In physics and cosmology, the anthropic principle encompasses diverse explanations about the structure of the universe that open the question of whether it exists with the purpose to permit the emergence of human life. It regards as significant the surprising coincidence of physical features that are—or at least seem to be—necessary and relevant to the existence on Earth of biochemistry, carbon-based life, and eventually human beings to observe such a universe. It has led some to a reconsideration of the centrality of human beings—who can observe and understand the universe—despite astronomers having long ago pushed humans to the edge of insignificance amidst the vastness of space.
The ultimate question here is whether there is or was some special intention or plan for the appearance of humans in the universe, and, if so, was there or is there some intending entity or intelligence or being or "creator" existing "behind" or "over" the universe and the particular qualities that occur within it. As M.A. Corey put it:
... the chief question now centers around whether humans in particular were deliberately intended from the very beginning. This is a difficult question that strikes at the very heart of the human dilemma, for no one seems to know who we really are or what our position actually is in the cosmos. (God and the New Cosmology, 224)
If the universe or cosmos is purely mechanistic, consisting only of matter and physical entities (forces, energy, fields, etc.), then it seems that the answer to that question of an intending entity or intelligence or creator would be "no." But then what is the source of those closely balanced features that are observed in the existing cosmos—are they just happenstance or fortuitous coincidences? Can coincidence or lucky happenstance be a sufficient answer to this problem?
There are many versions of the anthropic principle. At least one source has suggested that there may be as many as 30 of them. But they are all based on observation—or supposed observation anyway, because the question of the status and observational accuracy and neutrality of the observer is one of the points of discussion—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 something more, such as some kind of design or purpose or intention or teleology in the universe, is a central question of investigation and controversy.
The common (or "weak") form of the anthropic principle is a truism that begins with the observation that the universe appears surprisingly hospitable to the emergence of life—particularly complex, multicellular life—that has the ability make such an observation, and concludes that in only such a fine-tuned universe can such living observers exist. If one accepts the Big Bang theory and 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 biological entities is unexpected by any normal model of turbulence-driven structuring that science has so far been able to derive.
The first person to employ the phrase "anthropic principle" was the theoretical astrophysicist Brandon Carter, in his contribution to a 1973 symposium titled Confrontation of Cosmological Theories with Observational Data honoring Copernicus' 500th birthday. Carter articulated the anthropic principle as an ecological correction of what is now called the "cosmological principle."
Copernicus had argued that the Earth is not the center of the Solar System, but Carter noted that pure cosmological extensions of this idea led to "anticentrist dogma." 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’s anthropic principle was a development of earlier ideas. In 1957, Robert H. Dicke wrote 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."  Biological factors require that the age of the universe has to be roughly this "golden-age." If the universe were much younger, there would not have been time for sufficient interstellar levels of carbon to build up by nucleosynthesis; if it were much older, the golden age of main-sequence stars and stable planetary systems would have already ended.
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." Also, in the 1940s, an anthropic principle was formulated in relation to relativistic cosmology and physics constants by Abraham Zelmanov, a prominent scientist in the General Theory of Relativity and cosmology.
Anthropic coincidences  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—particularly, complex, multicellular, carbon-based life—at a specific time and "location"  in the history of the universe.
The observed values of the dimensionless parameters (such as the fine-structure constant) that govern the four fundamental 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 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 a change as infinitesimally small as one part in 1040 or even smaller would be sufficient—then the universe's structure and capacity for life as we now know it would disappear. The extreme precision of these constants is seen by some commentators as precluding simple chance or coincidence.
Around 1937, Paul Dirac, Lucasian Professor of Mathematics at Cambridge University and co-winner with Erwin Schrödinger in 1933 of the Nobel Prize in physics, became interested in a number of facts about the universe that, to him, go beyond mere coincidences: (1) The age of the universe in atomic time units—a much more basic and universal unit than years calculated by how long it takes the earth to circumnavigate the sun—is 1040 atomic time units. (2) The reciprocal of the gravitational constant is 1040. (3) The number of particles in the observable universe is, within an order of magnitude, 1080, or (1040)2. Thus the figure 1040 has appeared here three times, and these three facts come from different branches of science: cosmology dealing with the age of the universe, the gravitational constant dealing with the strength of gravity, and the atomic time unit which comes from the investigation of the microcosm and the smallest units of existence. Are these just coincidences—often called "cosmic coincidences"—or is there more to it? Dirac, for one, held that there must be some deeper connection here, and he tried to reformulate cosmic coincidences as scientific necessities.
The most thorough extant study of the anthropic principle is The Anthropic Cosmological Principle (1986) 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.
Barrow and Tipler then 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 even slightly 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... 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 time (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)
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 (see cosmological arguments and the anthropic principle below. Theorists who wish to avoid this conclusion commonly argue for the existence of many alternate universes as providing 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 have argued that the Anthropic Principle is consistent with evolution.
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. This finding would be supportive of the view that the emergence of life requires intelligent design. On the other hand, 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—a finding less consistent with the claims of intelligent design.
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.".
Still another possibility is the multiple universe hypothesis proposed by John Wheeler and others that holds that there are a large number (perhaps infinite number?) of universes, unknown to and unobservable to each other, each with a different set of fundamental constants. Thus nature might have tried all the very large number (perhaps infinite number?) of possibilities over a very long (perhaps infinite?) period of time, and we just happen to be on the one that happens to have the characteristics that make human life possible. A simple way of saying it would be that with this particular universe, nature got lucky—assuming that you view the existence of human life as a good; otherwise you might say that nature got especially unlucky with this one.
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"). 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 truism or 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 labeling 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."
Stephen 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 percent 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—that is, it could have come out of "nothing." This work, however, 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?" That "there is something instead of nothing" is a fundamental problem of metaphysics.
Nick Bostrom of Oxford University has written that anthropic arguments rely on what he calls observation selection effects. An observation selection effect occurs when an observer selects an improper or unrepresentative or otherwise biased class or set of things for his observation and then draws conclusions based on that mistake. Applying this to the anthropic principle, he writes on his website:
Anthropic reasoning, which seeks to detect, diagnose, and cure such biases, is a philosophical goldmine. Few fields are so rich in empirical implications, touch on so many important scientific questions, pose such intricate paradoxes, and contain such generous quantities of conceptual and methodological confusion that need to be sorted out. Working in this area is a lot of intellectual fun.
Let’s look at an example where an observation selection effect is involved: We find that intelligent life evolved on Earth. Naively, one might think that this piece of evidence suggests that life is likely to evolve on most Earth-like planets. But that would be to overlook an observation selection effect. For no matter how small the proportion of all Earth-like planets that evolve intelligent life, we will find ourselves on a planet that did (or we will trace our origin to a planet where intelligent life evolved, in case we are born in a space colony). Our data point—that intelligent life arose on our planet—is predicted equally well by the hypothesis that intelligent life is very improbable even on Earth-like planets as by the hypothesis that intelligent life is highly probable on Earth-like planets. This datum therefore does not distinguish between the two hypotheses, provided that on both hypotheses intelligent life would have evolved somewhere. 
In 2002, he wrote:
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.
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.
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 (theory) 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.
Although one can hold to the anthropic principle without necessarily holding to any notion of divine origin or divine causation, at least some people have used present-day scientific observation of the cosmological coincidences and/or an observation of the existence of what they call irreducible complexity in biological structures to construct a renewed version or versions of the cosmological argument for the existence of God.
The cosmological argument has a long history, going back to the ancient Greeks, especially Plato and Aristotle, who used versions of it. It came to full prominence in the work of the medieval theologian/philosopher Thomas Aquinas, who presented it as one of his Five Ways to prove—he thought—the existence of God. But in more recent philosophy, David Hume and others have seemed to destroy the cosmological argument by bringing devastating objections against it, especially the claim that any structure will appear designed to someone existing solely within that structure.
As noted above, one such group of theorists who have resurrected the cosmological argument consists of those who hold that at least some biological structures exhibit intelligent design. If that is so, it implies the existence of an intelligent designer behind the intelligent design of those biological structures because they do not come into existence solely through physicalist mechanistic processes, such as postulated by Darwin and the various forms of Darwinist evolution, which are purely mechanistic. Many—but not all—of those who subscribe to the notion of intelligent design hold that this intelligent designer is indeed God. (See the work of William Dembski, Michael Behe, Jonathan Wells, and others on intelligent design.)
Other people appeal to the fact that the cosmic coincidences are balanced on such an extremely small knife-edge as proving that this could not have happened by chance, so, they claim, there must be God as designer and creator to bring it all about. Two of the numerous people who have argued this are Paul Davies, Professor of Mathematical Physics at the University of Adelaide in Australia, and writer Michael A. Corey, who relies on the work of Davies and others. Corey (2001) presents the physical and cosmological facts clearly and with sufficient detail so that the cosmic coincidences become quite apparent, and he also shows that infinitesimally small changes in them would render the existence the universe as we know it, and probably any universe that would last for any extended period of time, impossible. In the beginning of his book Corey promises more than he finally can deliver by claiming that he will prove the existence of God, thus leading the reader to expect a logically sound deductive proof. But, in the end, all he is able to give is an inductive argument, based on the claim that these facts make the existence of God very highly probable and the most likely or cogent explanation for them. Nevertheless it is a strong, although not logically decisive, inductive argument.
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