The anthropic principle begins with the observation that the universe appears surprisingly hospitable to the emergence of life. In physics and cosmology, the anthropic principle observes, even complex multicellular life, in at least one particular place and time, namely the Earth. 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 ever been able to derive. The anthropic principle is a convenient heading for physical and cosmological reasoning that takes into account the existence of a biosphere on Earth in an essential way.
The anthropic principle is in part a truism, since any valid cosmology must be consistent with the existence on Earth of biochemistry and human beings. It is the balanced nature of the evolutionary physics defining anthropic significance that stands out. Similarly, all anthropic coincidences are balanced between the extremes of a spectrum, ranging from the Earth's ecosystem, to the near-perfect balance between the strength of gravitation and the cosmological constant governing the expansion of the universe.
Attempts to invoke the "principle" to develop scientific explanations has 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 favouring any particular time and location for the appearance of carbon-based life as we know it.
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 of the anthropic principle.
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 are modified sufficiently the universe's structure and capacity for life is greatly affected.
Direct observational evidence in support of the Anthropic Principle includes the Cosmic microwave background radiation, whose anthropic relevance has only been partially "explained-away".
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:
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.
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:.
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 of the anthropic principle.
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 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". 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.
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.
Another possibility suggested is that our universe is more likely than others, like in 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."
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 D. 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.