
'vary' of course.
In a multiverse scenario, all the universes in the multiverse are part of actual reality. Uniformity of nature should not allow laws of nature to vary across actual reality.
However, uniformity of nature says nothing about whether laws of nature can vary across possible reality--between our world and other possible worlds.
But the Multiverse hypotheses is about actual universes, not about (logically) possible worlds.
If nature is uniform then there is only one imetaphysically possible natural reality.
The multiverse Generator itself has to be an extremely intelligent First Cause (a bigger God), in order to create coherent universes that can support complex life.
Logic and the law of Causality, always requires a First Cause of Everything.
Adi
That's a different argument that is only indirectly related to the actual FTA.
it would apply to any sort of life-supporting universe, whether it's fine-tuned or not.
Walter, I think I now see where the part of the problem might lie in our communications. Let me try again.
1. Once the overarching laws are "such and such", there is a metaphysical necessity, barring miracle, for natural processes to follow them. BUT, there is no metaphysical necessity to the fundamental laws themselves. They are either chosen out of an infinite set of mathematically possible equations by an agent or they are a "brute fact" with no "reason" for their obtaining concretely. The former case grants no metaphysical necessity to the laws themselves (except "after the fact" of the choice) and the second grants no metaphysical explanation or status whatever, seeing all explanation stopping at the physical level. Your statement "If nature is uniform then there is only one metaphysically possible natural reality" should be changed to "If nature is uniform, there is only one natural reality by definition". But this tautology tells us precisely nothing about what is metaphysically possible unless we make the further assumption that physical nature is all of reality and has no cause. Natural uniformity could theoretically be observed whether this assumption is true or not, so uniformitarianism has no definitive metaphysical implications on its own.
2. An overarching singular TOE can still (theoretically) lead to universe-specific sets of (derived) laws that vary due to different histories of symmetry-breaking, etc. The derived relationships may be thought of as different "solutions" to the TOE. These are the variations that allow multiverse theories to explain fine-tuning, but they still need a TOE that allows one or more solutions that are compatible with organised and stable complexity in order to then appeal to "observer-selection" as the reason why we find ourselves in what appears an improbable universe. Since the assumption that any old TOE would do that is specious, multiverse explanations for fine-tuning are pointless even if we are part of a multiverse.
Fr Kirby
But a naturalist makes the further assumption that physical nature is all of reality and has no cause. That's the definition of a naturalist. Now, if this assumption is true, then there is no metaphysical possibility apart from what is possible in physical nature. IOW, metaphysics and physics would be identical.
And as to "different histories of symmetry-breaking", those would be impossible if the laws of nature didn't allow for them. And the point is, we do not really know what nature allows or doesn't allow.
But suppose that nature does allow for various histories of symmetry breaking, then why wouldn't it allow for "one or more solutions that are compatible with organised and stable complexity"? It seems you would have to account for why nature is restricted to histories of symmetery breaking that are not compatible with organised and stable complexity.
My point is, if there are no restrictions whatsoever, then there is no reason why there would not be a multiverse and there is no reason why one or more of the universes would not have organised and stable complexity.
The point is, we do not know and, as yet, we do not have a TOE. And there could be a TOE that doesn't allow for one or more solutions that are compatible ..., but for now, we do not now. So, let's not speculate on what would or would not be possible.
Obviously our TOE is biocompatible, whether we are in a multiverse or not, as we are here.
The question is how likely is it, a priori, for a TOE (out of the infinity of possible mathematical equations) to allow for any biocompatibility (or compatibility with any interesting structure) whatsoever? From what we know of the ease with which our laws are ultra-vulnerable to not being biocompatible with tiny changes, and how it seems easier mathematically to produce either boring or overly messy structures/environments that do not allow for the combination of stability and organised complexity evolution and life needs, there is no reason to think any old TOE would do the same job ours does, no matter how many iterations it had. Most equations aren't going to give you the particular kind of symmetry-breaking or iterative variety I mentioned, either. Even chemistry's possibility relies on a nomic and parametric balancing act, never mind biology.
If the answer is that only our TOE is possible anyway, there is either a reason for that or not.
If not, we are back to brute fact. Then the TOE happens to obtain for no reason at all, and, by the way, happens to be improbably bio-compatible. This is really no different to an appeal to magic.
If there is a reason, it can not be logical or mathematical, as neither restricts what equations are possible or restrains the concrete at all. And it can't be empirical, since that would be appealing to the effects of the TOE we have, and thus be circular reasoning. Since, for the naturalist, Hume's Fork or modern positivism restricts valid reasoning to the logical or the empirical only, we are out of options, and so back to brute fact.
Fr Kirby
But a TOE is a description of reality. If reality is not restricted in any way, then a biocompatible universe is obviously a possibility and if reality is not restricted, a Multiverse is also an actual possibilty. And whether that Multiverse is likely to contain at least one biocompatible universe fepends on thé number if universes in it. And we have no way of knowong how many there would be.
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