Podcast Teaser: The Anthropic Principle

Ever since I’ve heard of the anthropic principle (AP), many years ago, I thought that way too many smart people were endlessly going about a question akin to the old Scholastic issue of how many angels can dance on the head of a pin. In our next podcast Julia and I will take on the baffling variety of anthropic principles and comment on some of the bizarre ideas surrounding the issue (there indeed is an issue, though I don’t think the AP makes any contribution to resolve it).

Apparently, the first person to think of something like the AP (though certainly not in the modern context, and not using that term) was Alfred Russel Wallace, the co-discoverer of natural selection with Charles Darwin. He wrote:

“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.”

Wallace’s quote encapsulates what bothers many people about the AP: it seems to imply that life, and in particular human intelligence, is the pinnacle of creation, and that, by implication, there is a creator behind all the fuss. (Which of course immediately brings to mind Douglas Adams’ immortal quote: “In the beginning the Universe was created. This has made a lot of people very angry and has been widely regarded as a bad move.”) As we shall see, however, there is no necessary connection between various forms of the AP and intelligent design (ID).

The first modern author to raise AP-related issues was Robert Dicke in 1961. He noted that the current moment in the history of the universe is not a random sample of all possible historical moments, because the universe was inhospitable to life until relatively recently, and will be again in the distant future. Of course, this observation is rather trivial (a recurring problem with the AP), because it simply restates an obvious fact: life could not have been around before galaxies and planets were in a position to form, and it will not be around once the universe will be so old that stars will be too cold to support the thermodynamics of life. Duh.

It was Brandon Carter in 1973 that introduced the phrase Anthropic Principle, and who distinguished between a weak (WAP) and a strong (SAP) version. Here is the difference:

(Carter) WAP = “We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.”

(Carter) SAP = “The Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage. To paraphrase Descartes, cogito ergo mundus talis est. [I think, therefore the world is such as it is.]”

Oh boy, you know we are in trouble when physicists paraphrase philosophers! Carter’s WAP is a truism: it simply says that we have a privileged (i.e., non-random) position in the universe as observers, in virtue of the fact that there can be observers at all only in a small section of space-time. To which the proper comment is: yup. Carter’s SAP, however, is also a truism, because he meant “must” simply as a logical deduction from the fact that we exist, not that we must somehow exist. Again, nothing controversial here, and Carter later regretted the use of both the words “Anthropic” (because he meant the idea to go for any sentient self-aware life form) and “Principle” (a bit too grand a name for the restatement of a pretty obvious truth).

The real trouble arises with the 1986 book The Anthropic Cosmological Principle, by John Barrow and Frank Tipler (the latter went on to write such nonsensical books as The Physics of Christianity). They also — very unfortunately — used the terms WAP and SAP, but defined them in crucially different, and much more questionable, ways from Carter. Here it goes:

(B&T) 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.”

(B&T) SAP: “The Universe must have those properties which allow life to develop within it at some stage in its history.”

Notice here that Barrow and Tipler’s WAP is more restrictive than Carter’s, as it talks about carbon-based life forms, not just generic observers. This is often referred to as the (utterly unjustified) idea of “carbon chauvinism.” Also, they introduce talk of probability of cosmological quantities, as if we had any idea of what the respective probability distributions actually look like (we don’t). More importantly, the wording is ambiguous, because to say that the cosmological constants are “restricted” by certain “requirements” begins to smell suspiciously of intelligent design. Sure enough, it is hard not to interpret Barrow and Tipler’s SAP as a thinly disguised form of ID: why else must the universe be such that life has to develop?

But Barrow and Tipler aren’t done with the nonsense, proceeding to propose their 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.” Wow. This is very much akin to Teilhard de Chardin’s “omega principle,” a semi-Christian mystical notion aimed at providing a scientific basis for the religiously inspired idea of immortality. Do I smell Templeton prize?

Of course the FAP is what famously led to Martin Gardner’s sarcastic proposal of the 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.”

Ok, assuming you are not already hopelessly confused (for which state you would have plenty of good reasons), we got one more, John Wheeler’s Participatory Anthropic Principle (PAP): “Observers are necessary to bring the Universe into being.” Here we have moved from tautology (Carter) through wishful thinking (Barrow and Tipler) to just pure and simple bad logic: Wheeler’s principle obviously begs the question (i.e., it circularly assumes) of how is it possible for observers to be required in order to bring the universe into existence, and yet for those same observers to be part of that very universe. Unless, of course, the “observer” is actually a god existing outside space-time (whatever that means), in which case we are back to ID.

If this were the state of discourse concerning the AP, it would scarcely be worth bothering with. But recently a good number of high-level theoretical physicists jumped into the fray, including Steven Weinberg, Leonard Susskind and Lee Smolin. These are smart people, and we need to seriously entertain why exactly they are wasting neuronal power on any variant of the AP at all.

The answer is that of course discussions about the various APs do pick on one underlying serious question: why are the laws of physics the way they are? (Not to mention that old philosophical chestnut: why is there something instead of nothing?) And of course that’s precisely what fundamental physics is all about.

Still, one would not understand why people like Weinberg and co. bother with the AP if one did not grasp the real debate going on within the fundamental physics community these days: the one about string theory vs. possible alternatives to provide a “final theory of everything,” a theory that, presumably, would tell us why the fundamental physical constants do take the values that they do. [See our recent podcast on the problematic status of string theory for more.]

There are — to simplify grossly — essentially three types of answers currently on the table. String theorists hope that their theory will provide a unification of all forces of nature, a single equation with few if any free parameters, that will therefore show that the universe simply had to have the laws that it does, period. This will cut the legs out from under any non-trivial version of the AP (assuming that there is such a thing), because what sets the whole AP “argument” going is the (completely unproven) idea that the laws of physics and the physical constants could take any of an infinite number of values, thereby resulting in a spooky feeling once we realize that a tiny fraction of those values are compatible with us existing at all (though even that assumption has been questioned to some extent).

The second alternative is to invoke the multiverse, also a concept tightly related to string theory. This is the idea that there are infinite universes “out there,” each characterized by its own set of physical laws and constants. We just happened to be on one of those compatible with life, but this is now entirely unsurprising, as we obviously wouldn’t be having this discussion if our universe were incompatible with our existence. Notice, incidentally, that the multiverse is not at all the same thing as the many-worlds interpretation of quantum mechanics, with which it is so often confused: the latter simply says that our universe keeps splitting into an infinite number of sub-possibilities where individual events take a different course — but the laws of nature would be the same in all these universes, making the many-worlds interpretation irrelevant to discussions of APs.

Finally, we have Smolin’s controversial idea of cosmic natural selection (CNS), according to which universes with different sets of physical parameters keep being generated and compete for prevalence in the landscape of all possible universes. It has to be noticed that Smolin is one of the most vocal critics of string theory (see his excellent The Trouble with Physics), and that his bet is on a different approach known as loop quantum gravity.

There are many problems with Smolin’s idea of CNS, beginning with the obvious fact that it is not at all clear what the measure of fitness would be for CNS to get going (contra Smolin, in biology, fitness isn’t simply the rate of reproduction, it is the rate of differential reproduction, which implies competition for resources in a common environment — but parallel universes do not share a common environment, by definition). Another serious problem is that any theory of natural selection requires a mechanism of reliable inheritance, and again it is far from being clear how information about physical constants would be inherited from a parent to a baby universe. Moreover, that inheritance has to be imperfect so that variation may arise on which selection can act — and what would the mutation mechanism be, in the case of parallel universes? And what is the “mutation” frequency to new laws of physics, anyway?

There is much, much more that could be said about the AP, but we need to clear up yet another source of confusion before opening the discussion. Contrary to what is often stated, no version of the AP makes any testable prediction (which means that, whatever APs are, they're not science). The two alleged examples often brought up are Steven Weinberg’s prediction of the value of the cosmological constant and Fred Hoyle’s prediction of a particular resonance of Carbon 12, the element on which (terrestrial) life is based. The problem is that both predictions were actually based on standard science, with no need to consider the AP at all. Here I think the conclusive rebuttal goes to Smolin, who in a letter to Susskind puts it this way:

“The logic of [Weinberg-style] arguments is: A implies B; B is observed; B, together with theory C, implies D [therefore, A is true].”

Here A is any form of the Anthropic Principle or the Principle of Mediocrity, together with assumptions about priors, probability distributions on universes etc, plus our own existence, that leads to the conclusion that we should observe B.

B is that galaxies have formed. C is the theory of structure formation, D is that the cosmological constant is not too large.

The fallacy is not to recognize that the first line plays no role in the argument, and the prediction of D is equally strong if it is dropped. One can prove this by noting that if D were not seen, one would have to question the theory C [assuming the observation is correct, as it certainly is here.] One would have no reason to question either A or the assertion that A implies B.”

In other words, the AP adds nothing to the predictions made by good old-fashioned scientific theories, and our understanding of the ultimate question of life, the universe and everything therefore gains precisely nothing by the introduction of the Anthropic Principle. How many angels were dancing on that pin?