Accordingly, alternative laws and constants trivially have physical probability zero, whereas the actual laws and constants have physical probability one. If the laws and constants that physics has so far determined turned out to be merely effective laws and constants fixed by some random process in the early universe which might be governed by more fundamental physical laws, it would start to make sense to apply the concept of physical probability to those effective laws and constants Juhl However, the fine-tuning considerations as outlined in Section 1.
Attempts to apply the notion of logical probability to fine-tuning for life are beset with difficulties as well. Critics argue that, from a logical point of view, arbitrary real numbers are possible values of the constants McGrew et al. According to them, any probability measure over the real numbers as values of the constants that differs from the uniform measure would be arbitrary and unmotivated. The uniform measure itself, however, assigns zero probability to any finite interval.
By this standard, the life-permitting range, if finite, trivially has probability zero, which would mean that life-friendly constants are highly improbable whether or not fine-tuning in the sense of Section 1. This conclusion seems counterintuitive, but Koperski argues that it is not as unacceptable for proponents of the view that life-friendly conditions are improbable and require a response as it may initially seem. Motivated by the difficulties that arise in attempts to apply the physical and logical notions of probability to fine-tuning for life, contemporary accounts often appeal to an essentially epistemic notion of probability e.
According to these approaches, life-friendly conditions are improbable in that we would not rationally expect them. See Section 3. Views according to which life-friendly conditions are epistemically improbable face the challenge to provide reasons as to why we should not expect life-friendly conditions from an epistemic perspective which ignores that life exists. One response to this challenge is to point out that there is no clear systematic pattern in the actual, life-permitting, combination of values of the constants Donoghue sect. Another response is to appeal to the criterion of naturalness see Section 5 , which would lead one to expect values for at least two constants of nature—the cosmological constant and the mass of the Higgs particle—which differ radically from the actual ones.
Neither elegance and simplicity nor naturalness dictate any specific probability distribution over the values of the constants, however, let alone over the form of the laws itself. But proponents of the view that fine-tuning for life is epistemically improbable can appeal to these criteria to argue that life-friendly conditions will be ascribed very low probability by any probability distribution that respects these criteria. Even if fine-tuned conditions are improbable in some substantive sense, it might be wisest to regard them as primitive coincidences which we have to accept without resorting to such speculative responses as divine design or a multiverse.
It is indeed uncontroversial that being improbable does not by itself automatically amount to requiring a theoretical response. The same attitude is advocated by Gould and Carlson and Olsson with respect to fine-tuning for life. Cory Juhl argues along independent lines that we should not regard fine-tuning for life as calling for a response. The sense in which fine-tuning for life fails to be surprising according to Juhl differs from the sense in which it is surprising according to authors such as Leslie, van Inwagen, Bostrom, Manson and Thrush: while the latter hold that life-friendly conditions are rationally unexpected from an epistemic point of view which sets aside our knowledge that life exists, Juhl holds that— given our knowledge that life exists and is causally ramified—it is unsurprising that life depends sensitively, for its existence, on the constants and boundary conditions.
Biological fine-tuning for survival and reproduction, as marvellous as it often appears, is regarded as unmysterious by biologists because evolution as driven by natural and sexual selection can generate it see Section 1. One may hope that, similarly, future developments in fundamental physics will reveal principles or mechanisms which explain the life-friendly conditions in our universe.
There are two different types of scenarios of how future developments in physics could realize this hope: first, physicists may hit upon a so-called theory of everything according to which, as envisaged by Albert Einstein,. Einstein This would make considerations about alternative laws and constants obsolete, and thereby undermine any perspective according to which these are fine-tuned for life. Unfortunately, developments in the last few decades have not been kind to hopes of the sort expressed by Einstein. But according to our present understanding of string theory, the theory has an enormous number of lowest energy states, or vacua , which would manifest themselves at the empirical level in terms of radically different effective physical laws and different values of the constants.
These would be the laws and constants that we have empirical access to, and so string theory would not come close to uniquely determining the laws and constants in the manner envisaged by Einstein. Further claimed achievements of inflationary cosmology include its ability to account for the observed near-perfect isotropy of the universe and the absence of magnetic monopoles. The strongest empirical support for inflationary cosmology, however, is now widely believed to come from of its apparently correct predictions of the shape of the cosmic microwave background fluctuations PLANCK collaboration A classic response to the observation that the conditions in our universe seem fine-tuned for life is to infer the existence of a cosmic designer who created life-friendly conditions.
If one identifies this designer with some supernatural agent or God, the inference from fine-tuning for life to the existence of a designer becomes a version of the teleological argument. Indeed, many regard the argument from fine-tuning for a designer as the strongest version of the teleological argument that contemporary science affords. Expositions of the argument from fine-tuning for design are typically couched in terms of probabilities e. In contrast, it is highly likely according to them that the constants are right for life if there is indeed a designer.
This motivates the likelihood inequality. We could not possibly have existed in conditions that are incompatible with the existence of observers. The famous weak anthropic principle WAP Carter suggests that this apparently trivial point may have important consequences:. Carter , emphasis due to Carter. Our methods of empirical observation are unavoidably biased towards detecting conditions which are compatible with the existence of observers. For example, even if life-hostile places vastly outnumber life-friendly places in our universe, we should not be surprised to find ourselves in one of the relatively few places that are life-friendly and seek an explanation for this finding, simply because—in virtue of being living organisms—we could not possibly have found ourselves in a life-hostile place.
Biases that result from the fact that what we observe must be compatible with the existence of observers are referred to as observation selection effects. The observation selection effects emphasized by the weak anthropic principle with respect to location in the universe are emphasized by what Carter dubs the strong anthropic principle SAP with respect to the universe as a whole:. Carter But, as Carter himself highlights , see also Leslie — , the SAP is meant to highlight exactly the same type of bias as the WAP and is literally stronger than the WAP only when conjoined with a version of the multiverse hypothesis.
The so-called anthropic objection against the argument from fine-tuning for design argues that that argument breaks down once our biasedness due to the observation selection effects emphasized by the weak and strong anthropic principles is taken into account. Elliott Sober , advocates this objection. The firing squad scenario involves an observation selection effect because the prisoner cannot contemplate his post-execution situation unless he somehow survives the execution.
But, as Leslie, Weisberg and Kotzen argue, this recommendation seems very implausible. To resolve the difficulty of accommodating observation selection effects in likelihood arguments, Kotzen suggests that bias due to such effects be taken into account as evidence rather than background information. Views according to which life-friendly conditions are improbable in an epistemic sense due to the required fine-tuning are challenged to come to terms with the fact that, as a matter of fact, we have long known that our universe is life-friendly, which means that life-friendly conditions are not literally unexpected for us.
As a consequence of this fact, the Bayesian version of the argument from fine-tuning for a designer as outlined in Section 3. An obvious choice, endorsed by Monton , who is critical of the argument from fine-tuning for design, and Collins , who supports it, is the so-called counterfactual or ur-probability solution to the problem of old evidence, as defended by Howson Tentative suggestions concerning the background knowledge that can reasonably be ascribed to such an agent are developed by Monton sect. This assumption can be challenged, as already discussed in 2.
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This assumption can be challenged as well. Notably, life-friendly conditions, conditional on the existence of the God of traditional theism, do not have very low probability according to Swinburne, i. Criticisms of the view that life-friendly constants are to be expected if there is a designer have a long tradition and go back to John Venn and John Maynard Keynes More recently, Sober has voiced general reservations about our abilities to competently judge what a divine designer, if real, would do:.
Our judgements about what counts as a sign of intelligent design must be based on empirical information about what designers often do and what they rarely do.
As of now, these judgements are based on our knowledge of human intelligence. The more our hypotheses of intelligent designers depart from the human case, the more in the dark we are as to what the ground rules are for inferring intelligent design. Sober However, one may question whether such tailored versions of the designer hypothesis have sufficient independent motivation and plausibility to deserve serious consideration in the first place.
According to Swinburne ch. The argument from fine-tuning for design as reviewed in Section 3. An advantage of this alternative is that it fits better with our actual epistemic situation: that the conditions are right for life is something we have known for a long time; our actual new evidence is that the laws of physics—as White and Weisberg put it—are stringent rather than lax in the constraints that they impose on the constants and boundary conditions if there is to be life. Does it plausibly hold for reasonable probability assignments?
Crucial among them is the inequality.
Put differently, according to White, absent information that life exists, information that the laws are stringent does at least not speak against the existence of a designer. The design theorist holds a combination of views according to which, on the one hand, life is more probable if there is a designer than if there is no designer and life is less probable if the laws are stringent rather than lax.
If one adds to this combination of views the assumption that none of the possible life-friendly conditions has higher probability than the others, both if there is a designer and if there is no designer, it dictates that—bracketing knowledge that life exists—stringent laws speak against the existence of a designer, i. Absent any evidence that life exists, evidence that the laws are stringent speaks against the existence of life in that stringent laws make life unexpected.
A problem with this response, similar to the difficulties discussed in Section 3. According to the multiverse hypothesis, there are multiple universes, some of them radically different from our own. Many of those who believe that fine-tuning for life requires some theoretical response regard it as the main alternative beside the designer hypothesis. The idea that underlies it is that, if there is a sufficiently diverse multiverse in which the conditions differ between universes, it is only to be expected that there is at least one where they are right for life. As the strong anthropic principle highlights see Section 3.
This suggests that, on the assumption that there is a sufficiently diverse multiverse, it is neither surprising that there is at least one universe that is hospitable to life nor—since we could not have found ourselves in a life-hostile universe—that we find ourselves in a life-friendly one. Many physicists e. It is controversial, however, whether this characterization is adequate. Impressed by this and other coincidences, Dirac stipulated that they might hold universally and as a matter of physical principle.
He conjectured that the strength of gravity may decrease as the age of the universe increases, which would indeed make it possible for the coincidence to hold across all cosmic times.
Dicke , criticizing Dirac, argues that standard cosmology with time-independent gravity suffices to account for the coincidence, provided that we take into account the fact that our existence is tied to the presence of mainline stars like then sun and of various chemical elements produced in supernovae. As Dicke shows, this requirement dictates that we could only have found ourselves in that cosmic period in which the coincidence holds.
Accordingly, there is no need to assume, as suggested by Dirac, that gravity varies with time to make the coincidence unsurprising. Carter and Leslie , ch. More often than as an inference to the best explanation the argument from fine-tuning for the multiverse is formulated using probabilities, in analogy to the argument from fine-tuning for design see Section 3.
For the purposes of the discussion about fine-tuning for life, hypotheses according to which there is only a single universe with constants that vary across space-time qualify as versions of the multiverse hypothesis. They seem to be disfavoured by the available evidence, however, see Uzan for a review. If we assume that, on the assumption that there is only a single universe, it is improbable that it has the right conditions for life see Section 2. Unless we have prior reasons to dramatically prefer a single universe over the multiverse, i.
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Just as the argument from fine-tuning for design, the argument from fine-tuning for the multiverse must come to terms with the problem that the existence of life is old evidence for us. At least prima facie, it is unclear what background knowledge can be assumed for an agent in that curious condition see Section 3. According to them, the argument commits this fallacy by, as White puts it,. White Versions of this criticism are endorsed by Draper et al. In an early response to Hacking, McGrath argues that the analogy between the argument from fine-tuning for the multiverse and a person who randomly enters a casino and witnesses a remarkable outcome is misleading: while the person entering a casino could have found any arbitrary outcome, we could not have found ourselves in a universe with conditions that are not right for life.
enter The appropriate analogy to consider, according to McGrath, involves someone who is allowed to enter the casino only if and when some specific remarkable outcome occurs and who, upon being called in and finding that this outcome has occurred, infers the existence of other trials in the past. In that scenario, the inference to multiple trials in the past is indeed rational, and so, according to McGrath, is the inference from fine-tuned conditions to multiple universes.
However, if we had strong independent evidence for other universes with life-hostile conditions, attempts to account for why our own universe is life-friendly would most likely seem futile. Thus independent evidence for some multiverse scenario could have a strong impact on what we regard as a rational response to fine-tuning for life.
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