The anthropic principle in its most basic form states a truism: that any valid theory of the universe must be consistent with our existence as carbon-based human beings at this particular time and place in the universe. Attempts to apply this principle to develop scientific explanations in cosmology have led to some confusion and much controversy.
The term "anthropic principle" was first proposed in 1973 by Brandon Carter during the celebration of Copernicus’ 500th birthday, as if to proclaim that humanity does hold a special place in the universe after all.1
Proponents and versions
Proponents of the anthropic principle suggest that we live in a fine-tuned universe, i.e. a universe that appears to be "fine-tuned" to allow the existence of life as we know it. If any of the basic physical constants were different, then life as we know it would not be possible. Papers have been written arguing that the anthropic principle would explain the physical constants such as the fine structure constant, the number of dimensions in the universe, and the cosmological constant. Proponents also point out that these constants do not have "obvious" values. The universe we observe must be suitable for the development of intelligent life, for otherwise we could not be here to observe it.
The three primary versions of the principle, as stated by Barrow and Tipler (1986), are:
- 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."
- Strong Anthropic Principle (SAP): "The Universe must have those properties which allow life to develop within it at some stage in its history."
- 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."
The weak version has been criticized as an argument by lack of imagination for assuming no other forms of life are possible (see also carbon chauvinism). Furthermore, the range of constants allowing evolution of carbon-based life may be much less restricted than proposed (Stenger, "Timeless Reality"). The strong version is also criticized as being neither testable nor falsifiable, and unnecessary. The final version is discussed in more detail under Final Anthropic Principle; Barrow and Tipler state that, although it is a physical statement, it is nevertheless "closely connected with moral values".
Proponents of the intelligent design conjecture assert support from the anthropic principle. On the other hand, the existence of alternate universes is suggested for other reasons and the anthropic principle provides additional support for their existence. Assuming some possible universe would be capable of supporting intelligent life, some actual universes must do so, and ours clearly is one of those.
The Anthropic Cosmological Principle
In 1986, the controversial book The Anthropic Cosmological Principle by John D. Barrow and Frank J. Tipler (Oxford University Press) was published. In this book Barrow, a cosmological scientist, pioneered what he called the anthropic principle in order to deal with the seemingly incredible coincidences that allow for our presence in a universe that appears to be perfectly set up for our existence. Everything from the particular energy state of the electron to the exact level of the weak nuclear force seems to be tailored for us to exist. We appear to live in a universe dependent on several independent variables where only a slight change would render it inhospitable for any form of life. And yet, here we are. The anthropic principle states that the reason we are here to ponder this question at all, is due to the fact that all the correct variables are in place. According to critics, this is simply a very elaborate way of saying 'if things were different, they would be different'.
Brandon Carter presented his ideas about the anthropic principle in a 1974 publication of the International Astronomical Union. Later, in 1983, he claimed that, in its original form, the principle was meant only to caution astrophysicists and cosmologists of possible errors in the interpretation of astronomical and cosmological data unless the biological constraints of the observer were taken into account. In 1983 he also included the warning that the inverse was true for evolutionary biologists; Carter claimed that in interpreting the evolutionary record, one must take into account the astrophysical restraints of the process. Working with this in mind, Carter concluded that the evolutionary chain probably could include only one or two highly improbable links given the available time interval. A. Feoli and S. Rampone ("Is the Strong Anthropic Principle Too Weak," 1999) argued that the estimated size of our universe and number of planets allows a higher bound, indicating no evidence for intelligent design in evolution.
There was renewed scientific interest in the anthropic principle in the late-1990s motivated by observational cosmology and theoretical work in quantum gravity. The theoretical work involved attempting to unify gravity with the other forces. While there were a number of promising developments, they all seemed to suffer from the problem that the fundamental physical constants seemed to be unconstrained. The observational motivation came from cosmological observations which gave firm values for quantities such as the matter density of the universe. Contrary to expectations, the value was not zero, but 0.7, which is a non-obvious value.
Recent publications (2004) by Stephen Hawking suggest 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 universe of a type as ours will come from a Big Bang. Further, using the basic wavefunction of the universe as basis, Hawking's equations indicate that such a universe can come into existence without relation to anything prior to it, meaning that it could come out of nothing. In 2004, these publications and the theories in them are still subject to scientific debate.
Anthropic bias and anthropic reasoning
In 2002, Nick Bostrom asked "Is it possible to sum up the essence of observation selection effect s 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.
- Kane, Gordon L., Malcolm J. Perry, and Anna N. Zytkow, "The Beginning of the End of the Anthropic Principle". (arxiv.org)
- Anthropic Reasoning, Stephen Hawking Kavli-CERCA Conference Video Archive
¹ The principle had, however, been invoked before then, e.g. in 1957, R.H. Dicke wrote: '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.' (R.H. Dicke, Principle of Equivalence and Weak Interactions, Rev.Mod.Phys. 29, 355 (1957).) Even earlier statements of the principle may be found in Alfred Russel Wallace's book Man's Place in the Universe, which was first published in 1903. For example: "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." (pp. 256-7 in the 1912 edition).