Despite the growing popularity of the multiverse proposal, it must be admitted that many physicists remain deeply uncomfortable with it. The reason is clear: the idea is highly speculative and, from both a cosmological and a particle physics perspective, the reality of a multiverse is currently untestable. Indeed, it may always remain so, in the sense that astronomers may never be able to observe the other universes with telescopes a and particle physicists may never be able to observe the extra dimensions with their accelerators. The only way out would be if the effects of extra dimensions became 'visible' at the TeV scale, in which case they might be detected when the Large Hadron Collider becomes operational in 2007. This would only be possible if the extra dimensions were as large as a millimetre. However, it would be very fortunate (almost anthropically so) if the scale of quantum gravity just happened to coincide with the largest currently accessible energy scale.
For these reasons, some physicists do not regard these ideas as coming under the purvey of science at all. Since our confidence in them is based on faith and aesthetic considerations (for example mathematical beauty) rather than experimental data, they regard them as having more in common with religion than science. This view has been expressed forcefully by commentators such as Sheldon Glashow , Martin Gardner  and George Ellis , with widely differing metaphysical outlooks. Indeed, Paul Davies  regards the concept of a multiverse as just as metaphysical as that of a Creator who fine-tuned a single universe for our existence. At the very least the notion of the multiverse requires us to extend our idea of what constitutes legitimate science.
In some people's eyes, of course, cosmology has always bordered on metaphysics. It has constantly had to battle to prove its scientific respectability, fighting not only the religious, but also the scientific orthodoxy. For example, the prevalent view until well into the nineteenth century (long after the demise of the heliocentric picture) was that speculations about things beyond the Solar System was not proper science. This was reflected by Auguste Comte's comments on the study of stars in 1859 :
Never, by any means, will we be able to study their chemical compositions. The field of positive philosophy lies entirely within the Solar System, the study of the Universe being inaccessible in any possible science.
However, Comte had not foreseen the advent of spectroscopy, triggered by Gustav Kirchhoff's realization in the same year that the dark lines in the solar spectrum were absorption features associated with chemical elements.
For the first time this allowed astronomers to probe the composition of distant stars.
Cosmology attained the status of a proper science in 1915, when the advent of general relativity gave the subject a secure mathematical basis. The discovery of the cosmological expansion in the 1920s then gave it a firm empirical foundation. Nevertheless, it was many decades before it gained full scientific recognition. For example, when Ralph Alpher and Robert Herman were working on cosmological nucleosynthesis in the 1940s, they recall : 'Cosmology was then a sceptically regarded discipline, not worked in by sensible scientists.' Only with the detection of the microwave background radiation in 1965 was the hot Big Bang theory established as a branch of mainstream physics, and only with the recent results from the WMAP satellite (postdating the Stanford meeting which led to this book) has it become a quantitative science with real predictive power.
Nevertheless, cosmology is still different from most other branches of science; one cannot experiment with the Universe, and speculations about processes at very early and very late times depend upon theories of physics which may never be directly testable. Because of this, more conservative physicists still tend to regard cosmological speculations as going beyond the domain of science. The introduction of anthropic reasoning doubtless enhanced this view. On the other hand, other physicists have always held a more positive opinion, so there has developed a polarization of attitudes towards the anthropic principle. This is illustrated by the following quotes. The first is from the protagonist Freeman Dyson :
I do not feel like an alien in this Universe. The more I examine the Universe and examine the details of its architecture, the more evidence I find that the Universe in some sense must have known we were coming.
This might be contrasted with the view of the antagonist Heinz Pagels :
The influence of the anthropic principle on contemporary cosmological models has been sterile. It has explained nothing and it has even had a negative influence. I would opt for rejecting the anthropic principle as needless clutter in the conceptual repertoire of science.
An intermediate stance is taken by Brandon Carter , who might be regarded as one of the fathers of the anthropic principle:
The anthropic principle is a middle ground between the primitive anthropocentrism of the pre-Copernican age and the equally unjustifiable antithesis that no place or time in the Universe can be privileged in any way.
The growing popularity of the multiverse picture has encouraged a drift towards Carter's view, because it suggests that the anthropic fine-tunings can at least have a 'quasi-physical' explanation. To the hard-line physicist, the multiverse may not be entirely respectable, but it is at least preferable to invoking a Creator. Indeed anthropically inclined physicists like Susskind and Weinberg are attracted to the multiverse precisely because it seems to dispense with God as the explanation of cosmic design.2
In fact, the dichotomy in attributing anthropic fine-tunings to God or the multiverse is too simplistic. While the fine-tunings certainly do not provide unequivocal evidence for God, nor would the existence of a multiverse preclude God since - as emphasized by Robin Collins  - there is no reason why a Creator should not act through the multiverse. Neverethless, the multiverse proposal certainly poses a serious challenge to the theological view, so it is not surprising that it has commended itself to atheists. Indeed, Neil Manson has described the multiverse as 'the last resort for the desperate atheist' .
By emphasizing the scientific legitimacy of anthropic and multiverse reasoning, I do not intend to deny the relevance of these issues to the science-religion debate . The existence of a multiverse would have obvious religious implications , so contributions from theologians are important. More generally, cosmology addresses fundamental questions about the origin of matter and mind, which are clearly relevant to religion, so theologians need to be aware of the answers it provides. Of course, the remit of religion goes well beyond the materialistic issues which are the focus of cosmology. Nevertheless, in so much as religious and cosmological truths overlap, they must be compatible. This has been stressed by Ellis , who distinguishes between Cosmology (with a big C) - which takes into account 'the magnificent gestures of humanity' - and cosmology (with a small c), which just focuses on physical aspects of the Universe. In his view, morality is embedded in the cosmos in some fundamental way. Similar ideas have been expounded by John Leslie .
On the other hand, science itself cannot deal with such issues, and it seems unlikely that - even in the extended form required to accommodate the multiverse - science will ever prove or disprove the existence of God. Some people may see in the physical world some hint of the divine, but this can only provide what John Polkinghorne describes as 'nudge' factors .
2 It should be cautioned that the concept of 'cosmic design' being described here has nothing to do with the 'Intelligent Design' movement in the USA. Nevertheless, atheists might hope that the multiverse theory will have the same impact in the context of cosmic design as the theory of evolution did in the context of biological design.
Convictions about God's existence must surely come from 'inside' rather than 'outside' and even those eminent physicists who are mystically inclined do not usually base their faith on scientific revelations . For this reason, theology receives rather short shrift in this volume. The contributors are nearly all physicists, and even those of a theological disposition have generally restricted their remarks to scientific considerations.
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