Paper View: Cosmic Questions, Personal Implications
A good question provokes good thinking. It stimulates the imagination and inspires reasoning about profound issues. It focuses attention on problems, calls for clarification of assumptions, and leads to good follow-up questions, too. Such a good question was asked in four simple words by Sean M. Carroll1 (U of Chicago, Enrico Fermi Institute) this month in Nature.2 As part of a special issue focused on cosmology, Carroll asked, “Is our universe natural?”
It should become immediately apparent that this question invites discussion of what we mean by natural, and touches on issues of universal scope – even reaching beyond the universe, to put the cosmos in context. As we will see, the question also requires clarification of the nature, goals and limits of science. Don’t be deceived by the simplicity of his first sentence; the avenues he explores to answer the question are indeed profound.
It goes without saying that we are stuck with the Universe we have. Nevertheless, we would like to go beyond simply describing our observed Universe, and try to understand why it is that way rather than some other way. When considering both the state in which we find our current Universe, and the laws of physics it obeys, we discover features that seem remarkably unnatural to us. Physicists and cosmologists have been exploring increasingly ambitious ideas in an attempt to explain how surprising aspects of our Universe can arise from simple dynamical principles.
He gets right to the definition of “natural” –
What makes a situation ‘natural’? Ever since Newton, we have divided the description of physical systems into two parts: the configuration of the system, characterizing its particular state at some specific time, and the dynamical laws governing its evolution. For either part of this description, we have an intuitive notion that certain possibilities are more robust than others. When we come across a situation that seems unnatural or finely tuned, physicists seize upon it as a clue pointing towards some underlying mechanism that made it that way. Such clues can occasionally be misleading, but they often serve to guide our thinking about how we can extend our understanding into unknown domains.
This introduction makes it clear that Carroll views science as an attempt to get a handhold on observables, to reduce complex data to basic principles – to rationalize reality in terms accessible to the human mind, reducible to laws and equations. Implicit in this paragraph is a dislike for contingency or appeals beyond the “natural” (whatever that means). But what if reductionist approaches fail? What if some things in nature really are beyond the capabilities of natural explanations? How far can a committed naturalist go in “exploring increasingly ambitious ideas” before admitting defeat? Arthur C. Clarke once said, “The only way to discover the limits of the possible is to go beyond them into the impossible.” This can be healthy, like T. S. Eliot said; never cease from your explorations, and when you come back to where you started, you will understand the place for the first time. As we shall see, though, if naturalism goes too far afield, and never comes back, it morphs into its own nemesis: supernaturalism.
The introduction also hints that the naturalistic approach is built on faith. Scientists believe that even in the most puzzling phenomena there exist underlying physical or natural principles accessible to the human mind. Like the clues that lead a detective to solve a crime, puzzles spur scientists to discover underlying regularities, and to organize the observations into a unified, plausible account. Like Carroll said, scientists are not content to merely describe and catalog data; they want to be able to prove that, given certain initial conditions and natural laws, the phenomenon under investigation will follow. Carroll surveys several instances in the history of science where this approach has succeeded handsomely. It takes faith, however, to believe this approach can be extrapolated without bounds.
“If any system should be natural, it’s the Universe,” Carroll states as a truism. “Nevertheless, according to the criteria just described, the Universe that we observe seems remarkably unnatural.” Example: the entropy of the cosmos is remarkably low, compared to what it could be (everything could be in black holes, or uniformly distributed, for instance). This implies that, “for some reason, the early Universe was in a state of incredibly low entropy.” In addition, “our fundamental theories of physics involve huge hierarchies between the energy scales” of gravitation, particle physics, and “the recently discovered vacuum energy.” These hierarchies appear finely tuned: so much so, that he compares it to a ball perched on top of a hill. “Of course, it may simply be that the Universe is what it is, and these are brute facts that we have to live with,” he concedes. “More optimistically, however” (and here is where faith comes in), “ these apparently delicately tuned features of our Universe may be clues that can guide us towards a deeper understanding of the laws of nature.” There must be a point where the clues become expressible in equations, though, else this “deeper understanding” becomes gnosticism – a form of intuitive wisdom for the elite, or a mystery religion. Will Carroll succeed in bringing heaven down to earth?
A natural explanation should be testable to be considered scientific. Yet Carroll tells us that cosmologists have been increasingly open to radical ideas that seem untestable, even in principle. If so, the only law discovered may be one of Murphy’s – “every solution breeds new problems.”
Given this situation, physicists have been exploring dramatic extensions of our known theories, in an attempt to provide a larger context in which our apparently unnatural Universe is seen to make perfect sense. Interestingly, attempts to account for both the low entropy of the early Universe and the disparate energy scales of fundamental physics lead us to a similar idea: that the local Universe we observe is part of a much larger ensemble. This casts new light on the problems of naturalness, while raising vexing issues of its own; considerable advances in both theory and experiment will be necessary before we can decide whether we are learning the appropriate lessons from the clues provided by nature.
Attempts to explain the fine-tuning of the universe through natural causes are not new. Inflation theory, for example, got rid of the flatness problem and horizon problem by positing an exponential expansion in the first second of the universe. Or did it? In a surprise revelation, Carroll shows that the solution was worse than the problem:
It is worth emphasizing that the only role of inflation is to explain the initial conditions of the observable Universe. And at this it does quite a good job: inflation predicts that the Universe should be spatially flat, and should have a scale-free spectrum of adiabatic density perturbations, both of which have been verified to respectable precision by observations of the cosmic microwave background. But we are perfectly free to imagine that these features are simply part of the initial conditions—indeed, both spatial flatness and scale-free perturbations were investigated long before inflation. The only reason to invoke inflation is to provide a reason why such an initial condition would be natural.
However, as Penrose and others have argued, there is a skeleton in the inflationary closet, at least as far as entropy is concerned. The fact that the initial proto-inflationary patch must be smooth and dominated by dark energy implies that it must have a very low entropy itself; reasonable estimates for this entropy SI range from about 1 to 1020. Thus, among randomly chosen initial conditions, the likelihood of finding an appropriate proto-inflationary region is actually much less than simply finding the conditions of the conventional Big Bang model (or, for that matter, of our Universe ten minutes ago). It would seem that the conditions required to start inflation are less natural than those of the conventional Big Bang.
Contrary to popular accounts, therefore, inflation didn’t solve the fine-tuning problem at all. Nor has it been solved since by more exotic forms of inflationary theory, such as chaotic inflation, spontaneous inflation or eternal inflation – because these also rely on unobservable parts of the universe. “Needless to say, proposals of this type are extremely speculative, and may well be completely wrong,” he says; regardless of the model proposed, “it is crucial to understand whether inflation plays a role in explaining how our observed configuration could be truly natural.”
Carroll then investigates whether the laws of physics are natural. It would seem the constants of physics could take any arbitrary values, though the laws and equations be tightly constrained. Could the particular values of these constants reflect mere environmental conditions, like the apparently arbitrary number of planets in our solar system? “As mentioned in the introduction, that is not what we observe,” he reminds us. The values are separated by huge hierarchies. What’s more, “In contemplating the nature of these hierarchies, a complicating factor arises: we could not exist without them.” We are back to the anthropic principle (08/16/2005), and the only way out, to make the universe a natural consequence of physics, is to propose an ensemble of universes – a multiverse (12/18/2005):
In the first case [i.e., we were lucky], there are two separate possibilities: either we are really lucky, in the sense that the observed hierarchies are truly unnatural and have no deeper explanation, or there exist unknown dynamical mechanisms that make these hierarchies perfectly natural. The latter possibility [environmental selection] is obviously more attractive, although it is hard to tell whether such dynamical explanation will eventually be forthcoming. Environmental selection, sometimes discussed in terms of the ‘anthropic principle’, has received renewed attention since the discovery of the dark energy. The basic idea is undeniably true [sic]: if our observable Universe is only a tiny patch of a much larger ‘multiverse’ with a wide variety of local environments, there is a selection effect due to the fact that life can only arise in those regions that are hospitable to the existence of life. Of course, to give this tautology any explanatory relevance, it is necessary to imagine that such a multiverse exists.
Carroll’s brief digression into the properties of a multiverse, one that might yield our universe with its finely-tuned cosmological constant, ends in despair: “At present, then, there is no reliable environmental explanation for the observed value of the cosmological constant.“ Moreover, “other attempts to use anthropic reasoning seem to lead to predictions that are in wild disagreement with observations.” But that does not mean the multiverse proposal has been falsified, he says, whether or not it is falsifiable. Yet if we cannot observe something or test it, and if we cannot falsify it – if it is an appeal to a mystery world that someone finds “attractive” – then have we not abandoned the goals of science?
More importantly, limitations in our current ability to calculate expectation values in the multiverse are not evidence that there is no truth to the idea itself. If we eventually decide that environmental selection plays no important role in explaining the observed parameters of nature, it will be because we come to believe that the parameters we measure locally are also characteristic of regions beyond our horizon, not because the very concept of the multiverse is aesthetically unacceptable or somehow a betrayal of the Enlightenment project of understanding nature through reason and evidence.
But then, how could one know that the observed parameters hold true for regions beyond our horizon, or whether those regions even exist? How does this differ from appeals to angels and demons or any other unseen entity as a proxy for observable effects? “The ideas discussed here involve the invocation of multiple inaccessible domains within an ultra-large-scale multiverse,” Carroll admits. “For good reason, the reliance on the properties of unobservable regions and the difficulty in falsifying such ideas make scientists reluctant to grant them an explanatory role.” He also admits that extrapolating our parameters into unseen realms is just as untestable. Maybe the multiverse concept will be testable some day; for now, it is not.
We are back to definitions. What is natural? Is our universe natural? Is our cosmology natural? Can science even claim that a “natural” explanation is better than an unnatural one? It all seems in the eye of the beholder:
Naturalness is an ambiguous guide in the quest to understand our Universe better. The observation that a situation seems unnatural within a certain theoretical context does not carry anything like the force of an actual contradiction between theory and experiment. And despite our best efforts, naturalness is something that is hard to quantify objectively.
Carroll ends with appeals to the future, in effect saying that we are a long way from coming back to where we started and understanding it for the first time.
The ultimate goal is undoubtedly ambitious: to construct a theory that has definite consequences for the structure of the multiverse, such that this structure provides an explanation for how the observed features of our local domain can arise naturally, and that the same theory makes predictions that can be directly tested through laboratory experiments and astrophysical observations. To claim success in this programme, we will need to extend our theoretical understanding of cosmology and quantum gravity considerably, both to make testable predictions and to verify that some sort of multiverse picture really is a necessary consequence of these ideas. Only further investigation will allow us to tell whether such a programme represents laudable aspiration or misguided hubris.
1Not to be confused with Sean B. Carroll (molecular biologist at U of Wisconsin-Madison) and several other scientists named Sean Carroll.
2Sean M. Carroll, “Is our universe natural?”, Nature 440, 1132-1136 (27 April 2006) | doi:10.1038/nature04804.
This article is filled with fodder for philosophers of science, historians of science, and theologians. Modern cosmology has followed the Enlightenment dream, only to end up in the middle of nowhere, bankrupt. The attempt to naturalize everything has pushed them out of scientific bounds; they have no equations, no predictions, no falsification criteria, no confirming data, and no reason to continue the quest other than that they find “natural” explanations more “attractive.” This basically admits that naturalism, as opposed to theism, is only a preference. Carroll cannot even explain what natural means; he debunks the word as ambiguous and unquantifiable in one paragraph, only to hope, a few sentences later, that science will some day find a natural explanation. Substitute a nonsense word for natural in those sentences, to see that this makes scientific naturalism a meaningless and futile quest.
This paper arms the intelligent design movement in the current fight over the definition of science. The Darwinists and other materialists insist that the rules of science require only naturalistic explanations, regardless of one’s personal religious beliefs. But if scientific naturalists cannot even define what natural means, they have no case for insisting on that rule. Why should materialistic cosmologists be permitted to speculate about unobservable entities beyond the reach of observation and testability, and get their speculations published in Nature, without competition? And, why could not a theistic cosmologist turn a meaningless word to his advantage, and call intelligent design a “natural” explanation?
For the most part, Carroll wrote thoughtfully and perceptively, except for one thing: he totally ignored theism as an option. He is like Robert Jastrow’s mountain climber, scrambling over the last highest peak, only to find a band of theologians who have been sitting there for centuries. Yet he doesn’t even bother to say Howdy. Instead, he walks over to them and tries to describe them with equations, and puzzles about how they emerged by a natural process. As he does this, one of the theologians taps on his head and says, “Hello? Anybody home?” yet Carroll continues, now trying to naturalize the pain he feels in his skull.
You have to feel sorry for the Enlightenment secular scientist. Granted a fair measure of success explaining physical phenomena by natural causes, he has attempted to extrapolate this programme to all the universe, only to find, as with Gödel’s theorem, it cannot be done from within: one cannot prove the consistency of a system from within the system. Extending the system into a hypothetical “natural” multiverse does nothing to change the predicament. Naturalism has led to a self-refuting absurdity.
Whenever a path of inquiry leads to absurdity, it may be a clue all right, but a clue that real understanding lies elsewhere. Accelerating on the wrong road only accelerates lostness. Progress may require a new direction, even backtracking. By analogy, though naturalism may have done well explaining the operation of the car, it cannot justify the direction it is traveling. A futile insistence on naturalism is like the stubborn driver with “misguided hubris” insisting he doesn’t need to stop for directions, though he may be an excellent mechanic.
To complete T. S. Eliot’s circuit – to arrive back from where you started and to understand it for the first time – consider taking a fresh look at an old, musty book on the home coffee table. It just might, after all you’ve learned, seem remarkably perceptive and insightful – you know, that book that begins, “In the beginning, God created the heavens and the earth.”* Maybe it’s no coincidence that most of your fellow sentient beings have found that explanation, without the long detour, perfectly natural.