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What is the anthropic principle of cosmology?
David H. Bailey
Updated 31 March 2019 (c) 2019
Definition of the anthropic principle
The term "anthropic principle" was first introduced into the scientific literature in 1974 by Brandon Carter [Carter1974, pg. 291]. In discussing the "large number" coincidences then evident in physics and cosmology, Carter used the term to refer to the fact that our making any scientific observation is necessarily contingent on our existence. In 1986, astronomer John Barrow and astrophysicist Frank Tipler published the landmark work The Anthropic Cosmological Principle, which elaborated on the anthropic principle and the "cosmic coincidences" in considerable technical detail [Barrow1986]. Carter himself mentioned two variations of the term, and Barrow and Tipler defined three [Barrow1986]:
As a single example of the weak anthropic principle, consider the force of gravitation. It must be balanced very closely to the expansion of the universe to provide a universe that fosters intelligent beings. If it were slightly weaker, then in the early stages of the big bang, matter would have dispersed too rapidly to permit stars, much less galaxies, to form. On the other hand, if gravitation were slightly stronger, then the universe would have reached a maximum extent and then recollapsed in a big crunch long before carbon-based could have arisen. Consider for a moment the fact that carbon, and elements heavier than carbon, appear essential to any conceivable form of intelligent life -- the handful of lighter elements do not form any complex atomic structures among themselves. But carbon and all elements heavier than carbon were not formed in the big bang, but instead are formed in the explosion of first-generation stars at the end of their lifetimes. Thus scientists note, in the spirit of the weak anthropic principle, that we should not be surprised to be living in a universe that is some 13.8 billion years old, because if it were much younger, say less than about three billion years old, carbon atoms could not exist, and thus carbon-based sentient creatures like us could not exist, much less be reading articles like this one and contemplating the meaning of their existence. In other words, our very existence places stringent boundaries on several aspects of the design of our universe.
- Weak anthropic principle. 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 requirement that the universe be old enough for it to have already done so.
- Strong anthropic principle. The universe must have those properties which allow life to develop within it at some stage in its history.
- Final anthropic principle. Intelligent information-processing must come into existence in the universe, and, once it comes into existence, it will never die out.
Leading scientists on the anthropic principle
Some scientists consider the weak anthropic principle to be a virtual tautology -- a statement of little or no substance -- and dismiss the "cosmic coincidences" as, well, coincidences. They argue that the gravitation "coincidence" mentioned above is now explained by the inflationary scenario of the big bang cosmology, wherein the very early universe underwent an incredible expansion. But other leading scientists disagree with this assessment of the anthropic principle, arguing that numerous other aspects of our universe, such as the cosmological constant paradox, are so wildly improbable and so utterly inexplicable from currently known physical theories as to beg a more fundamental explanation, and they see the weak anthropic principle as salvation. (For a list of some of the more striking cosmic coincidences, see
Cosmic, and for discussion of the cosmological constant paradox, see
Cosmological constant paradox.)
Just a few of the eminent scientists who now see the weak anthropic principle as essential include the following:
The strong anthropic principle is even more interesting. This stems from some interpretations of the laws of quantum mechanics, wherein a phenomenon cannot truly be said to "exist" until it is observed. For example, a light beam passing through two slots forms an interference pattern on a screen behind the slotted medium. Strangely, this happens even when the beam is reduced to just a discrete sequence of individual photons. Yet as soon as some experimental means is used to identify which slot individual photons pass through, the interference pattern disappears. In other words, one cannot really say that an individual photon exists anywhere in the experimental setup until it is measured and/or observed. In an similar vein, a number of leading physicists suggest that a similar principle applies to the entire universe -- it cannot really be said to exist in a scientific sense, unless at some point in its history it spawns conscious observers. Many scientists are very reluctant to accept this assertion, and believe that once we have a clearer understanding of the laws of physics, such extrapolations will prove to be unfounded speculations. But others, such as these eminent scientists, disagree:
- Stephen Hawking: "[T]he Anthropic Principle is essential, if one is to pick out a solution to represent the universe, from the whole zoo of solutions allowed by M theory." [Susskind2005, pg. 353].
- Andrei Linde: "Those who dislike anthropic principles are simply in denial. This principle is not a universal weapon, but a useful tool, which allows us to concentrate on the fundamental problems of physics by separating them from the purely environmental problems, which may have an anthropic solution. One may hate the Anthropic Principle or love it, but I bet that eventually everyone is going to use it." [Susskind2005, pg. 353].
- Leonard Susskind: "The fact that [the cosmological constant] is not absent is a cataclysm for physicists, and the only way that we know how to make any sense of it is through the reviled and despised Anthropic Principle." [Susskind2005, pg. 22].
The final anthropic principle, the most speculative form of the anthropic principle, goes even further than the strong anthropic principle. Some scientists have speculated that in the distant future mankind (or mankind joined with other sentient civilizations throughout the universe) will form a kind of supermind that will in some sense unite with the universe, forming a god-like entity. As researcher David Deutsch describes it [Williams2006]:
- Christian de Duve: "My reasons for seeing the universe as meaningful lie in what I perceive as its built-in necessities. Monod stressed the improbability of life and mind and the preponderant role of chance in their emergence, hence the lack of design in the universe, hence its absurdity and pointlessness. My reading of the same facts is different. It gives chance the same role, but acting within such a stringent set of constraints as to produce life and mind obligatorily, not once but many times. To Monod's famous sentence 'The universe was not pregnant with life, nor the bisosphere with man,' I reply: 'You are wrong. They were.'" [DeDuve1995, pg. 300].
- Freeman Dyson: "As we look out into the universe and identify the many accidents of physics and astronomy that have worked together to our benefit, it almost seems as if the universe must in some sense have known we were coming." [Dyson1979, pg. 250].
- Stuart Kauffman: "We are at home in the universe." [Kauffman1995].
- Andre Linde: "The universe and the observer exist as a pair. ... The moment you say that the universe exists without any observers, I cannot make any sense out of that. I cannot imagine a consistent theory of everything that ignores consciousness. A recording device cannot play the role of an observer, because who will read what is written on this recording device? In order for us to see that something happens, and say to one another that something happens, you need to have a universe, you need to have a recording device, and you need to have us. It's not enough for the information to be stored somewhere, completely inaccessible to anybody. It's necessary for somebody to look at it. You need an observer who looks at the universe. In the absence of observers, our universe is dead." [Folger2002; see also Linde2004, pg. 449].
- Simon Conway Morris: "One always worries in science that one's being too narrow, one always worries that one's not been imaginative enough even in the case of evolutionary convergence say look N equals 1, we only have one earth and maybe there are initial conditions which in a sense predispose everything in one set of inevitable directions. I don't actually think that is the case because really the building blocks of the universe are things like carbon and various other elements and in point of fact it maybe even from that stage, which of course then leads us to the Big Bang itself, there is, if you like, seeded into the initiation of the universe itself the inevitability of intelligence." [ConwayMorris2005].
In the final anthropic principle or if anything like an infinite amount of computation taking place is going to be true, which I think is highly plausible one way or another, then the universe is heading towards something that might be called omniscience. Although there never will be a moment of omniscience, at any one moment we'll have infinitely less knowledge than omniscience. But yes, there's something like that, the concept that we've found that is most like a religious concept is providence. The fine-tuning of the universe, whatever it's due to, is very like providence. But again, the role that this providence plays in physics is very different from the role that religious providence plays in religion, because in religion providence is supposed to be an explanation for why things are as they are. And that's no good, because you've got to explain why providence did this and it just makes matters worse not better. In thinking about fine-tuning and trying to explain it, what we're looking for is something that explains the fine-tuning. In other words, providence there is not a proposed solution, it's an interesting problem, which is going to be explained by something else, if at all.
The latest physics findings, the multiverse, cosmic coincidences and the anthropic principle are nicely summarized in a feature article on the Simons Foundation news site [Wolchover2013].
The anthropic principle and Judeo-Christian theology
From the beginning, the significance of these discussions has not been lost on theologians and theologically-minded scientists. George Greenstein is one of the many who have commented [Greenstein1988; Susskind2005, pg. 8]:
As we survey all the evidence, the thought insistently arises that some supernatural agency, or rather Agency, must be involved. Is it possible that suddenly, without intending to, we have stumbled upon scientific proof of the existence of a supreme being? Was it God who stepped in and so providentially created the cosmos for our benefit?
Similarly, British theologian Keith Ward has explained [Williams2006]:
The sort of religious view which is very compatible with modern science is a vast cosmic evolution of the finite towards the infinite: the growth of consciousness from the unconscious; the growth of intelligence from that which has no intelligence. So that indeed it seems to make sense if you're looking at it in the most broad terms, to see the universe as a finite reflection of the intelligence of ultimate mind, a reflection which becomes one with its source.
In short, the anthropic principle of cosmology has emerged as the centerpiece of an intense debate among leading physicists, astronomers, cosmologists and theologians, as to the fundamental meaning and ultimate fate of the universe. While many are eager to see the current debates as a "solution" to the age-old debate between science and religion, clearly considerable caution is in order. More than once, both theologians and scientists have been captivated by some development, only later to see it succumb to a more prosaic explanation. But it will be interesting to see how all of this plays out.
For additional discussion, see