The topic I would like to discuss in this blog post is the relationship between the so-called anthropic principle and the postulated existence of a multiverse. Before going further in my discussion, I think I need to define what I mean by "the anthropic principle" and "the multiverse," especially for the benefit of those who are not familiar with these terms. The "anthropic principle" refers to the idea that the universe seems especially well suited for the emergence of life, particularly human life (hence the term "anthropic"). In recent years (as I understand it) scientists have discovered that many of the physical conditions existing in the universe make it quite hospitable to life. For example, the earth is just the right distance from the sun to harbor life--if it were a little closer, it would be too hot, like Venus, and if it were a little further away, it would be too cold, like Mars. It is also my understanding that there are many other characteristics of the universe that make it just the right environment for life to exist. Of course, the big question is why this should be so. As for "the multiverse," this term refers to the idea that the universe we live in is not the only one; rather, it is merely one of many universes making up a "multiverse," each universe having its own characteristics. However, the existence of a multiverse is still highly speculative since so far there is no conclusive evidence for it.
What then is the relationship between the anthropic principle and the notion of a multiverse? It is simply this--some scientists (and others) have argued that the anthropic principle can be explained by the existence of a multiverse. From their perspective, all sorts of conditions must exist in the various universes that make up the multiverse; we live in the one in which those conditions just happened to be right for the emergence of life.
I suggested earlier that the big question to be asked about the anthropic principle is why it should be that the universe has just the right characteristics for life to exist. The existence of a multiverse is certainly one possible answer to this question. Of course, there is another possible answer--perhaps the universe is so well-suited for life because it was supposed to be that way. This would seem to imply the existence of something or someone consciously guiding the emergence of the universe. For many people, that something or someone would be God.
However, many scientists (and others) resist such a conclusion. In their view, science should be grounded in philosophical naturalism/materialism--the related ideas that nothing exists outside of nature and that matter is the only thing that exists. Thus, science must reject the notion of an entity (God) that exists outside of nature (that is, is supernatural) and has no material existence (being spirit). For some of these scientists (and like-minded nonscientists), the multiverse provides an appropriately naturalistic/materialistic explanation for the anthropic principle.
The problem, though, as I (and I believe others) see it, is that invoking the multiverse to "explain away" the anthropic principle seems to be based primarily on a desire to avoid the seemingly theistic implications of what we might call the single-universe model--even if that is not publicly acknowledged as the rationale. Thus, it would seem to me that for the notion of a multiverse to be legitimately a scientific theory, it would have to be shown that there are one or more scientific reasons to prefer it over the single-universe model. One such reason could be, for example, that it (undeniably) provides an explanation for certain phenomena that the single-universe model fails to (to the best of my knowledge this has not been the case so far). Otherwise, the argument for a multiverse is essentially philosophical/metaphysical in nature, not scientific. If one argues that for a scientific theory to be valid it must be consistent with philosophical naturalism/materialism and for that reason the multiverse model is to be preferred, that too is a philosophical/metaphysical argument.
In fact, I would argue (and I suspect this is not an original argument) that the claim that science must be based on naturalism/materialism is somewhat self-defeating because it cannot itself be verified by the methods of science. As I understand it, evidence in science is obtained through observation of and/or experimentation with what exists in nature (and has a material existence so that it can be observed and/or experimented upon). Thus, any alleged entity/phenomenon existing outside nature and/or having no material existence cannot be detected through the methods of science. Consequently, a failure to find evidence for supernatural/nonmaterial entities or phenomena could be due to the fact that they do not exist, but could also be due instead to the fact that, while existing, they are undetectable by scientific methods. How could we be sure which of those possibilities is correct? And yet, for philosophical naturalism/materialism to be shown to be true, we would have to demonstrate somehow that only the first possibility is true and not the second, which could only be done by assuming that naturalism/materialism is true, thus ending up with a circular argument. (This is assuming that the existence of a supernatural/nonmaterial being could not be proven scientifically, which not everyone would necessarily agree with.)
To conclude, unless it can be demonstrated for purely scientific reasons that a multiverse must exist, any argument using the putative existence of a multiverse to negate the apparently theistic implications of the anthropic principle must be suspect. Moreover, even the existence of a multiverse would not necessarily undermine the theistic implications of the anthropic principle: as others have argued, we would then have to ask why a multiverse existed in which one universe had exactly the right conditions for life. Indeed, we might even ask: whence the multiverse? One thing, at least, is certain--debate over this topic will continue to rage for the foreseeable future.