In this essay, Sean Carroll:
Dissolves the problem of "creation from nothing":
A provocative way of characterizing these beginning cosmologies is to say that "the universe was created from nothing." Much debate has gone into deciding what this claim is supposed to mean. Unfortunately, it is a fairly misleading natural-language translation of a concept that is not completely well-defined even at the technical level. Terms that are imprecisely defined include "universe," "created," "from," and "nothing." (We can argue about "was.")
The problem with "creation from nothing" is that it conjures an image of a pre-existing "nothingness" out of which the universe spontaneously appeared – not at all what is actually involved in this idea. Partly this is because, as human beings embedded in a universe with an arrow of time, we can't help but try to explain events in terms of earlier events, even when the event we are trying to explain is explicitly stated to be the earliest one. It would be more accurate to characterize these models by saying "there was a time such that there was no earlier time."
To make sense of this, it is helpful to think of the present state of the universe and work backwards, rather than succumbing to the temptation to place our imaginations "before" the universe came into being. The beginning cosmologies posit that our mental journey backwards in time will ultimately reach a point past which the concept of "time" is no longer applicable. Alternatively, imagine a universe that collapsed into a Big Crunch, so that there was a future end point to time. We aren't tempted to say that such a universe "transformed into nothing"; it simply has a final moment of its existence. What actually happens at such a boundary point depends, of course, on the correct quantum theory of gravity.
The important point is that we can easily imagine self-contained descriptions of the universe that have an earliest moment of time. There is no logical or metaphysical obstacle to completing the conventional temporal history of the universe by including an atemporal boundary condition at the beginning. Together with the successful post-Big-Bang cosmological model already in our possession, that would constitute a consistent and self-contained description of the history of the universe.
Nothing in the fact that there is a first moment of time, in other words, necessitates that an external something is required to bring the universe about at that moment. As Hawking put it in a celebrated passage:
So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really self-contained, having no boundary or edge, it would have neither beginning nor end, it would simply be. What place, then, for a creator?
Uses Bayesian reasoning to judge possible explanations:
Nevertheless, for the sake of playing along, let's imagine that intelligent life only arises under a very restrictive set of circumstances. Following Swinburne, we can cast the remaining choices in terms of Bayesian probability. The basic idea is simple: we assign some prior probability – before we take into account what we actually know about the universe – to each of the three remaining scenarios. Then we multiply that prior probability by the probability that intelligent life would arise in that particular model. The result is proportional to the probability that the model is correct, given that intelligent life exists. Thus, for option #2 (a single universe, no supernatural intervention), we might put the prior probability at a relatively high value by virtue of its simplicity, but the probability of life arising (we are imagining) is extremely small, so much so that this model could be considered unlikely in comparison with the other two.
We are left with option #3, a "multiverse" with different conditions in different regions (traditionally called "universes" even if they spatially connected), and #4, a single universe with parameters chosen by God to allow for the eventual appearance of life. In either case we can make a plausible argument that the probability of life arising is considerable. All of the heavy lifting, therefore, comes down to our prior probabilities – our judgments about how a priori likely such a cosmological scenario is. Sadly, prior probabilities are notoriously contentious objects.
I will consider more carefully the status of the "God hypothesis," and its corresponding prior probability, in the final section. For now, let's take a look at the multiverse.
Correctly describes parsimony in terms of Kolmogorov complexity:
What prior likelihood should we assign to such a scenario? One popular objection to the multiverse is that it is highly non-parsimonious; is it really worth invoking an enormous number of universes just to account for a few physical parameters? As Swinburne says:
To postulate a trillion trillion other universes, rather than one God in order to explain the orderliness of our universe, seems the height of irrationality.
That might be true, even with the hyperbole, if what one was postulating were simply "a trillion trillion other universes." But that is a mischaracterization of what is involved. What one postulates are not universes, but laws of physics. Given inflation and the string theory landscape (or other equivalent dynamical mechanisms), a multiverse happens, whether you like it or not.
This is an important point that bears emphasizing. All else being equal, a simpler scientific theory is preferred over a more complicated one. But how do we judge simplicity? It certainly doesn't mean "the sets involved in the mathematical description of the theory contain the smallest possible number of elements." In the Newtonian clockwork universe, every cubic centimeter contains an infinite number of points, and space contains an infinite number of cubic centimeters, all of which persist for an infinite number of separate moments each second, over an infinite number of seconds. Nobody ever claimed that all these infinities were a strike against the theory. Indeed, in an open universe described by general relativity, space extends infinitely far, and lasts infinitely long into the future; again, these features are not typically seen as fatal flaws. It is only when space extends without limit and conditions change from place to place, representing separate "universes," that people grow uncomfortable. In quantum mechanics, any particular system is potentially described by an infinite number of distinct wave functions; again, it is only when different branches of such a wave function are labeled as "universes" that one starts to hear objections, even if the mathematical description of the wave function itself hasn't grown any more complicated.
A scientific theory consists of some formal (typically mathematical) structure, as well as an "interpretation" that matches that structure onto the world we observe. The structure is a statement about patterns that are exhibited among the various objects in the theory. The simplicity of a theory is a statement about how compactly we can describe the formal structure (the Kolmogorov complexity), not how many elements it contains. The set of real numbers consisting of "eleven, and thirteen times the square root of two, and pi to the twenty-eighth power, and all prime numbers between 4,982 and 34,950" is a more complicated set than "the integers," even though the latter set contains an infinitely larger number of elements. The physics of a universe containing 1088 particles that all belong to just a handful of types, each particle behaving precisely according to the characteristics of its type, is much simpler than that of a universe containing only a thousand particles, each behaving completely differently.
Discusses "meta-explanatory accounts":
For convenience I am brutally lumping together quite different arguments, but hopefully the underlying point of similarity is clear. These ideas all arise from a conviction that, in various contexts, it is insufficient to fully understand what happens; we must also provide an explanation for why it happens – what might be called a "meta-explanatory" account.
It can be difficult to respond to this kind of argument. Not because the arguments are especially persuasive, but because the ultimate answer to "We need to understand why the universe exists/continues to exist/exhibits regularities/came to be" is essentially "No we don't." That is unlikely to be considered a worthwhile comeback to anyone who was persuaded by the need for a meta-explanatory understanding in the first place.
Granted, it is always nice to be able to provide reasons why something is the case. Most scientists, however, suspect that the search for ultimate explanations eventually terminates in some final theory of the world, along with the phrase "and that's just how it is." It is certainly conceivable that the ultimate explanation is to be found in God; but a compelling argument to that effect would consist of a demonstration that God provides a better explanation (for whatever reason) than a purely materialist picture, not an a priori insistence that a purely materialist picture is unsatisfying.
Why are some people so convinced of the need for a meta-explanatory account, while others are perfectly happy without one? I would suggest that the impetus to provide such an account comes from our experiences within the world, while the suspicion that there is no need comes from treating the entire universe as something unique, something for which a different set of standards is appropriate.
States of affairs only require an explanation if we have some contrary expectation, some reason to be surprised that they hold. Is there any reason to be surprised that the universe exists, continues to exist, or exhibits regularities? When it comes to the universe, we don't have any broader context in which to develop expectations. As far as we know, it may simply exist and evolve according to the laws of physics. If we knew that it was one element of a large ensemble of universes, we might have reason to think otherwise, but we don't. (I'm using "universe" here to mean the totality of existence, so what would be called the "multiverse" if that's what we lived in.)
Likewise for the universe. There is no reason, within anything we currently understand about the ultimate structure of reality, to think of the existence and persistence and regularity of the universe as things that require external explanation. Indeed, for most scientists, adding on another layer of metaphysical structure in order to purportedly explain these nomological facts is an unnecessary complication. This brings us to the status of God as a scientific hypothesis.
Points out the theory-saving in and the predictive issues of God as a hypothesis:
Similarly, the apparent precision of the God hypothesis evaporates when it comes to connecting to the messy workings of reality. To put it crudely, God is not described in equations, as are other theories of fundamental physics. Consequently, it is difficult or impossible to make predictions. Instead, one looks at what has already been discovered, and agrees that that's the way God would have done it. Theistic evolutionists argue that God uses natural selection to develop life on Earth; but religious thinkers before Darwin were unable to predict that such a mechanism would be God's preferred choice.
This is a venerable problem, reaching far beyond natural theology. In numerous ways, the world around us is more like what we would expect from a dysteleological set of uncaring laws of nature than from a higher power with an interest in our welfare. As another thought experiment, imagine a hypothetical world in which there was no evil, people were invariably kind, fewer natural disasters occurred, and virtue was always rewarded. Would inhabitants of that world consider these features to be evidence against the existence of God? If not, why don't we consider the contrary conditions to be such evidence?
- And more!
See also his blog entry for more discussion of the essay.
Edit: added the bullet point about "meta-explanatory accounts."