Originally proposed as a rule of reasoning, the term has since been extended to cover supposed "superlaws" that in various ways require the universe to support intelligent life, usually assumed to be carbon-based, and occasionally to be specifically human beings. Anthropic reasoning involves assessing these constraints by analyzing the properties of universes with different fundamental parameters or laws of physics from the current one, and has frequently concluded that essential structures, from atomic nuclei to the whole universe, depend, for stability, on delicate balances between different fundamental forces; balances which occur only in a small minority of possible universes — so that ours seems to be fine-tuned for life. Anthropic reasoning also attempts to explain and quantify this fine tuning. Within the scientific community the usual approach is to invoke selection effects from a real ensemble of alternate universes, which cause an anthropic bias in what can be observed.
The anthropic principle has led to more than a little confusion and controversy, partly because several distinct ideas carry this label. All versions of the principle have been accused of providing simplistic explanations which undermine the search for a deeper physical understanding of the universe. The invocation of either multiple universes or an intelligent designer are highly controversial, and both ideas have received criticism for being untestable and therefore outside the purview of contemporary science.
Robert Dicke noted that the age of the universe as seen by living observers is not random, but is constrained by biological factors that require it to be roughly a "golden age". Ten times younger, and there would not have been time for sufficient interstellar levels of carbon to build up by nucleosynthesis; but ten times older, and the golden age of main sequence stars and stable planetary systems would have already come to an end. This explained away a rough coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, which had inspired Dirac's varying-G theory.
Later, Dicke reasoned that the density of matter in the universe must be almost exactly the critical density needed to stop the universe from recollapsing (the "Dicke coincidences" argument, see article on Robert Dicke). It seems he was wrong: latest estimates are that matter has about 30% of the critical density, with the rest contributed by a cosmological constant. Steven Weinberg gave an anthropic explanation: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than expected from particle physics (often described as the worst prediction in physics ). However, if the cosmological constant were more than about 10 times the observed value, the universe would suffer catastrophic inflation, preventing the formation of stars, and, presumably, life.
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. A list of cosmological, chemical and physical "anthropic coincidences" is given by Hugh Ross.
One of the best known examples of anthropic reasoning used in the prediction of cosmological phenomena was by Fred Hoyle. He calculated and then reasoned that there must be an excited state at an energy of 7.6 million electron volts in the nucleus of carbon-12 since if he, Fred Hoyle, a life form based upon carbon molecules, existed, then the resonance must also exist to create the carbon.
Carter defined two forms of the Anthropic principle, a "weak" one which referred only to anthropic selection of privileged space-time locations in the universe, and a more controversial "strong" form which referred to the fundamental parameters of physics.
Roger Penrose explains the weak form:
One reason this is plausible is that there are plenty of other places and other times in which we can imagine finding ourselves. But when applying the strong principle, we only have one Universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: first, we can use our own existence to make "predictions" about the parameters. But second, "as a last resort", we can convert these predictions into explanations by assuming that there is more than one Universe, in fact a large or infinite collection of universes; what is now called a multiverse ("world ensemble" is Carter's term), in which the parameters (and perhaps the laws of physics) do vary from universe to universe. The strong principle then becomes an example of a selection effect, exactly analogous to the weak principle. Postulating a multiverse is certainly a radical step, but as a pay-off it offers at least a partial answer to a question which seems to be out of the reach of normal science: "why do the fundamental laws take that particular form and not another?"
Since Carter's original paper, the term "Anthropic Principle" has been extended to cover a number of ideas which are different in important ways from the ones he espoused. Particular confusion was caused by the influential book The Anthropic Cosmological Principle by John D. Barrow and Frank Tipler, which makes a very different distinction between "weak" and "strong" anthropic principle, as discussed in the next section.
Brandon Carter was not the first to invoke some form of the anthropic principle. For instance, 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 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.
Quite different definitions of these terms were offered by Barrow, and variants of these were used in his book with Tipler:
The first of these has of course been welcomed by proponents of intelligent design. Carter has protested that such teleological readings "are quite different from, and even contradictory with, what I intended". The Barrow and Tipler SAP has also been rejected as a fundamental misreading of Carter by the philosophers John Leslie and Nick Bostrom. For Bostrom, Carter's anthropic principle just warns us to make allowance for anthropic bias, that is, the bias created by anthropic selection effects (called "observation" selection effects by Bostrom) — the necessity for observers to exist in order to get a result. He writes:
But how seriously can we take the multiverse? And which specific multiverse should we assume? — this question must be answered before any quantitative anthropic predictions can be made. Although philosophers have discussed related concepts for centuries, in the early 1970s the only genuine physical theory giving a multiverse of sorts was the many worlds interpretation of quantum mechanics. This would allow variation in initial conditions, but not in truly fundamental constants. Since that time a number of mechanisms for producing a multiverse have been suggested: see the review by Max Tegmark, and the multiverse (science) article. An important development in the 1980s was the combination of inflation theory with the idea that some parameters are determined by symmetry breaking in the early universe, which allows parameters previously thought of as "fundamental constants" to vary over very large distances, eroding the distinction between Carter's weak and strong principles. At the beginning of the 21st century, the concept of the string landscape gave a mechanism for varying essentially all the constants, including the number of spatial dimensions.
The anthropic idea that fundamental parameters are selected from a multitude of different possibilities (each actual in some universe or other) contrasts with the traditional hope of physicists for a theory of everything with no free parameters: as Einstein said, "What really interests me is whether God had any choice in the creation of the world". Quite recently, proponents of the leading candidate "theory of everything", String theory, proclaimed "the end of the anthropic principle since there would be no free parameters to select. Ironically, string theory now seems to offer no hope of predicting fundamental parameters, and some of its exponents have resorted to invoking the anthropic principle (see below).
Opponents of intelligent design are not limited to hypothesizing the existence of alternate universes: they may argue anti-anthropically that the universe is less fine-tuned than often claimed, or that accepting fine tuning as a brute fact is less astonishing than the idea of an intelligent creator. Furthermore, even accepting fine tuning, Sober (2005) and Ikeda and Jefferys, argue that the Anthropic Principle as conventionally stated actually undermines intelligent design. This is discussed in more detail in fine-tuned universe.
Paul Davies has discussed fine-tuning at length, and in his book The Goldilocks Enigma (2006) he summarises the current state of the debate in detail. He concludes by enumerating the alternative responses:
Omitted here 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).
Clearly all of these resolve some aspects of the puzzle, and leave other questions unanswered. Followers of Carter would allow only option 3 as an anthropic explanation, whereas 3 through 6 are covered by different versions of Barrow and Tipler's SAP (and also 7 if considered a variant of 4).
The anthropic principle, at least as conceived by Carter, can be applied on scales much smaller than the whole universe. For instance Carter (1983) inverted the usual line of reasoning and pointed out that 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, the evolutionary chain probably can allow only one or two low probability links. This conclusion has been disputed by Feoli and Rampone. They argued that the estimated size of our universe and number of planets allows a higher bound, also indicating no evidence for intelligent design in evolution.
John Leslie makes a number of predictions from the point of view of the Carter SAP (with multiverse):
Hogan has emphasised that it would be very strange if all fundamental constants were strictly determined, since this would leave us with no ready explanation for apparent fine tuning. In fact we might have to resort to something like Barrow and Tipler's SAP: there would be no option for such a universe not to support life.
Probabilistic predictions of parameter values can be made given (i) a particular multiverse with a "measure", i.e. a well defined "density of universes" (so, for parameter X, one can calculate the prior probability P(X0) dX that X is in the range X0 < X < X0 + dX), and (ii) an estimate of the number of observers in each universe, N(X) (e.g. this might be taken as proportional to the number of stars). The probability of observing value X is then proportional to N(X) P(X). (A more sophisticated analysis is offered by Nick Bostrom). A generic feature is that the expected values should not be "over tuned", i.e. if there is some perfectly tuned value (e.g. zero) we don't expect to be much closer to it than needed to allow life. The small but (apparently) finite value of the cosmological constant is often regarded as a successful prediction in this sense.
One thing that would not count as evidence for the Anthropic principle is anti-Copernican evidence that the Earth or the Solar System were literally in a special position in the universe (for possible hints of this see Copernican principle), unless there was some reason to think that said position was a necessary condition for our existence as observers.
The book begins with an extensive review of many topics in the history of ideas the authors deem relevant to the anthropic principle, because the authors believe that principle has important antecedents in the notions of intelligent design, the writings of Fichte, Hegel, Bergson, and Alfred North 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).
Seeing little sense in a principle requiring intelligent life to emerge while remaining indifferent to the possibility of its eventual extinction, Barrow and Tipler propose the:
Barrow and Tipler submit that the FAP is both a valid physical statement and "closely connected with moral values." FAP places strong constraints on the structure of the universe, constraints developed further in Tipler's The Physics of Immortality. One such constraint is that the universe must end in a big crunch, which seems unlikely in view of the tentative conclusions drawn since 1998 about dark energy, based on observations of very distant supernovas.
In his review of Barrow and Tipler, Martin Gardner ridiculed the FAP by quoting the last two sentences of their book as defining a Completely ridiculous anthropic principle (CRAP):
In fairness to the authors, they state at the outset that they are not necessarily committed to the ideas they describe, and admit that the SAP and FAP are "quite speculative".
A common criticism of Carter's SAP is that it is an easy deus ex machina which discourages searches for physical explanations. To quote Penrose again: "it tends to be invoked by theorists whenever they do not have a good enough theory to explain the observed facts.
Some applications 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. For instance, Harnik et al. propose a Weakless Universe in which the Weak nuclear force is eliminated apparently without significant effect, provided some adjustments are made in the other forces. However, some of the fine-tuned details of our universe would rule out complex structures of any kind — stars, planets, galaxies etc — if violated.
Carter's SAP and Barrow and Tipler's WAP are truisms or tautologies, stating something not immediately obvious to everyone yet true. As such they are often criticized as an elaborate way of saying "if things were different, they would be different", which may not implicitly provide proof of alternatives, but are nonetheless valid arguments. Discussion of anthropic principles implicitly posits that our ability to ponder cosmology at all is contingent on one or more fundamental physical parameters having numerical values falling within quite a narrow range, which is not a tautology, and neither is the postulate of an actual multiverse. Moreover, working out the consequences of a change in the fundamental parameters for our existence is far from trivial, and, as we have seen, can lead to quite unexpected constraints on carbon-based life as we currently understand it, though not necessarily demonstrating that alternate forms of life are not possible under these transposed fundamental parameters.
Critics of the Barrow and Tipler SAP claim that it is neither testable nor falsifiable, and thus is not science. The same criticism has been leveled against the multiverse idea, although we have seen that proponents argue that it does make falsifiable predictions, albeit not very strong ones. A modified version of this criticism is that such calculations are in practice impossible because we understand so little about the emergence of life, especially intelligent life, that it is effectively impossible to calculate the number of observers in each universe; moreover the prior distribution of universes as a function of parameters is too easy to modify to get any desired result.
Carter himself has frequently regretted his own choice of the word anthropic as conveying the misleading impression that the principle involves humans specifically, rather than intelligent observers in general. Others have criticised the word principle as too grandiose for a relatively straightforward application of selection effects.
Steven Jay Gould , Michael Shermer and others have observed that known causes and effects seem to have been reversed in the Anthropic Principle. Dr. Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to claiming that sausages were originally made long and narrow so that they would fit modern hotdog buns, or that ships had been invented to provide homes for barnacles. These critics cite the vast store of physical and evolutionary evidence which shows that life has been fine-tuned by the universe, through natural selection, to match the conditions in which life exists. Fossil, genetic and other biological evidence abundantly supports the observation that life adapts to physics, not the other way around.
The paleophysicist Caroline Miller has said:
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 Paul 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. That the tiny patch of space from which our observable 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 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. Nobel Prize-winning physicist and atheist Steven Weinberg, believes AP may be appropriated by cosmologists committed to nontheism; he refers to the Anthropic Principle as a "turning point" in modern science since, applied to the string landscape, it "may explain how the constants of nature that we observe can take values suitable for life without being fine-tuned by a benevolent creator." Others, most notably David Gross but also Lubos Motl, Peter Woit and Lee Smolin, argue that this is not predictive. Max Tegmark, Mario Livio and Martin Rees respond that various ingredients of well-accepted theories will never be testable, and that the test of a physical theory is not that every aspect of it should be observable and/or testable, but rather that enough is observable and testable to give confidence in the theory's correctness. Jürgen Schmidhuber (2000-2002) points out that Ray Solomonoffs theory of universal inductive inference and its extensions already provide the optimal framework for maximizing this confidence, given a limited physical observation sequence and some prior distribution on the set of possible alternative explanations of the universe.