Anthropic principle


In cosmology and philosophy of science, the anthropic principle, also known as the observation selection effect, is the proposition that the range of possible observations that could be made about the universe is limited by the fact that observations are only possible in the type of universe that is capable of developing observers in the first place. Proponents of the anthropic principle argue that it explains why the universe has the age and the fundamental physical constants necessary to accommodate intelligent life. If either had been significantly different, no one would have been around to make observations. Anthropic reasoning has been used to address the question as to why certain measured physical constants take the values that they do, rather than some other arbitrary values, and to explain a perception that the universe appears to be finely tuned for the existence of life.
There are many different formulations of the anthropic principle. Philosopher Nick Bostrom counts thirty, but the underlying principles can be divided into "weak" and "strong" forms, depending on the types of cosmological claims they entail.

Definition and basis

The principle was formulated as a response to a series of observations that the laws of nature and parameters of the universe have values that are consistent with conditions for life as it is known rather than values that would not be consistent with life on Earth. The anthropic principle states that this is an a posteriori necessity, because if life were impossible, no living entity would be there to observe it, and thus it would not be known. That is, it must be possible to observe some universe, and hence, the laws and constants of any such universe must accommodate that possibility.
The term anthropic in "anthropic principle" has been argued to be a misnomer. While singling out the currently observable kind of carbon-based life, none of the finely tuned phenomena require human life or some kind of carbon chauvinism. Any form of life or any form of heavy atom, stone, star, or galaxy would do; nothing specifically human or anthropic is involved.
The anthropic principle has given rise to some confusion and controversy, partly because the phrase has been applied to several distinct ideas. All versions of the principle have been accused of discouraging the search for a deeper physical understanding of the universe. Critics of the weak anthropic principle point out that its lack of falsifiability entails that it is non-scientific and therefore inherently not useful. Stronger variants of the anthropic principle which are not tautologies can still make claims considered controversial by some; these would be contingent upon empirical verification.

Anthropic observations

In 1961, Robert Dicke noted that the age of the universe, as seen by living observers, cannot be random. Instead, biological factors constrain the universe to be more or less in a "golden age", neither too young nor too old. If the universe was one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of metallicity especially carbon, by nucleosynthesis. Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the main sequence and would have turned into white dwarfs, aside from the dimmest red dwarfs, and stable planetary systems would have already come to an end. Thus, Dicke explained the coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence that inspired Dirac's varying-G theory.
Dicke later reasoned that the density of matter in the universe must be almost exactly the critical density needed to prevent the Big Crunch. The most recent measurements may suggest that the observed density of baryonic matter, and some theoretical predictions of the amount of dark matter, account for about 30% of this critical density, with the rest contributed by a cosmological constant. Steven Weinberg gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than the value particle physics predicts. However, if the cosmological constant were only several orders of magnitude larger than its observed value, the universe would suffer catastrophic inflation, which would preclude the formation of stars, and hence life.
The observed values of the dimensionless physical constants governing the four fundamental interactions are balanced as if fine-tuned to permit the formation of commonly found matter and subsequently the emergence of life. A slight increase in the strong interaction would bind the dineutron and the diproton and convert all hydrogen in the early universe to helium; likewise, an increase in the weak interaction also would convert all hydrogen to helium. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as it is known, would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.

Origin

The phrase "anthropic principle" first appeared in Brandon Carter's contribution to a 1973 Kraków symposium. Carter, a theoretical astrophysicist, articulated the anthropic principle in reaction to the Copernican principle, which states that humans do not occupy a privileged position in the Universe. Carter said: "Although our situation is not necessarily central, it is inevitably privileged to some extent." Specifically, Carter disagreed with using the Copernican principle to justify the Perfect Cosmological Principle, which states that all large regions and times in the universe must be statistically identical. The latter principle underlies the steady-state theory, which had recently been falsified by the 1965 discovery of the cosmic microwave background radiation. This discovery was unequivocal evidence that the universe has changed radically over time.
Carter defined two forms of the anthropic principle, a "weak" one which referred only to anthropic selection of privileged spacetime locations in the universe, and a more controversial "strong" form that addressed the values of the fundamental constants of physics.
Roger Penrose explained the weak form as follows:
One reason this is plausible, is that there are many other places and times in which humans could have evolved. But when applying the strong principle, there is only one universe, with one set of fundamental parameters. Thus, Carter offers two possibilities: First, humans can use their own existence to make "predictions" about the parameters. But second, "as a last resort", humans can convert these predictions into explanations by assuming that there is more than one universe, in fact a large and possibly infinite collection of universes, something that is now called the multiverse, in which the parameters vary across universes. The strong principle then becomes an example of a selection effect, analogous to the weak principle. Postulating a multiverse is a radical step that could provide at least a partial insight, seemingly out of the reach of normal science, regarding why the fundamental laws of physics take the particular form we observe and not another.
Since Carter's 1973 paper, the term anthropic principle has been extended to cover a number of ideas that differ in important ways from his. Particular confusion was caused by the 1986 book The Anthropic Cosmological Principle by John D. Barrow and Frank Tipler, which distinguished between a "weak" and "strong" anthropic principle in a way different from Carter's, as discussed in the next section.
Carter was not the first to invoke some form of the anthropic principle. The evolutionary biologist Alfred Russel Wallace anticipated the anthropic principle as long ago as 1904: "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." In 1957, Robert Dicke wrote: "The age of the Universe 'now' is not random but conditioned by biological factors would preclude the existence of man to consider the problem."
Ludwig Boltzmann may have been one of the first in modern science to use anthropic reasoning. Prior to knowledge of the Big Bang, Boltzmann's thermodynamic concepts painted a picture of a universe that had inexplicably low entropy. Boltzmann suggested several explanations, one of which relied on fluctuations that could produce pockets of low entropy or Boltzmann universes. While most of the universe is featureless in this model, to Boltzmann, it is unremarkable that humanity happens to inhabit a Boltzmann universe, as that is the only place that could develop and support intelligent life.

Variants

According to Brandon Carter, the weak anthropic principle states that "... our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers." For Carter, "location" refers to our location in time space. Carter goes on to define the strong anthropic principle as the idea that:
The Latin tag makes it clear that "must" indicates a deduction from the fact of our existence; the statement is thus a truism.
In their 1986 book, The anthropic cosmological principle, John Barrow and Frank Tipler depart from Carter and define the WAP and SAP differently. According to Barrow and Tipler, the WAP is the idea that:
Unlike Carter, they restrict the principle to "carbon-based life" rather than just "observers". A more important difference is that they apply the WAP to the fundamental physical constants, such as the fine-structure constant, the [|number of spacetime dimensions], and the cosmological constant—topics that fall under Carter's SAP.
According to Barrow and Tipler, the SAP states that "the Universe must have those properties which allow life to develop within it at some stage in its history." While this looks very similar to Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:
  • "There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers'." This can be seen as simply the classic design argument restated in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental physical constants set to ensure that life emerges and evolves.
  • "Observers are necessary to bring the Universe into being." Barrow and Tipler believe that this is a valid conclusion from quantum mechanics, as John Archibald Wheeler has suggested, especially via his idea that information is the fundamental reality and his participatory anthropic principle which is an interpretation of quantum mechanics associated with the ideas of Eugene Wigner.
  • "An ensemble of other different universes is necessary for the existence of our Universe." By contrast, Carter merely says that an ensemble of universes is necessary for the SAP to count as an explanation.
Philosophers John Leslie and Nick Bostrom reject the Barrow and Tipler SAP as a fundamental misreading of Carter. For Bostrom, Carter's anthropic principle just warns us to make allowance for "anthropic bias"—that is, the bias created by anthropic selection effects —the necessity for observers to exist in order to get a result. He writes:
Bostrom defines a concept called the "strong self-sampling assumption", the idea that "each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its reference class." Analyzing an observer's experience into a sequence of "observer-moments" like this helps avoid certain paradoxes, but the main ambiguity is the selection of the appropriate "reference class". For Carter's WAP, this might correspond to all real or potential observer-moments in our universe. As for his SAP, this might correspond to all in the multiverse. Bostrom's mathematical development shows that choosing too broad or too narrow a reference class leads to counter-intuitive results, but he is not able to prescribe an ideal choice.
According to Jürgen Schmidhuber, the anthropic principle essentially just says that the conditional probability of finding yourself in a universe compatible with your existence is always one. It does not allow for any additional nontrivial predictions such as "gravity won't change tomorrow". To gain more predictive power, additional assumptions on the prior distribution of alternative universes are necessary.
Playwright and novelist Michael Frayn describes a form of the strong anthropic principle in his 2006 book The Human Touch, which explores what he characterises as "the central oddity of the Universe":