Anthropic principle

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In cosmology, the anthropic principle in its most basic form states the truism that any valid theory of the universe must be consistent with our existence as carbon-based human beings at this particular time and place in the universe. In other words, "If something must be true for us, as humans, to exist, then it is true simply because we exist." Attempts to apply this principle to develop scientific explanations in cosmology have led to some confusion and much controversy.

1 Origin
2 Proponents and versions
3 The Anthropic Cosmological Principle
4 Anthropic principle in string theory
5 Anthropic bias and anthropic reasoning
6 See also
7 References
8 External links
9 Footnote

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Origin

The term "anthropic principle" was first proposed in 1973 by theoretical physicist Brandon Carter in his contribution to a symposium titled "Confrontation of Cosmological Theories with Observational Data" honouring Copernicus's 500th birthday. He took this opportunity to articulate the anthropic principle as the contrary of what has come to be called the Copernican principle (which Copernicus emphatically did not articulate), which denies that the position of human beings in the cosmological order is in any way privileged. (Just as Copernicus argued that the Earth is not the centre of the universe, we now know that the sun is a typical star located in a typical galaxy.) 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" (IAUS 63 (1974) 291). <ref>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." (Dicke 1957, "Principle of Equivalence and Weak Interactions," Rev.Mod.Phys. 29: 355) Alfred Russel Wallace anticipated the WAP 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." (Wallace 1912, Man's Place in the Universe: 256-7). The WAP is also 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." (The German Ideology, chpt. 1.)</ref>

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Proponents and versions

Proponents of the anthropic principle suggest that we live in a fine-tuned universe, i.e. a universe that appears to be "fine-tuned" to allow the existence of life as we know it. 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 this fine-tuned universe we live in. Papers have been written arguing that the anthropic principle would explain the physical constants such as the fine structure constant, the number of dimensions in the universe, and the cosmological constant.

The three primary versions of the principle, as stated by John D. Barrow and Frank J. Tipler (1986), are:

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 now simplifies this definition to - conditions that are observed in the universe must allow the observer to exist.

Strong anthropic principle (SAP): "The Universe must have those properties which allow life to develop within it at some stage in its history."

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."

The weak version has been criticized as an argument by lack of imagination for assuming no other forms of life are possible (sometimes called "carbon chauvinism", see also alternative biochemistry). Furthermore, the range of constants allowing evolution of carbon-based life may be much less restricted than proposed (Stenger, "Timeless Reality"). The weak anthropic principle has also been cited by both critics and supporters as a tautology, stating something trivially true, but not readily obvious.

The strong version is also criticized as being neither testable nor falsifiable, and unnecessary. The final version is discussed in more detail under final anthropic principle; Barrow and Tipler state that, although it is a physical statement, it is nevertheless "closely connected with moral values".

Proponents of intelligent design assert support from the anthropic principle. On the other hand, the existence of the multiverse or alternate universes is hypothesized for other reasons and the anthropic principle provides a plausible explanation for the fine tuning of our universe. Assuming some possible universe would be capable of supporting intelligent life, some actual universes must do so, and ours clearly is one of those. However, alternatives of the intelligent design conjecture are not limited to proposing the existence of alternate universes. Also, it has been argued that the anthropic principle as conventionally stated actually undermines the intelligent design conjecture (discussed in more detail under fine tuning). In other words, proponents of evolution also assert support from the anthropic principle.

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The Anthropic Cosmological Principle

A very detailed study of the anthropic principle can be found in the controversial book The Anthropic Cosmological Principle by John D. Barrow, a cosmologist, and Frank J. Tipler, a mathematical physicist. They invoke the anthropic principle in order to explain the seemingly incredible coincidences that characterize our universe and that permit human beings to evolve in it. 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. The anthropic principle implies that our ability to ponder cosmology at all is contingent on all parameters having numerical values falling in a quite narrow range. Critics reply that this is simply tautological reasoning, a 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.

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 ("Is the Strong Anthropic Principle Too Weak," 1999) 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.

Some alternatives to the anthropic principle do exist, 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.

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 (a mathematization of physics some argue is not clearly understood even by its inventor, Schrodinger) 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.

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Anthropic principle in string theory

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String theory predicts a large number of possible universes, called the backgrounds or vacua. The set of these of vacua is often called the anthropic landscape or string landscape. Some physicists, such as Leonard Susskind, argue that the existence of a large number of vacua puts the anthropic reasoning on firm ground. Others, most notably David Gross but also Lubos Motl and Peter Woit, argue that this is not predictive. Steven Weinberg described the anthropic principle as a "turning point" in the history of science in his 2005 paper on the string landscape, "Living in the multiverse".

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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, in spite of the fact that 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.

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References

John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press. ISBN 0192821474
Nick Bostrom, 2002. Anthropic Bias: Observation Selection Effects in Science and Philosophy. Routledge. ISBN 0415938589
Simon Conway Morris, 2003. Life's Solution: Inevitable Humans in a Lonely Universe. Cambridge Univ. Press.
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Ward, P. D., and Brownlee, D., 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Springer Verlag.

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External links

{{cite journal

| author = Kane, Gordon L., Malcolm J. Perry, and Anna N. Zytkow
| title=The Beginning of the End of the Anthropic Principle
| journal = New Astron. 
| volume = 7 
| year = 2002
| pages = 45–53
| id=Template:Arxiv
}}

debate among scientists on arxiv.org

Anthropic Reasoning, Stephen Hawking Kavli-CERCA Conference Video Archive

{{cite journal

| author = A. Feoli, and S. Rampone
| id = Template:Arxiv
| title = Is the Strong Anthropic Principle too weak?
|journal = Nuovo Cim.
| volume = B114
|  year = 1999
| pages = 281–289
}}