Anthropic principle

In cosmology, the anthropic principle in its most basic form asserts the truism that any valid theory of the universe must be consistent with the existence of human beings and of organic chemistry, here and now in the universe.

In simple terms, the anthropic principle says, if a billion universes existed, with a multitude of laws of nature, then humans would only be aware of those in which humans could emerge, and (no matter how many versions of laws of nature existed) the laws of nature humans saw would only be the laws of nature of those universes in which humans as we know them could emerge. In other words, "If something must be true for us, as humans, to exist, then it is true because we exist." This is an effect known as selection bias.

Attempts to apply this principle to develop scientific explanations in cosmology have led to a little confusion and controversy.

Origin

The first known occurrence of the phrase "anthropic principle" appears to have been by 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. His article articulated the anthropic principle as the contrary of what has come to be called the Copernican principle (which Copernicus did not articulate), which denies that the situation of humans 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). 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 ... 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 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." (The German Ideology, chpt. 1.)

Proponents and versions

Proponents of the anthropic principle suggest that we live in a universe that 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.

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 universe we live in. Papers have been written arguing that the (weak) anthropic principle would explain the physical constants such as the fine structure constant, the number of dimensions in the universe, and the cosmological constant.

It is necessary to distinguish between a variety of weak, strong, final and other versions of the anthropic principle. A slight change in the wording leads to major changes in implication. The primary versions of the anthropic principle include:

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." (John D. Barrow and Frank J. Tipler, 1986)
- The Merriam-Webster Dictionary offers this definition: conditions that are observed in the universe must allow the observer to exist.

Strong anthropic principle (SAP):
- The version of SAP held by Barrow and Tipler is that "The Universe must have those properties which allow life to develop within it at some stage in its history."
- Another version of "the strong anthropic principle is simply the classic design argument dressed in the modern garb of cosmology. It implies that the production of life is part of the intent of the universe, with the laws of nature and their fundamental constants set to ensure the development of life as we know it." ("The Rejection of Pascal's Wager")

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." (Barrow and Tipler, 1986)

Martin Gardner ridicules the most extreme extension of the anthropic principle considered by Barrow and Tipler in his review of their book, with: Completely ridiculous anthropic principle (CRAP): "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."

In Carter's original definition the weak anthropic principle 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 one reason for continuing disagreements about the status of the various anthropic principles.

Proponents of intelligent design claim support from the strong anthropic principle. On the other hand, the existence of the multiverse or alternate universes is hypothesized for other reasons and the weak anthropic principle 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. Yet there are advocates of evolution who also claim support from the anthropic principle. For example, Ikeda and Jefferys (2006) argue that the anthropic principle as conventionally stated actually undermines intelligent design (discussed in more detail under fine tuning).

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 a 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 ("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.

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 (Stenger 2000).

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 anthropic principle implies that our ability to ponder cosmology at all is contingent on all 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. Another, obvious, criticism of the anthropic principle is that the direction of causality it asserts is mistaken; humans have evolved to adapt to the universe as it currently is, cosmological constants and all, and not the converse. That is, we exist because we are adapted to the physical universe; the physical universe is not adapted specifically for us.

Critics of the SAP claim that it is neither testable nor falsifiable, and thus is unnecessary. 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".

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.

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.