Anthropic principle

In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe and the existence of human observers within that universe by way of noting the coincidentally balanced features that are necessary and relevant to the existence on Earth of biochemistry, carbon-based life, and eventually human beings to observe such a universe. There are many versions of the anthropic principle; at least one source has suggested that there may be as many as thirty of them. All of them, however, begin with observance (or supposed observance anyway, because the question of the status and observational accuracy of the observer is one of the points of discussion and disagreement) of a number of facts of cosmology, physics, and other features of the known universe. Whether these facts are simple coincidences or whether they are evidence of some kind of design or purpose or intention or teleology in the universe is a central question of investigation and controversy.

Anthropic Coincidences

The common (and "weak") form of the anthropic principle is a truism or tautology that begins with the observation that the universe appears surprisingly hospitable to the emergence of life, particularly complex multicellular life, that can make such an observation and concludes with that premise that in only such a fine-tuned universe can such living observers be. Given the extreme simplicity of the universe at the start of the Big Bang, the friendliness of the universe to complex structures such as galaxies, planetary systems, and biology is unexpected by any normal model of turbulence driven structuring that science has been able to derive.

The idea evolved from the so-called "Dicke's coincidence", and has subsequently been reinforced by the discovery of many more anthropic coincidences since Robert Dicke first noted that the evolution of the universe is not random, but is coincidentally constrained by biological factors that require that the age of the universe had to be roughly this "golden-age" . Much younger, and there would not have been time for sufficient interstellar levels of carbon to build up by nucleosynthesis, but much older, and the golden age of main sequence stars and stable planetary systems would have already come to an end.

The anthropic principle also acts as a convenient category for physical and cosmological reasoning that takes into account the existence of a biosphere on Earth in an essential way. Attempts to invoke the "anthropic principle" to develop scientific explanations has led to more than a little confusion and controversy.

The relevant anthropic coincidences [2] occur in complete unexpected contrast to all attempts to model the evolution of the universe, and have been "unfolding" since the moment of the big bang, ranging in magnitude from our local ecosystem, all the way up the ladder to the near-perfectly balanced, "flat" structuring of the universe, itself, so the universe appears to be surprisingly hospitable to the emergence of life at a specific time and "location" [3] in the history of the universe, particularly, complex multicellular carbon-based life.

Given the extreme simplicity of the universe at the start of the Big Bang, the friendliness of the universe to complex structures such as galaxies, planetary systems, and biology is unexpected by any normal model of turbulence driven structuring that we have been able to derive. It is therefore possible to use anthropic reasoning to make testable predictions about the features of the observed universe from the average of extreme opposing runaway tendencies that are known to be inherent to the anthropic coincidences, as well as to choose from a multiverse of different configurations, in lieu of a more viable stability mechanism.

Carbon-based life is either a likely and thus necessary outcome of the observed structure and stability of the universe, or it is a part of an engine of creation that favors such outcomes. So a proven identification of a stability mechanism may be expected to settle this question of causality, which means that the strong anthropic principle (SAP) can be further supported or falsified. If falsified, only the weak anthropic principle (WAP), essentially a tautology, remains.

Attempts to invoke the "principle" to develop scientific explanations have led to more than a little confusion, and controversy.

Origin of the anthropic cosmological principle

The first to employ the phrase "anthropic principle" appears to have been the theoretical astrophysicist Brandon Carter, in his contribution to a 1973 symposium titled Confrontation of Cosmological Theories with Observational Data honouring Copernicus's 500th birthday. Carter articulated the Anthropic Principle as an ecological correction, so called, of what is now called the Cosmological Principle. This Principle extends the principle of relativity so as to require that all observers experience the same laws of physics uniformly throughout the universe. Hence at any given time, the universe will be both homogeneous and isotropic, (in 3-D space). This defines a non-applicable principle of mediocrity, one precluding the existence of a mechanism favoring any particular time and location for the appearance of carbon-based life as we know it.

An anthropic principle was as well formulated already in the 1940's, in relation to relativistic cosmology and physics constants, by Abraham Zelmanov, a prominent scientist in the General Theory of Relativity and cosmology.

Copernicus argued that the Earth is not the centre of the solar system, but Carter noted that pure cosmological extensions of this idea are what he called the anticentrist dogma, that led to cosmological formulations like the Perfect Cosmological Principle, which does not result from the evolutionary physics that derives the cosmic coincidences and the otherwise unexplained large scale structuring of the universe that becomes absurdly apparent with the cosmological constant problem. This vexing problem is why the Anthropic Principle has acquired a following among string theorists trying to choose the correct vacuum solution from the landscape, since no other stability mechanism explaining why this is so has been proposed. Carter's symposium paper, "Large Number Coincidences and the Anthropic Principle in Cosmology," included the statement: "Although our situation is not necessarily central, it is inevitably privileged to some extent".

Carter was not the first to invoke some form of the anthropic principle. For instance, Robert H. Dicke wrote in 1957 that: "The age of the Universe 'now' is not random but conditioned by biological factors ... [changes in the values of the fundamental constants of physics] would preclude the existence of man to consider the problem." Alfred Russel Wallace anticipated the weak anthropic principle as long ago as 1903: "Such a vast and complex universe as that which we know exists around us, may have been absolutely required ... in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man." The WAP is perhaps even echoed by Karl Marx's theory of historical materialism: "The first premise of all human history is, of course, the existence of living human individuals. Thus the first fact to be established is the physical organisation of these individuals and their consequent relation to the rest of nature."

Observational evidence

The observed values of the dimensionless parameters (such as the fine-structure constant) that govern the four forces of nature are finely balanced. A slight increase in the strong nuclear force would bind the dineutron and the diproton and all the hydrogen in the early universe would have been converted to helium. There would be no water or the long-lived stable stars that are essential for the development of life. Similar relationships are evident in each of the four force strengths. If they were to be modified even slightly — some commentators have noted that as little as one part in 10⁴⁰ or even smaller would be sufficient to make the difference — then the universe's structure and capacity for life as we now know it would disappear.

Variants of the anthropic principle

Proponents of the anthropic principle submit that the universe appears "fine-tuned" so as to permit life as we know it to exist, because were the universe not fine tuned in this fashion, human beings would not exist and hence could not observe the universe.

Barrow and Tipler (1986) propose three important variants of the Anthropic Principle, Weak, Strong, and Final, listed below in order of increasing strength:

• Weak anthropic principle (WAP): "The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe be old enough for it to have already done so."
  • The Merriam-Webster dictionary defines WAP as conditions that are observed in the universe must allow the observer to exist.

If any of the fundamental physical constants were sufficiently different, then life as we know it would not be possible and no one would be around to contemplate the universe we live in. Barrow and Tipler, among others, argue that the WAP explains the fundamental physical constants, such as the fine structure constant, the number of dimensions in the universe, and the cosmological constant.

From Roger Penrose:

Is there something special about our particular location in the universe, either in space or in time? These are the kinds of question that are addressed by what has become known as the anthropic principle. This principle has many forms. (See Barrow and Tipler 1986.) The most clearly acceptable of these addresses merely the spatiotemporal location of conscious (or 'intelligent') life in the universe. This is the weak anthropic principle. The argument can be used to explain why the conditions happen to be just right for the existence of (intelligent) life on the earth at the present time. For if they were not just right, then we should not have found ourselves to be here now, but somewhere else, at some other appropriate time. This principle was used very effectively by Brandon Carter and Robert Dicke to resolve an issue that had puzzled physicists for a good many years. The issue concerned various striking numerical relations that are observed to hold between the physical constants (the gravitational constant, the mass of the proton, the age of the universe, etc.). A puzzling aspect of this was that some of the relations hold only at the present epoch in the earth's history, so we appear, coincidentally, to be living at a very special (line (give or take a few million years!). This was later explained, by Carter and Dicke, by the fact that this epoch coincided with the lifetime of what are called main-sequence stars, such as the sun. At any other epoch, so the argument ran, there would be no intelligent life around in order to measure the physical constants in question-so the coincidence had to hold, simply because there would be intelligent life around only at the particular time that the coincidence did hold! (From The Emperor's New Mind)

• Strong anthropic principle (SAP): "The Universe must have those properties which allow life to develop within it at some stage in its history." Barrow and Tipler then propose three elaborations of the SAP:
  • "There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers.' " This can be seen as simply the classic design argument dressed in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental constants set to ensure that life as we know it will emerge and evolve. ("The Rejection of Pascal's Wager")
  • "Observers are necessary to bring the Universe into being." Barrow and Tipler believe that this can be validly inferred from quantum mechanics.
  • "An ensemble of other different universes is necessary for the existence of our Universe." Thus Barrow and Tipler sympathize with the many worlds interpretation of quantum mechanics.

• Final anthropic principle (FAP): "Intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out."

In his review of Barrow and Tipler in the New York Review of Books', Martin Gardner ridiculed the FAP by quoting the last two sentences of their book:

• Completely ridiculous anthropic principle (CRAP): "At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know. And this is the end."

In Carter's original definition, the WAP referred only to certain "cosmological" parameters, namely our space/time location within the universe, and not to values of the fundamental physical constants, which would fall under the SAP according to him. He also refers to "observers" rather than "carbon-based life". This ambiguity is a reason for the ongoing disagreements about the status of the various Anthropic Principles.

Proponents of intelligent design claim support from the SAP. On the other hand, the existence of the multiverse or alternate universes is hypothesized for other reasons and the WAP provides a plausible explanation for the fine tuning of our universe. Assuming there are possible universes capable of supporting intelligent life, some actual universes must do so and ours clearly is one of those. However, alternatives to intelligent design are not limited to hypothesizing the existence of alternate universes, and some advocates of evolution claim support from the Anthropic Principle. On the other hand, Ikeda and Jefferys (2006) argue that the Anthropic Principle as conventionally stated actually undermines intelligent design. This is discussed in more detail in fine-tuned universe.

The Anthropic Cosmological Principle

The most thorough extant study of the anthropic principle is the controversial book The Anthropic Cosmological Principle by John D. Barrow, a cosmologist, and Frank J. Tipler, a mathematical physicist. This book contains an extensive review of the relevant history of ideas, because its authors believe that the anthropic principle has important antecedents in the notions of intelligent design, the philosophies of Fichte, Hegel, Bergson, and Whitehead, and the omega point cosmology of Teilhard de Chardin. Barrow and Tipler carefully distinguish teleological reasoning from eutaxiological reasoning; the former asserts that order must have a consequent purpose; the latter asserts more modestly that order must have a planned cause. They attribute this important but nearly always overlooked distinction to Hicks (1883).

Barrow and Tipler set out in great detail the seemingly incredible coincidences that characterize our universe and that permit human beings to evolve in it. They then maintain that only the anthropic principle can make sense of this raft of coincidences. Everything from the energy states of the electron to the exact strength of the weak nuclear force seems tailored for us to exist. That our universe contains carbon-based life is contingent upon the values of several independent parameters, and were the value of any of those parameters to vary slightly, carbon-based life could not exist. While Barrow and Tipler (1986) is primarily a work of theoretical physics, it also discusses a variety of related topics in chemistry and earth science.

In 1983, Brandon Carter, qualifying his 1974 paper, stated that the anthropic principle, in its original form, was meant only to caution astrophysicists and cosmologists about possible errors in the interpretation of astronomical and cosmological data if they failed to take into account constraints arising from the biological nature of the observer. Carter also warned that the inverse was true for evolutionary biologists; in interpreting the evolutionary record, one must take into account cosmological and astrophysical considerations. With this in mind, Carter concluded that, given the best estimates of the age of the universe (then about 15 billion years, now 13.7 billion years), the evolutionary chain probably can allow only one or two low probability links. A. Feoli and S. Rampone argue for a higher number of low probability links, given the size of our universe and the likely number of planets. The higher number of low probability links is less consistent with the claim that the emergence of life and its subsequent evolution requires intelligent design.

Recent work in observational cosmology and the theory of quantum gravity has led to renewed interest in the anthropic principle. Quantum gravity attempts to unify gravity with the other forces. While there have been a number of promising developments, all such theories suffer from the problem that the fundamental physical constants are unconstrained. The observational motivation comes from more precise estimates of quantities such as the matter density of the universe. Recent estimates of this density are about 0.3, while cosmological theory generally predicts a value indistinguishable from one.

There are alternatives to the anthropic principle, the most optimistic being that a Theory of everything will ultimately be discovered, uniting all forces in the universe and deriving from scratch all properties of all particles. Candidate "theories of everything" include M-Theory and various theories of quantum gravity, although all theories of this nature are currently deemed speculative. Another possibility is Lee Smolin's model of cosmological natural selection, also known as fecund universes, which proposes that universes have "offspring" which are more plentiful if they happen to have features common to our universe. Also see Gardner (2005) and his "selfish biocosm hypothesis."

Criticisms

Some forms of the anthropic principle have been criticized as an argument by lack of imagination for assuming that the only possible chemistry of life is one based on carbon compounds and liquid water (sometimes called "carbon chauvinism", see also alternative biochemistry). The range of fundamental physical constants allowing evolution of carbon-based life may also be much less restrictive than proposed.

The WAP has been criticized, by its supporters as well as its critics, for being a tautology, stating something not readily obvious yet trivially true. The weak anthropic principle implies that our ability to ponder cosmology at all is contingent on all fundamental physical parameters having numerical values falling within quite a narrow range. Critics reply that this is simply tautological reasoning, an elaborate way of saying "if things were different, they would be different". If this is granted, the WAP becomes a truism saying nothing and explaining nothing, because in order for us to be here to ponder the universe, that universe has to be structured so that we can exist. Peter Schaefer denies that labelling the WAP a truism invalidates it, on the grounds that one cannot refute a statement merely by saying that it is true.

Critics of the SAP claim that it is neither testable nor falsifiable, and thus is not science. The FAP is discussed in more detail under final anthropic principle; Barrow and Tipler (1986) state that while the FAP is a valid physical statement, it is also "closely connected with moral values".

The anthropic principle at first glance seems to discourage research into a theory of everything, however it only suggests that progress made regarding a theory of everything must allow the observer of such progress to exist.

Hawking (2004) suggests that our universe is much less 'special' than the proponents of the anthropic principle claim it is. According to Hawking, there is a 98% chance that a Big Bang will result in a universe of the same type as ours. However, some question whether the equations Hawking employs to reach this conclusion are scientifically meaningful, and what sort of universe can be said to be of the "same type as ours".

Hawking's wave function of the universe, he and others have claimed, shows how our universe could have come into existence without any relation to anything existing prior to it, i.e., could have come out of "nothing." As of 2004, however, this work remains debatable. Moreover, as Hawking wrote in 1988, "What is it that breathes fire into the equations and makes a universe for them to describe?...Why does the universe go to all the bother of existing?" (Hawking 1988). That "there is something instead of nothing" is the fundamental problem of metaphysics.

Anthropic bias and anthropic reasoning

In 2002, Nick Bostrom asked "Is it possible to sum up the essence of observation selection effects in a simple statement?" He concluded that it might be, but that:

Many 'anthropic principles' are simply confused. Some, especially those drawing inspiration from Brandon Carter's seminal papers, are sound, but... they are too weak to do any real scientific work. In particular, I argue that existing methodology does not permit any observational consequences to be derived from contemporary cosmological theories, though these theories quite plainly can be and are being tested empirically by astronomers. What is needed to bridge this methodological gap is a more adequate formulation of how observation selection effects are to be taken into account.

His Self-Sampling Assumption is "that you should think of yourself as if you were a random observer from a suitable reference class." This he expands into a model of anthropic bias and anthropic reasoning under the uncertainty introduced by not knowing your place in our universe - or even who "we" are. This may also be a way to overcome various cognitive bias limits inherent in the humans doing the observation and sharing models of our universe using mathematics, as suggested in the cognitive science of mathematics.

Anthropic principle in cosmic inflation

A critique of cosmic inflation, questioning the very premise of the theory, was offered by Don N. Page who emphasized the point that initial conditions which made it possible that a thermodynamic arrow of time in a Big Bang type of theory must necessarily include a low entropy initial state of the Universe and therefore to be extremely improbable. The critique was rebutted by P. C. W. Davies who used an inflationary version of the anthropic principle. While accepting the premise that the initial state of the visible Universe (originally a microscopic amount of space before the inflation) had to possess a very low entropy value — due to random quantum fluctuations — to account for the observed thermodynamic arrow of time, he deemed it not a problem of the theory but an advantage. The fact that the small fragment of space from which our Universe grew had to be extremely orderly to allow inflation resulting in a universe with an arrow of time makes it unnecessary to adopt any ad-hoc hypotheses about the initial entropy state which are necessary in other Big Bang theories.

Anthropic principle in string theory

String theory predicts a large number of possible universes, called the backgrounds or vacua. The set of these universes or vacua is often called the "multiverse" or "anthropic landscape" or "string landscape". Leonard Susskind has argued that the existence of a large number of vacua puts the anthropic reasoning on firm ground; only universes with the remarkable properties sufficient to allow observers to exist are beheld while a possibly much larger set of universes without such properties go utterly unnoted. Others, most notably David Gross but also Lubos Motl, Peter Woit and Lee Smolin, argue that this is not predictive. In his paper on the string landscape, Steven Weinberg refers to the Anthropic Principle as a "turning point" in modern science.

See also

• Anthropic landscape
• Big Bounce
• Cosmological natural selection
• Doomsday argument
• Fine-tuned universe
• Inverse gambler's fallacy
• metaphysical naturalism
• Perfect Cosmological Principle
• Principle of mediocrity
• Rare Earth hypothesis
• Teleology
• Weakless Universe

References

ISBN links support NWE through referral fees

• Nick Bostrom, 2002. Anthropic Bias: Observation Selection Effects in Science and Philosophy. Routledge. ISBN 0-415-93858-9. 5 chapters available online.
• Brandon Carter, "Large Number Coincidences and the Anthropic Principle" in Longair, M. S., ed., IAU Symposium 63: Confrontation of Cosmological Theory with Astronomical Data. Reidel (Dordrecht, Netherlands): 291-98.
• ------, 2004, "Anthropic Principle in Cosmology." Paper presented at the conference "Cosmology: Facts and Problems," hosted by the College de France.
• Craig, William Lane, 1987, "Critical review of The Anthropic Cosmological Principle," International Philosophical Ouarterly 27: 437- 47.
• Gardner, James N., 2005, "The Physical Constants as Biosignature: An anthropic retrodiction of the Selfish Biocosm Hypothesis," International Journal of Astrobiology.
• Martin Gardner, "WAP, SAP, PAP, and FAP," The New York Review of Books 23, No. 8 (May 8, 1986): 22-25.
• Hawking, Stephen W. (1988). A Brief History of Time. New York: Bantam Books, p.174. ISBN 0-553-34614-8. 
• Kane, Gordon L., Malcolm J. Perry, and Anna N. Zytkow (2002). The Beginning of the End of the Anthropic Principle. New Astron. 7: 45–53. Astrophysics, Abstract.
• Simon Conway Morris, 2003. Life's Solution: Inevitable Humans in a Lonely Universe. Cambridge Univ. Press.
• Sober, Elliott, 2005, "The Design Argument" in Mann, W. E., ed., The Blackwell Guide to the Philosophy of Religion. Blackwell Publishers.
• Stenger, Victor J., 1999, "Anthropic design," The Skeptical Inquirer 23 (August 31 1999): 40-43
• Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press.
• Max Tegmark, 1997, "On the dimensionality of spacetime," Classical and Quantum Gravity 14: L69-L75. A simple anthropic argument for why there are 3 spatial and 1 temporal dimensions.
• Max Tegmark, 1998, "Is `the theory of everything' merely the ultimate ensemble theory?" Annals of Physics 270: 1-51.
• Frank J. Tipler, 2003, "Intelligent Life in Cosmology," International Journal of Astrobiology 2: 141-48.
• Ward, P. D., and Brownlee, D., 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Springer Verlag.
• A Universe without Weak interactions - Roni Harnik, Graham Kribs, Gilad Perez - Phys. Rev.D74:035006,2006.

External links

• Nick Bostrom: web site devoted to the Anthropic Principle.
• Chown, Marcus, Anything Goes, New Scientist, 6 June 1998. On Max Tegmark's work.
• Stephen Hawking, Anthropic Reasoning Kavli-CERCA Conference Video Archive.
• Ikeda, Michael, and Jefferys, Bill, 2006, The Anthropic Principle Does Not Support Supernaturalism
• Tobin, Paul N., 2000, "Is the Universe Fine-Tuned for Life?" A critique of the Anthropic Principle from an atheist viewpoint.
• Steven Weinberg, 2005, Living in the multiverse
• "Anthropic Coincidence" — the anthropic controversy as a segue to Lee Smolin's theory of cosmological natural selection.
• Leonard Susskind and Lee Smolin debate the Anthropic Principle.
• debate among scientists on arxiv.org.
• Evolutionary Probability and Fine Tuning
• Critical review of "The Privileged Planet"
• Cosmologists apply anthropic reasoning different from Steven Weinberg's to universes different from ours and arrive at different(lower) "optimal" values for the cosmological constant

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