Anthropic Principle

The Anthropic Principle asserts that the existence of human life places certain necessary constraints on cosmological and metaphysical theories. It is an ex post facto methodological tool that attempts to relate the structure of the universe to the underlying conditions that are necessary for the existence of observers.

The Anthropic Principle attempts to explain the universe's many life-supporting "coincidences" in two distinct ways: 1) by appealing to an allencompassing selection effect amongst a variety of universes (e.g., the Weak Anthropic Principle); 2) by asserting that the evolution of life is the necessary outcome of the laws of nature (e.g., the Strong Anthropic Principle). It is this latter form that suggests the possible creative activity of an Intelligent Designer.

Formulated in 1974 by the British astrophysicist Brandon Carter, the Anthropic Principle is an attempt to limit the Copernican dogma, which asserts that the Earth does not occupy a privileged central position in the universe. However, while the Earth may not be special or privileged in every way, this does not mean that it cannot be privileged in any way. Indeed, Carter pointed out that the location of the Earth in space is "necessarily privileged to the extent of being compatible with our existence as observers" (p. 291).

The Anthropic Principle is controversial because it implies a teleological link between the structure of the universe and the existence of human beings. Several theorists have taken this idea one step further by incorporating the Anthropic Principle into a larger design argument for the existence of God.

Teleology and fine-tuning

The Anthropic Principle makes this type of goal-directed argument possible by highlighting the various prerequisites for the existence of life. When these prerequisites are duly examined, a striking number of "cosmic coincidences" are discovered to exist between distant branches of physics. These anthropic coincidences are noteworthy because they are essential for the existence of life and because they require tremendous "fine-tuning" before they can be operational. The gravitational constant (G ), for instance, appears to be exceedingly fine-tuned for the existence of life. If it were slightly larger, stars would have burned too hot and much too quickly to support the fragile needs of life; if it were slightly smaller, the intrastellar process of nuclear fusion would have never initiated, and life would have been incapable of arising on the Earth.

This same rationale can also be applied to the expansion rate of the nascent universe. This crucial factor is determined by the cooperative interplay between several distinct cosmic parameters, including the mass density of the universe, the explosive vigor of the Big Bang, and the strength of the gravitational constant. If the resulting cosmic expansion rate happened to be slightly greater than the presently observed value, life-supporting galaxies would have been unable to form; if it were slightly smaller, the early universe would have collapsed back in on itself shortly after the Big Bang. Either way, no life forms would have been possible.

This is significant, because the various parameters that comprise the cosmic expansion rate also had to be fine-tuned to better than one part in 1060 in order to generate a "flat" universe, so that normal Euclidian geometry (in which the sum of a triangle's three angles adds up to 180 degrees) could then become applicable. A similar degree of fine-tuning can be found throughout the remainder of nature's fundamental parameters.

The challenge is to find a plausible explanation for this fine-tuning. According to the British mathematical physicist Roger Penrose, the odds that a fine-tuned biocentric universe could have accidentally evolved are an astounding one in ten to the 10123, a number so vast that it could not be written on a piece of paper the size of the entire visible universe. This is why many theorists have posited the existence of a "supercalculating intellect" to account for this fine-tuning.

Others, however, have scoffed at this teleological interpretation of cosmological history. They point out that this fine-tuning could have been generated randomly over billions of years if the universe turns out to be merely one of many. In this case, life would have evolved only in those regions that happened to possess the "correct" configuration of fundamental parameters, and human beings would then find themselves living in this special region as a straightforward selection effect. Critics, however, charge that this position is question-begging by its very nature, since it assumes the prior existence of these unexplained worlds.

Definitions

The Anthropic Principle comes in a variety of permutations, each with its own set of implications.

Weak Anthropic Principle. The broadest and least controversial permutation is known as the Weak Anthropic Principle (WAP). Given the reality of human life, the physical universe must contain areas that are compatible with the existence of human beings as observers. The WAP states that humans never could expect to observe a universe that is significantly different from their own, because human existence depends on the prior existence of just such a universe. The WAP thus doesn't try to explain how or why the universe came to be life-supporting. It merely notes that, while the universe is biocentric for unknown reasons, given the current existence of humans it couldn't possibly have been otherwise.

One of the advantages of the WAP is that it highlights the many diverse structural parameters that are necessary for the existence of life. Nevertheless, many people find the WAP deeply unsatisfying because it merely states what is already known to be true; namely, that the universe has to be structured in its present form before it can be capable of supporting carbon-based life. The WAP is thus incapable of explaining why the universe is structured in this biocentric manner.

Strong Anthropic Principle. The more potent Strong Anthropic Principle (SAP) attempts to explain why the universe has a biocentric structure. According to the SAP, the universe must have properties that will allow life to develop within it at some stage of its history. The key element is the word must; it means that the universe had to be life-supporting at some stage of its history. This possibility is suggested by the many astonishing coincidences between distant branches of physics that all work together, against all the odds, to make life possible. The conventional SAP, however, does not attempt to explain why the universe must be biocentric. It simply states that this must be so.

Design-Centered Anthropic Principle. The SAP thus comes close to positing the existence of a cosmic designer because there doesn't seem to be any other plausible way of explaining why the universe had to be life-supporting. For this reason the physicist Heinz R. Pagels (1939–1988) once quipped that the SAP is "the closest that some atheists can get to God." One interpretation of the SAP explicitly credits a designer for the Earth's many biocentric features. This interpretation, which can be called the Design-Centered Anthropic Principle (DCAP), holds that the universe is biocentric because it was deliberately designed to be this way by a higher power.

Participatory Anthropic Principle. A second version of the SAP, derived from the findings of modern theoretical physics, has been dubbed the Participatory Anthropic Principle (PAP) by physicist John Wheeler (b. 1911). This version holds that observers are necessary to bring the universe into being. The PAP follows from the standard Copenhagen interpretation of quantum mechanics, in which some type of living consciousness is required to make events "real." According to this interpretation, developed by physicist Neils Bohr (1885–1962), there is no such thing as a concrete quantum reality until a living observer exists to "collapse" the appropriate quantum wave function. Without this act of observation, reality seems to be held in a paralyzing state of indecision.

Some theorists have gone so far as to argue that life is necessary to make the universe itself real. The physicist George Greenstein (b. 1940) has conceived of a "symbiotic universe" in which both life and the universe exist in a classic state of symbiosis; the universe provides the physical foundation for the existence of life, and life symbiotically responds by imparting a concrete state of reality to the cosmos.

The problem with this conceptualization is that life did not evolve until billions of years after the Big Bang. In order for Greenstein's theory to be plausible, a noncorporeal form of life had to have been responsible for observing the universe into being long ago. The only candidate for this role would be the "Ultimate Observer" spoken of by John Barrow and Frank Tipler. This observer alone would have been in a position to observe the entire universe into being.

Final Anthropic Principle. A third version of the SAP has been dubbed the Final Anthropic Principle (FAP). According to FAP, intelligent life must come into existence in the universe, and, once it comes into existence, it will survive forever and become infinitely knowledgeable as it strives to mold the universe to its will. The FAP thus possesses an obvious religious quality because it states that there is a positive universal purpose to human life that cannot be thwarted by any possible power. In this sense, the FAP is analogous to the tenets of generic theism, particularly in its affirmation of an afterlife. However, the FAP does not explain why intelligent life will endure forever. It merely states that it will do so.

Anthropic coincidences

It is important to distinguish between the Anthropic Principle and a curious set of physical facts known as anthropic coincidences. The Anthropic Principle proper is a speculative hypothesis regarding the possible role of humanity in the cosmos, whereas the various anthropic coincidences are empirical observations that relate the apparent fine-tuning of the universe to the needs of life. This, in turn, seems to provide some degree of empirical support for certain forms of the Anthropic Principle.

The value of the gravitational constant G, the mass density of the universe, and the explosive vigor of the Big Bang have all seemingly been fine-tuned to cooperate with one another to generate a smoothly expanding universe of coherent galaxies, each containing an abundance of medium-sized biocentric stars like the sun. Numerous other fine-tuned anthropic coincidences are also at work in the universe to make life possible. A partial list includes the following:

1. the values of nature's fundamental constants;
2. the existence of three spatial dimensions;
3. the ratio of the electromagnetic force constant to the gravitational constant;
4. the mass ratio of the electron and proton;
5. the ratio of protons to electrons;
6. the cosmic entropy level;
7. the speed of light;
8. the age of the universe;
9. the mass excess of the neutron over the proton;
10. the initial excess of matter over antimatter; and
11. the sun's historical change in luminosity, which happened to coincide with the specific needs of Earth-based life forms.

One of the most notable anthropic coincidences was discovered in 1953 by the British astronomer Fred Hoyle (1915–2001), a former atheist. Hoyle had been researching the intrastellar process of carbon synthesis when he stumbled upon a remarkable series of coincidences pertaining to the stepwise assembly of the carbon atom. To his great surprise, Hoyle discovered that the nuclear resonance levels of both carbon and its immediate precursors (helium and beryllium) were fine-tuned to work together to encourage carbon synthesis. He also found that oxygen's nuclear resonance level is half a percent too low to encourage the nuclear conversion of carbon into oxygen. The result of this remarkable series of coincidences is that carbon can be manufactured inside dying stars in sufficient quantities to make organic life possible. Hoyle concluded that the universe is a "put up job," and that a "supercalculating intellect" had to have "monkeyed" with the basic parameters of physics and cosmology. Otherwise, one would never expect so many unrelated and improbable coincidences to work seamlessly together to generate a biocentric universe.

The Anthropic Design argument

Given the many intercoordinated steps that are required to generate a fine-tuned biocentric universe, many theorists find it astonishing that any form of life could have evolved on this planet. There are simply too many ways in which cosmic evolution could have gone wrong with respect to life, particularly given the universality of the Second Law of Thermodynamics, which states that the total amount of disorder in the universe is always increasing. It is the Second Law that leads one to expect a non-biocentric outcome at each stage of the universe-building process, yet the correct biocentric result nevertheless happened at each bifurcation point.

It is the fine-tuning of nature's fundamental constants at the Big Bang that probably enabled this to happen. Indeed, given the brute fact of human existence, it is necessarily the case that the universe be fine-tuned enough for it to overcome the many thermodynamic hurdles that naturally exist on the way to life. This, in turn, seems to suggest a strong element of necessity in the universe's underlying ability to generate life. Insofar as this is so, it constitutes evidence in favor of the Strong Anthropic Principle.

Moreover, since the general cosmic tendency is always towards an increased amount of disorder, some thinkers conclude that there must have been some type of constraining force at work in the past. Otherwise, this predisposition towards disorder would likely have put the universe on a nonbiocentric path long ago, despite the fact that order can sometimes be generated within an open thermodynamic system by adding energy to it.

Traditional cosmology has been unable to account for this mystery, except insofar as it has used the principle of cumulative selection to explain the successive preservation of small instances of order, each of which possibly could have been random in origin. The problem with this hypothesis is that the universe had to have evolved to a relatively advanced stage before any type of cumulative selection could have taken place. For this reason many find the Strong Anthropic Principle to be compelling. How else can one explain the trillions of correct choices on the way to life, despite the Second Law, if it weren't structurally necessary for the universe to evolve life at some point in its history?

The Weak Anthropic Principle is typically invoked to refute this conclusion. According to this view, humans shouldn't be surprised at their own existence because they are merely experiencing a selection effect, since it is not possible for them to have observed a non-biocentric universe. While this may be so, it does not necessarily follow that human existence is not surprising. In the same way that a condemned criminal facing a one hundred-man firing squad would naturally be surprised if all one hundred rifles misfired simultaneously, it is also appropriate for human beings to be astonished at their own existence.

Many Worlds Interpretation. A potent counterargument to this anthropic viewpoint has been provided by Hugh Everett's (1930–1982) Many Worlds Interpretation of quantum mechanics. According to this hypothesis, there are an infinite number of "compartments" or worlds in existence within a much larger "multiverse," each possessing its own randomly varying set of fundamental constants. Humans therefore shouldn't be surprised at their own existence, because it is only natural for life to evolve in the one region of the multiverse that is capable of supporting its existence. This is a prime example of how the Weak Anthropic Principle can be used within a nontheistic worldview to account for the existence of life.

There are three problems with the Many Worlds approach, however. First, there is no evidence for any of these other possible worlds, nor can there be any such evidence in the future because these alternative domains are believed to be utterly beyond human observational powers, even in principle. Secondly, this approach begs the question, since it assumes the prior, unexplained existence of the multiverse itself. Finally, the use of an infinite number of unobservable worlds to explain the existence of our own world is an unprecedented violation of Ockham's Razor, which states that the simplest explanation in any set of natural circumstances is probably the correct one.

Anthropic explanations

Critics of the Anthropic Principle believe it to be scientifically sterile, since it doesn't initially seem to explain much about the cosmos in which humans live. Supporters of the Anthropic Principle, by contrast, believe that it holds the key to an intriguing relationship between the structure of the universe and the existence of human observers. The size and age of the universe provide an excellent case in point. Prior to the advance of modern cosmological science, it was believed that both the physical and temporal dimensions of the universe were unrelated to the existence of living observers. The mathematician and philosopher Bertrand Russell, for instance, believed that the universe's enormous size and age naturally rendered the concept of intelligent design implausible, since one would naturally expect a deity to have created the best things in the world (e.g., human beings) first rather than last.

This viewpoint has been supplanted by modern cosmological findings that indicate that a certain minimum time frame is inherently required for the intrastellar synthesis of carbon by natural evolutionary pathways. The amount of time that is necessary for this outcome amounts to several billion years, which is roughly equivalent to the time required to synthesize carbon and other heavy elements inside dying red giant stars. During this entire carbon-making epoch, though, the universe itself has been relentlessly expanding. Therefore, it is only in a universe that is sufficiently old, and hence sufficiently large, that carbon-based observers can evolve. The enormous size of the visible universe (approximately fifteen billion light years in spatial extent) is thus directly related to the time required for intrastellar carbon synthesis, due to the ongoing cosmic expansion. This is a genuine anthropic explanation because it links several aspects of the universe to the conditions necessary to generate living observers.

Anthropic versus biocentric

The Anthropic Principle is actually a philosophical misnomer, since it is primarily an argument about the centrality of biological life in general. As such, it could legitimately be called the "Biocentric Principle." A separate argument is thus required to generate an Anthropic Principle from the biocentric evidence. The Greek word anthropos, however, refers to uniquely human life, so the possible existence of intelligent beings elsewhere would technically invalidate the Anthropic Principle. In order to allow for this possibility, it has been suggested that the Anthropic Principle be renamed the Humanoid Principle.

Three distinct arguments are thus conflated within the Anthropic Principle: (1) a biocentric argument, which refers to the centrality of biological life forms in general; (2) a humanoid argument, which refers to the centrality of intelligent humanoid life; and (3) a specific anthropic argument, which argues for the exclusivity of Earth-based intelligent life. These conflations, however, are widely deemed to be irrelevant to the central thrust of the Anthropic Principle, since it is generally assumed that human life would be the ultimate goal of any cosmic intention to evolve Earth-based life. It is also assumed that the possible existence of other humanoid life forms would not invalidate the Anthropic Principle itself. Instead, it would simply provide other cosmic loci by which the biocentric nature of the universe could be explained.

Conclusion

The basic purpose of the Anthropic Principle is to relate the underlying structure of the universe to the fact of human existence. Although many thinkers find this goal unrealistic, others believe that the uniqueness of human consciousness is a fact of fundamental significance in the cosmos. For it is primarily through the vehicle of human awareness that the universe has somehow become aware of itself, and no other known entity appears to possess this marvelous capacity.

See also Anthropocentricism; Copenhagen Interpretation; Cosmology, Physical Aspects; Design; Entropy; Geocentricism; Many-worlds Hypothesis; Physics, Quantum; Thermodynamics, Second Law of

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