Originally posted on sciy.org by Ron Anastasia on Sat 07 Oct 2006 01:13 AM PDT
The Physical Constants as Biosignature An anthropic retrodiction of the Selfish Biocosm Hypothesis
Originally published in the International
Journal of Astrobiology May 2005.
Reprinted on KurzweilAI.net February 28, 2006. Abstract
Introduction
The notion that we inhabit a universe whose laws and physical constants are fine-tuned in such a way as to make it hospitable to carbon-based life is an old idea (Gardner, 2003). The so-called "anthropic" principle comes in at least four principal versions (Barrow and Tipler, 1988) that represent fundamentally different ontological perspectives. For instance, the "weak anthropic principle" is merely a tautological statement that since we happen to inhabit this particular cosmos it must perforce by life-friendly or else we would not be here to observe it. As Vilenkin put it recently (Vilenkin, 2004), "the ‘anthropic' principle, as stated above, hardly deserves to be called a principle: it is trivially true." By contrast, the "participatory anthropic principle" articulated by Wheeler and dubbed "it from bit" (Wheeler, 1996) is a radical extrapolation from the Copenhagen interpretation of quantum physics and a profoundly counterintuitive assertion that the very act of observing the universe summons it into existence. All anthropic cosmological interpretations share a common theme: a recognition that key constants of physics (as well as other physical aspects of our cosmos such as its dimensionality) appear to exhibit a mysterious fine-tuning that optimizes their collective bio-friendliness. Rees noted (Rees, 2000) that virtually every aspect of the evolution of the universe—from the birth of galaxies to the origin of life on Earth—is sensitively dependent on the precise values of seemingly arbitrary constants of nature like the strength of gravity, the number of extended spatial dimensions in our universe (three of the ten posited by M-theory), and the initial expansion speed of the cosmos following the Big Bang. If any of these physical constants had been even slightly different, life as we know it would have been impossible: The [cosmological] picture that emerges—a map in time as well as in space—is not what most of us expected. It offers a new perspective on a how a single "genesis event" created billions of galaxies, black holes, stars and planets, and how atoms have been assembled—here on Earth, and perhaps on other worlds—into living beings intricate enough to ponder their origins. There are deep connections between stars and atoms, between the cosmos and the microworld.... Our emergence and survival depend on very special "tuning" of the cosmos—a cosmos that may be even vaster than the universe that we can actually see.
As stated recently by Smolin (Smolin, 2004), the challenge is to provide a genuinely scientific explanation for what he terms the "anthropic observation":
It is a daunting puzzle indeed. The strangely (and apparently arbitrarily) biophilic quality of the physical laws and constants poses, in Greene's view, the deepest question in all of science (Greene, 2004). In the words of Davies (Gardner, 2003), it represents "the biggest of the Big Questions: why is the universe bio-friendly?" Modern History of Anthropic ReasoningModern statements of the cosmological anthropic principle date from the publication of a landmark book by Henderson in 1913 entitled The Fitness of the Environment (Henderson, 1913). Henderson's book was an extended reflection on the curious fact that there are particular substances present in the environment—preeminently water—whose peculiar qualities rendered the environment almost preternaturally suitable for the origin, maintenance, and evolution of organic life. Indeed, the strangely life-friendly qualities of these materials led Henderson to the view that "we were obliged to regard this collocation of properties in some intelligible sense a preparation for the process of planetary evolution.... Therefore the properties of the elements must for the present be regarded as possessing a teleological character." Thoroughly modern in outlook, Henderson dismissed this apparent evidence that inanimate nature exhibited a teleological character as indicative of divine design or purpose. Indeed, he rejected the notion that nature's seemingly teleological quality was in any way inconsistent with Darwin's theory of evolution through natural selection. On the contrary, he viewed the bio-friendly character of the inanimate natural environment as essential to the optimal operation of the evolutionary forces in the biosphere. Absent the substrate of a superbly "fit" inanimate environment, Henderson contended, Darwinian evolution could never have achieved what it has in terms of species multiplication and diversification. The mystery of why the physical qualities of the inanimate universe happened to be so oddly conducive to life and biological evolution remained just that for Henderson—an impenetrable mystery. The best he could do to solve the puzzle was to speculate that the laws of chemistry were somehow fine-tuned in advance by some unknown cosmic evolutionary mechanism to meet the future needs of a living biosphere:
Henderson's iconoclastic vision was far ahead of its time. His potentially revolutionary book was largely ignored by his contemporaries or dismissed as a mere tautology. Of course there should be a close match-up between the physical requirements of life and the physical world that life inhabits, contemporary skeptics pointed out, since life evolved to survive the very challenges presented by that pre-organic world and to take advantage of the biochemical opportunities it offered. While lacking broad influence at the time, Henderson's pioneering vision proved to be the precursor to modern formulations of the cosmological anthropic principle. One of the first such formulations was offered by British astronomer Fred Hoyle. A storied chapter in the history of the principle is the oft-told tale of Hoyle's prediction of the details of the triple-alpha process (Mitton 2005). This prediction, which seems to qualify as the first falsifiable implication to flow from an anthropic hypothesis, involves the details of the process by which the element carbon (widely viewed as the essential element of abiotic precursor polymers capable of autocatalyzing the emergence of living entities) emerges through stellar nucleosynthesis. As noted by Livio (Livio, 2003):
Other chapters in the modern history of the anthropic principle are treated comprehensively by Barrow and Tipler (Barrow and Tipler, 1988) and will not be revisited here. The New Urgency of Anthropic InvestigationTwo recent developments have imparted a renewed sense of urgency to investigations of the anthropic qualities of our cosmos. The first is the discovery that the value of dark energy density is exceedingly small but not quite zero—an apparent happenstance, unpredictable from first principles, with profound implications for the bio-friendly quality of our universe. As noted recently by Goldsmith (Goldsmith, 2004):
The second development is the realization that M-theory—arguably the most promising contemporary candidate for a theory capable of yielding a deep synthesis of relativity and quantum physics—permits, in Bjorken's phrase (Bjorken, 2004), "a variety of string vacuua, with different standard-model properties." M-theorists had initially hoped that their new paradigm would be "brittle" in the sense of yielding a single mathematically unavoidable solution that uniquely explained the seemingly arbitrary parameters of the Standard Model. As Susskind has put it (Susskind, 2003):
This hope has been dashed by the recent discovery that the number of different solutions permitted by M-theory (which correspond to different values of Standard Model parameters) is, in Susskind's words, "astronomical, measured not in millions or billions but in googles or googleplexes." This development seems to deprive our most promising new theory of fundamental physics of the power to uniquely predict the emergence of anything remotely resembling our universe. As Susskind puts it, the picture of the universe that is emerging from the deep mathematical recesses of M-theory is not an "elegant universe" but rather a Rube Goldberg device, cobbled together by some unknown process in a supremely improbable manner that just happens to render the whole ensemble fit for life. In the words of University of California theoretical physicist Steve Giddings, "No longer can we follow the dream of discovering the unique equations that predict everything we see, and writing them on a single page. Predicting the constants of nature becomes a messy environmental problem. It has the complications of biology."[1] Two Contemporary Restatements of the Weak Anthropic Principle: Eternal Inflation Plus M-Theory and Many-Worlds Quantum Cosmology
There have been two principal approaches to the task of enlisting the weak anthropic principle to explain the mysteriously small (and thus bio-friendly) value of the density of dark energy and the apparent happenstance by which our bio-friendly universe was selected from the enormously large "landscape" of possible solutions permitted by M-theory, only a tiny fraction of which correspond to anything resembling the Standard Model prevalent in our cosmos. Eternal Inflation Meets M-Theory The first approach, favored by Susskind (Susskind, 2003). Linde (Linde, 2002), Weinberg (Weinberg, 1999), and Vilenkin (Vilenkin, 2004) among others, overlays the model of eternal inflation with the key assumption that M-theory-permitted solutions (corresponding to different values of Standard Model parameters) and dark energy density values will vary randomly from bubble universe to bubble universe within an eternally expanding ensemble variously termed a multiverse or a meta-univers. Generating a life-friendly cosmos is simply a matter of randomly reshuffling the set of permissible parameters and values a sufficient number of times until a particular Big Bang yields, against odds of perhaps a googleplex-to-one, a permutation that just happens to possess the right mix of Standard Model parameters to be bio-friendly. Sum-Over-Histories Quantum Cosmological ModelThe second approach invokes a quantum theory-derived sum-over-histories cosmological model inspired by Everett's "many worlds" interpretation of quantum physics. This approach, which has been prominently embraced by Hawking (Hawking and Hertog, 2002), was summarized as follows by Hogan (Hogan, 2004):
Hawking and Hertog (Hawking and Hertog, 2002) have explicitly characterized this "top down" cosmological model as a restatement of the weak anthropic principle:
Critique of Contemporary Restatements of the Weak Anthropic PrincipleApart from the objections on the part of those who oppose in principle any use of the anthropic principle in cosmology, there are at least three reasons why both the Hawking/Hogan and the Susskind/Linde/Weinberg restatements of the weak anthropic principle are objectionable. First, both approaches appear to be resistant (at the very least) to experimental testing. Universes spawned by Big Bangs other than our own are inaccessible from our own universe, at least with the experimental techniques currently available to science. So too are quantum wavefunction branches that we cannot, in principle, observe. Accordingly, both approaches appear to be untestable—perhaps untestable in principle. For this reason, Smolin recently argued (Smolin, 2004) "not only is the Anthropic Principle not science, its role may be negative. To the extent that the Anthropic Principle is espoused to justify continued interest in unfalsifiable theories, it may play a destructive role in the progress of science." Second, both approaches violate the mediocrity principle. The mediocrity principle, a mainstay of scientific theorizing since Copernicus, is a statistically based rule of thumb that, absent contrary evidence, a particular sample (Earth, for instance, or our particular universe) should be assumed to be a typical example of the ensemble of which it is a part. The Susskind/Linde/Weinberg approach, in particular, flouts this principle. Their approach simply takes refuge in a brute, unfathomable mystery—the conjectured lucky roll of the dice in a crap game of eternal inflation—and declines to probe seriously into the possibility of a naturalistic cosmic evolutionary process that has the capacity to yield a life-friendly set of physical laws and constants on a nonrandom basis. Third, both approaches extravagantly inflate the probabilistic resources required to explain the phenomenon of a life-friendly cosmos. (Think of a googleplex of monkeys typing away randomly until one of them, by pure chance, accidentally composes a set of equations that correspond to the Standard Model.) This should be a hint that something fundamental is being overlooked and that there may exist an unknown natural process, perhaps functionally akin in some manner to terrestrial evolution, capable of effecting the emergence and prolongation of physical states of nature that are, in the abstract, vanishingly improbable. The Darwinian PrecedentHogan (Hogan, 2004) has analogized the quantum theory-inspired sum-over-histories version of the weak anthropic principle to Darwinian theory:
Ironically, Hogan misses the key point that neither the branching wavefunction nor the eternal inflation-plus-M-theory versions of the weak anthropic principle hypothesize the existence of anything corresponding to the main action principle of Darwin's theory: natural selection. Both restatements of the weak anthropic principle are analogous, not to Darwin's approach, but rather to a mythical alternative history in which Darwin, contemplating the storied tangled bank (the arresting visual image with which he concludes The Origin of Species), had confessed not a magnificent obsession with gaining an understanding of the mysterious natural processes that had yielded "endless forms most beautiful and most wonderful," but rather a smug satisfaction that of course the earthly biosphere must have somehow evolved in a just-so manner mysteriously friendly to humans and other currently living species, or else Darwin and other humans would not be around to contemplate it. Indeed, the situation that confronts cosmologists today is reminiscent of that which faced biologists before Darwin propounded his revolutionary theory of evolution through natural selection. Darwin confronted the seemingly miraculous phenomenon of a fine-tuned natural order in which every creature and plant appeared to occupy a unique and well-designed niche. Refusing to surrender to the brute mystery posed by the appearance of nature's design, Darwin masterfully deployed the art of metaphor[2] to elucidate a radical hypothesis—the origin of species through natural selection—that explained the apparent miracle as a natural phenomenon. A significant lesson drawn from Darwin's experience is important to note at this point. Answering the question of why the most eminent geologists and naturalists had, until shortly before publication of The Origin of Species, disbelieved in the mutability of species, Darwin responded that this false conclusion was "almost inevitable as long as the history of the world was thought to be of short duration." It was geologist Charles Lyell's speculations on the immense age of Earth that provided the essential conceptual framework for Darwin's new theory. Lyell's vastly expanded stretch of geological time provided an ample temporal arena in which the forces of natural selection could sculpt and reshape the species of Earth and achieve nearly limitless variation. The central point for purposes of this paper is that collateral advances in sciences seemingly far removed from cosmology (complexity theory and evolutionary theory among them) can help dissipate the intellectual limitations imposed by common sense and naïve human intuition. And, in an uncanny reprise of the Lyell/Darwin intellectual synergy, it is a realization of the vastness of time and history that gives rise to the novel theoretical possibility to be discussed subsequently. Only in this instance, it is the vastness of future time and future history that is of crucial importance. In particular, sharp attention must be paid to the key conclusion of Wheeler: most of the time available for life and intelligence to achieve their ultimate capabilities lie in the distant cosmic future, not in the cosmic past. As Tipler (Tipler, 1994) has stated, "Almost all of space and time lies in the future. By focusing attention only on the past and present, science has ignored almost all of reality. Since the domain of scientific study is the whole of reality, it is about time science decided to study the future evolution of the universe." The next section of this paper describes an attempt to heed these admonitions. The Selfish Biocosm HypothesisIn a paper published in Complexity (Gardner, 2000), I first advanced the hypothesis that the anthropic qualities which our universe exhibits might be explained as incidental consequences of a cosmic replication cycle in which the emergence of a cosmologically extended biosphere could conceivably supply two of the logically essential elements of self-replication identified by von Neumann (von Neumann, 1948): a controller and a duplicating device. The hypothesis proposed in that paper was an attempt to extend and refine Smolin's conjecture (Smolin, 1997) that the majority of the anthropic qualities of the universe can be explained as incidental consequences of a process of cosmological replication and natural selection (CNS) whose utility function is black hole maximization. Smolin's conjecture differs crucially from the concept of eternal inflation advanced by Linde (Linde, 1998) in that it proposes a cosmological evolutionary process with a specific and discernible utility function—black hole maximization. It is this aspect of Smolin's conjecture rather than the specific utility function he advocates that renders his theoretical approach genuinely novel. As demonstrated previously (Rees, 1997; Baez, 1998), Smolin's conjecture suffers from two evident defects: (1) the fundamental physical laws and constants do not, in fact, appear to be fine-tuned to favor black hole maximization and (2) no mechanism is proposed corresponding to two logically required elements of any von Neumann self-replicating automaton: a controller and a duplicator.[3] The latter are essential elements of any replicator system capable of Darwinian evolution, as noted by Dawkins (Gardner, 2000) in a critique of Smolin's conjecture:
Theories of cosmological eschatology previously articulated (Kurzweil, 1999; Wheeler, 1996; Dyson, 1988) predict that the ongoing process of biological and technological evolution is sufficiently robust and unbounded that, in the far distant future, a cosmologically extended biosphere could conceivably exert a global influence on the physical state of the cosmos. A related set of insights from complexity theory (Gardner, 2000) indicates that the process of emergence resulting from such evolution is essentially unbounded. A synthesis of these two sets of insights yielded the two key elements of the Selfish Biocosm (SB) hypothesis. The essence of that synthesis is that the ongoing process of biological and technological evolution and emergence could conceivably function as a von Neumann controller and that a cosmologically extended biosphere could, in the very distant future, function as a von Neumann duplicator in a hypothesized process of cosmological replication. In a paper published in Acta Astronautica (Gardner, 2001) I suggested that a falsifiable implication of the SB hypothesis is that the process of the progression of the cosmos through critical epigenetic thresholds in its life cycle, while perhaps not strictly inevitable, is relatively robust. One such critical threshold is the emergence of human-level and higher intelligence, which is essential to the eventual scaling up of biological and technological processes to the stage at which those processes could conceivably exert a global influence on the state of the cosmos. Four specific tests of the robustness of the emergence of human-level and higher intelligence were proposed. In a subsequent paper published in the Journal of the British Interplanetary Society (Gardner, 2002) I proposed that an additional falsifiable implication of the SB hypothesis is that there exists a plausible final state of the cosmos that exhibits maximal computational potential. This predicted final state appeared to be consistent with both the modified ekpyrotic cyclic universe scenario (Khoury, Ovrut, Seiberg, Steinhardt, and Turok, 2001; Steinhardt and Turok, 2001) and with Lloyd's description (Lloyd, 2000) of the physical attributes of the ultimate computational device: a computer as powerful as the laws of physics will allow. Key Retrodiction of the SB Hypothesis: A Life-Friendly CosmosThe central assertions of the SB hypothesis are: (1) that highly evolved life and intelligence play an essential role in a hypothesized process of cosmic replication and (2) that the peculiarly life-friendly laws and physical constants that prevail in our universe—an extraordinarily improbable ensemble that Pagels dubbed the cosmic code (Pagels, 1983)—play a cosmological role functionally equivalent to that of DNA in an earthly organism: they provide a recipe for cosmic ontogeny and a blueprint for cosmic reproduction. Thus, a key retrodiction of the SB hypothesis is that the suite of physical laws and constants that prevail in our cosmos will, in fact, be life-friendly. Moreover—and alone among the various cosmological scenarios offered to explain the phenomenon of a bio-friendly universe—the SB hypothesis implies that this suite of laws and constants comprise a robust program that will reliably generate life and advanced intelligence just as the DNA of a particular species constitutes a robust program that will reliably generate individual organisms that are members of that particular species. Indeed, because the hypothesis asserts that sufficiently evolved intelligent life serves as a von Neumann duplicator in a putative process of cosmological replication, the biophilic quality of the suite emerges as a retrodicted biosignature of the putative duplicator and duplication process within the meaning of Goal 7 of the NASA Astrobiology Roadmap, which provides in pertinent part:
Does this retrodiction qualify as a valid scientific test of the validity of the SB hypothesis? I propose that it may, provided two additional qualifying criteria are satisfied:
Retrodiction as a Tool for Testing Scientific HypothesesThere is a lively literature debating the propriety of employing retrodiction as a tool for testing scientific hypotheses (Cleland, 2002; Cleland, 2001; Gee, 1999; Oldershaw, 1988). Oldershaw (Oldershaw, 1988) has discussed the use of falsifiable retrodiction (as opposed to falsifiable prediction) as a tool of scientific investigation:
As he went on to note, "Retrodictions usually represent falsification tests; the theory is probably wrong if it fails the test, but should not necessarily be considered right if it passes the test since it does not involve a definitive prediction." Despite their legitimacy as falsification tests of hypotheses, falsifiable retrodictions are qualitatively inferior to falsifiable predictions, in Oldershaw's view:
A less sympathetic view concerning the validity of retrodiction as a scientific tool was offered by Gee (Gee, 1999), who dismissed the legitimacy of all historical hypotheses on the ground that "they can never be tested by experiment, and so they are unscientific.... No science can ever be historical." This viewpoint, in turn, has been challenged by Cleland (Cleland, 2001) who contends that "when it comes to testing hypotheses, historical science is not inferior to classical experimental science" but simply exploits the available evidence in a different way:
Cleland's approach has the merit of preserving as "scientific" some of the most important hypotheses advanced in such historical fields of inquiry as geology, evolutionary biology, cosmology, paleontology, and archaeology. As Cleland has noted (Cleland, 2002):
In a paper presented to the 2004 Astrobiology Science Conference (Cleland, 2004), Cleland extended this analytic framework to the consideration of putative biosignatures as evidence of the past or present existence of extraterrestrial life. Acknowledging that "because biosignatures represent indirect traces (effects) of life, much of the research will be historical (vs. experimental) in character even in cases where the traces represent recent effects of putative extant organisms," Cleland concluded that it was appropriate to employ the methodology that characterizes successful historical research:
From the perspective of the evidentiary standards applicable to historical science in general and astrobiology in particular, the key retrodiction of the SB hypothesis—that the fundamental constants of nature that comprise the Standard Model as well as other physical features of our cosmos (included the number of extended physical dimensions and the extremely low value of dark energy) will be collectively bio-friendly—appears to constitute a legitimate scientific test of the hypothesis. Moreover, within the framework of Goal 7 of the NASA Astrobiology Roadmap, the retrodicted biophilic quality of our universe appears, under the SB hypothesis, to constitute a possible biosignature. Caution Regarding the Use of Retrodiction to Test the SB HypothesisBecause the SB hypothesis is radically novel and because the use of falsifiable retrodiction as a tool to test such an hypothesis creates at least the appearance of a "confirmatory argument resemble[ing] just-so stories (Rudyard Kipling's fanciful stories, e.g., how leopards got their spots)" (Cleland, 2001), it is important (as noted previously) that two additional criteria be satisfied before this retrodiction can be considered a legitimate test of the hypothesis:
As argued at length elsewhere (Gardner, 2003), the SB hypothesis is both consilient with central concepts in these "adjoining" fields and fully capable of generating falsifiable predictions. Concluding RemarksIn his book The Fifth Miracle (Davies, 1 Attachment: |