Many people are aware of the Weak and Strong Anthropic Principles. The Weak One says, basically, that it was jolly amazing of the universe to be constructed in such a way that humans could evolve to a point where they make a living in, for example, universities, while the Strong One says that, on the contrary, the whole point of the universe was that humans should not only work in universities but also write for huge sums books with words like “Cosmic” and “Chaos” in the titles.

The UU [Unseen University] Professor of Anthropics had developed the Special and Inevitable Anthropic Principle, which was that the entire reason for the existence of the universe was the eventual evolution of the UU Professor of Anthropics. But this was only a formal statement of the theory which absolutely everyone, with only some minor details of a “Fill in name here” nature, secretly believes to be true.
–Terry Pratchett, Hogfather

Antidotes to Narcissism

One of the most difficult ideas to expunge from human thinking is that we are somehow the most important thing around. Earth or the Sun is the center of the Universe (which is quite small), we are the masters of creation (and have been since the beginning), even that evolution is a linear progression, starting with bacteria and culminating with us. All things exist for our use, and if they have no apparent purpose, then there is no harm in expurgating them from existence. There are many variations of this self-centeredness, of course, depending on the era and culture. (One particularly odd one, at least to my way of thinking, says that God is in complete control of creation, so we can’t actually do any meaningful damage—so what we do to other species or to the environment doesn’t really matter.) The reality, of course, is less flattering: Earth and the Universe have been around a lot longer than our species, Earth is a small planet in a fairly ordinary star system (judging by recent exoplanet discoveries), evolution is undirected, with each species succeeding or not based on its adaptation to its environment. The “tree of life” is a much better metaphor than an ascending ladder of being, or the commonly-reproduced picture of hominid ancestors beginning with ape-like creatures and ending with us.

Of course, it’s equally obvious that most of the Universe is extremely hostile to life. If I travel about 100 miles north, I’m in Washington, DC; if I travel 100 miles down, I’m in Earth’s mantle, undergoing crushing pressure and killingly high temperatures; if I travel 100 miles up, I’m in space, with effectively no air and a lot less shielding from dangerous solar radiation. Even if there are billions of habitable planets in the Milky Way (and comparable numbers in every galaxy), they occupy a tiny volume, compared with the hostile empty reaches. In other words, Earth does kind of occupy a special place: one which allows liquid water to exist, and is far from dangerous astronomical bodies like black holes, pulsars, or even very hot stars (which pump out huge amounts of ultraviolet radiation). Many studies suggest that we live in a galactic habitable zone, and others say perhaps not every galaxy may be habitable. Our place in the Universe may not be unique, but it’s still somewhat privileged. Certainly no one can deny the effective way our species has spread to every part of the planet, showing our adaptability and resourcefulness—as well as our all-too-frequent shortsightedness with regard to resources.

Anthropic Principles

The recognition that the Universe as a whole contains at least one habitable world, and all the factors that came together to make this possible, is known as the weak anthropic principle (WAP). From a scientific point of view, the WAP places limits on the kinds of cosmological theories we can produce. For example, there must be sufficient time for stars to fuse the primordial elements (hydrogen and helium) into the stuff we’re made of (carbon, oxygen, sulfur, phosphorous, iron, calcium). The fundamental forces of nature—gravity, electromagnetism, and the two nuclear forces—must be consistent with life as well, as variations in their relative strengths would have profound implications for chemistry. However, the WAP isn’t really predictive: it’s more a set of simple criteria that must be satisfied before we accept a particular cosmological model, in addition to other similar criteria from other observations. In other words, the WAP isn’t controversial (and isn’t as tautological as the Pratchett quote above would indicate).

Other versions of the anthropic principle aren’t quite so nice, however. At one extreme, we have the strong anthropic principle (SAP), which says the Universe is constructed in such a way that life—and intelligent life—is inevitable, possibly even by some kind of design. Suffice to say most scientists reject this idea, and I’m not interested in debunking it any more than I am taking on modern-day geocentrists. Another version is a bit worrisome, due to its increasing popularity; I’ll call it the multiverse anthropic principle (MAP), since I haven’t run across another name for it. (If you have, please let me know in the comments.) The MAP isn’t entirely crazy: its genesis lies with cosmic inflation. While inflation isn’t perfect as a theory (coming as it does in too many flavors), I put my money on its correctness: it helps explain why the cosmic microwave background is so smooth, and why the geometry of the Universe is mostly flat, without needing to make everything perfectly flat. (See my earlier post on inflation for a lot more information.)

However, the central principle of inflation adds complications: in its early moments, the Universe expands very rapidly, meaning that many regions of the cosmos may become separated from each other. These disconnected parallel universes may not have the same physical properties as the observable Universe, and therefore may not be able to make stars, planets…or life. Parallel universes are an unfortunate consequence of inflation—unfortunate to my way of thinking at least, because we can’t observe them. Since science is based on testable predictions, it makes me itch to have a plethora of unreachable and unobservable universes as an inevitable consequence of a theory.

(By the way, this is a separate notion from the uncontroversial idea that the Universe we inhabit has regions we can’t see. While this also bothers me a little, it’s highly probable those regions have the same physical properties as the observable portion, since it’s part of the same “bubble”. If we could transport to a galaxy far far away, we would observe similar stars, galaxies, and other familiar objects. A parallel universe with different properties might not have stars, and the spectrum of atoms might even be different due to variations in the relative strengths of the fundamental forces.)

The String Theory Version

When string theory is added to the mix, things get even wilder for the multiverse. The simplest versions of string theory are all incorrect; fixing the problems results in a huge number of possible vacuum states for the Universe, each corresponding to a different “compactification” of the extra dimensions in the theory. (String theory, if you recall, requires 7 additional dimensions beyond the 4 spacetime dimensions. The extra dimensions must be rolled up—compactified—in some way, since they aren’t observable under ordinary conditions; understanding how compactification occurs is the primary challenge of string theory.) No obvious way within string theory exists to say one of these possible vacuum states should correspond to reality while the others don’t. All the possibilities together are known as the string theory landscape (or the energy landscape).

Some string theorists are bothered by that, and are working on alternative ideas that would avoid the landscape. Others, however, embrace the landscape and make it central to string theory cosmology. The idea is that every one of the possible vacuum states in the landscape arose during inflation (experts will notice I’m simplifying here), one of which contains our observable Universe, with the particular physical properties we see, while others may vary wildly. Again, there’s nothing that dictates why our particular universe is different than others—all possible universes in the landscape exist, and this just happens to be ours. This particular universe has all the right properties, so here we are; this is what I mean by the MAP. The string theory landscape (at least in the version I know) requires resorting to the anthropic principle, since nothing in the theory dictates why physical properties are what they are.

Many cosmologists don’t like this idea, and I’m one of them. This is apart from my skepticism about string theory in general, by the way: string theory may be correct and the landscape may be wrong, and some version of the landscape exists in a way even within inflationary theory. As Paul Steinhardt and Neil Turok point out in their book Endless Universe (which has many points I don’t agree with, so this isn’t a blanket endorsement of everything they write), usually when scientists invoke something that isn’t testable—including the anthropic principle—to explain something fundamental, they’ve shown the limits of the theory they’re working with. In other words, invoking something like the anthropic principle is giving up on improving the theory: it says science has hit its limits, and we must fall back on a metaphysical argument.

I prefer to think we haven’t solved the major problem yet of why our Universe has the physical properties it possesses. While I reluctantly accept that parallel universes may exist, and even that they may have different properties than our own, my hope is that cosmologists can figure out ways to explain our cosmos without invoking a kind of anthropic just-so story. Our understanding of our place in the Universe has changed with time and new discovery; our understanding of our place in the multiverse is still evolving, but I am hopeful we will yet achieve clarity.