A suggestion, due originally to Fred Hoyle in the 1950s, that humans exist because physical laws governing the universe exhibit special features. For example, if gravitation were stronger relative to electromagnetism than it actually is, stars such as our Sun would burn out too quickly for life to evolve nearby. If the strong nuclear force were slightly weaker relative to electromagnetism, the only stable element would be hydrogen. Some propound the idea of multiple universes which may or may not be stable. Others hold a stronger version of the principle, that a direct link exists between human existence and the actual form of the laws of nature. The anthropic principle is difficult to reconcile with unified theories in physics, which attempt a complete specification of the laws of nature.
In physics and cosmology, the anthropic principle is an umbrella term for various dissimilar attempts to explain the structure of the universe by way of coincidentally balanced features that are necessary and relevant to the existence of carbon-based life. The idea evolved from "Dicke's Coincidence", and has subsequently been reinforced by the discovery of many more anthropic coincidences since Robert Dicke first noted that the evolution of the universe is not random, but is coincidentally constrained by biological factors that require that the age of the universe had to be roughly the "golden-age" .
More anthropic coincidences
The relevant 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 at a specific time and "location" in the history of the universe, particularly, complex multicellular carbon-based life.
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 been able to derive. It is therefore possible to use anthropic reasoning to make testable predictions about the features of the observed universe from the average of extreme opposing runaway tendencies that are known to be inherent to the anthropic coincidences, as well as to choose from a multiverse of different configurations, in lieu of a more viable stability mechanism.
Carbon-based life is either a likely and thus necessary outcome of the observed structure and stability of the universe, or it is a part of an engine of creation that favors such outcomes.
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.
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. 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. Dicke wrote in 1957 that: "The age of the Universe 'now' is not random but conditioned by biological factors ... 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 ... 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".
Variants of the anthropic principle
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:
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 defines WAP as conditions that are observed in the universe must allow the observer to exist.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.
Strong anthropic principle (SAP): "The Universe must have those properties which allow life to develop within it at some stage in its history." It implies that the purpose of the
universe is to give rise to intelligent life, with the laws of nature and their fundamental constants set to ensure that life as we know it will emerge and evolve. 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."
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:
Completely ridiculous anthropic principle (CRAP): "At the instant the Omega Point is reached, 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. And this is the end."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. 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.
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 and Tipler set out in great detail the seemingly incredible coincidences that characterize our universe and that permit human beings to evolve in it. 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.
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.
Recent work in observational cosmology and the theory of quantum gravity has led to renewed interest in the anthropic principle.
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. 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.
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). 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.
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.
Hawking (2004) suggests that our universe is much less 'special' than the proponents of the anthropic principle claim it is.
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." 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?"
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?" 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 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. 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". 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.
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