One of the more extraordinary things about the universe is that it has produced beings who can observe it — namely, us. Its laws and constants are so precise that, if they were even slightly modified, no human would be here to see it. Many cosmologists and philosophers have wondered if we should read anything into all this preciseness: Are the finely-tuned physical laws that surround us mere coincidence, or does it imply that we are somehow meant to be here? That's where the Anthropic Principle comes into play.
Top image via Luc Perrot.
The Anthropic Principle (AP) is that hazy grey area where philosophy meets science. And in fact, many scientists loathe it for this very reason. It's untestable, they argue, and tautological — a skewed form of reasoning in which the principle is basically being used to prove itself.
And indeed, the AP does seem like a strange concept at first. It essentially states that we will only find ourselves in a universe that's capable of giving rise to us. Put another way, observations of the universe must be compatible with the conscious life that observes it.
It's a principle that makes perfect sense — and for some, no sense at all. But like so many things in science and philosophy, the devil is in the details.
The AP forces us to take a giant step back and evaluate the conditions of the universe in consideration of our presence within it. For scientists, it's a kind of ‘40 foot perspective' that can help illuminate — and even possibly explain — some of the more surprising aspects of cosmology. And at the very least, it serves as a constant reality check to remind us that we will always be subject to observational selectional effects; no matter where we go, we will always be there.
A good thought experiment in this regard comes from the Canadian philosopher John Leslie. In his book, Universes, he asks us to imagine a man facing a firing squad of fifty expert marksman. After aiming and firing, the executioners miss their mark.
Now, there are two ways in which we can evaluate this surprising outcome. We can either shrug our shoulders and point to the obvious, that they they simply missed. Or we can come up with some explanations as to why they all missed. This latter point is very much at the heart of anthropic reasoning.
The AP has been around for quite some time, though it only really took on its modern form in the last forty years.
Early efforts to come to grips with observational effects were expressed in Hume's Dialogues Concerning Natural Religion, and Kant's ideas about how our experience of the world is formulated by our sensory and intellectual faculties. Back in the 1920s, James Jeans observed that, "the physical conditions under which life is possible form only a tiny fraction of the range of physical conditions which prevail in the universe as a whole." Likewise, his contemporary, Arthur Eddington, speculated about "selective subjectivism," the idea that the laws of nature are indirectly imposed by the human mind, which in turn determines (and constrains) what we know about the universe.
More recently, some scientists have used it to explain the series of bizarre "large-number coincidences" in physics and cosmology. These are the surprisingly large order-of-magnitude connections that exist between (apparently) unrelated physical constants and cosmological parameters.
For example, the electromagnetic force is 39 orders of magnitude stronger than gravity. If it was any closer in strength, stars would have collapsed long before life could emerge. Or, the universe's vacuum energy density is about 120 orders of magnitude lower than some theoretical estimates, which, if any higher, would have blown the universe apart. And the neutron is heavier than the proton — but not so heavy that neutrons cannot be bound in nuclei where conservation of energy prevents the neutrons from decaying. Without neutrons, we wouldn't have the heavier elements needed for building complex life. There are many other examples, each one pointing to extreme specificity.
In 1961, Robert. H. Dickie used a prototypical version of the AP to explain away these coincidences, saying that physicists were reading too much into it. These large numbers, he argued, are a necessary coincidence (or prerequisite) for the presence of intelligent beings. If these parameters were not so, life would not have arisen. And in turn, we wouldn't be here to marvel at the ‘surprisingness' of these physical constants and laws.
Enter Brandon Carter
Then, in 1974, the philosopher Brandon Carter kindled the modern interpretation of these ideas, what he dubbed the Anthropic Principle. But rather than settle on just one perspective or definition, he said there were two different ways we can approach the issue.
Specifically, he proposed the Weak Anthropic Principle (WAP) and the Strong Anthropic Principle (SAP). Both approaches imply that these anthropic coincidences were not the result of chance, but were instead built directly into the structure of the universe.
Of the WAP he said:
We must be prepared to take into account the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.
And of the SAP he said:
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.
Indeed, the SAP is a bit of a mind frak. Carter essentially argued that, if the SAP is true, the universe must to give rise to intelligent observers. The WAP, on the other hand, simply implies that the universe we observe must have the conditions to support intelligent life, but that life doesn't necessarily have to arise.
So, if the SAP is true, then the universe is indeed here for us.
Keep in mind that these are philosophical thought experiments, and not scientific statements per se. To a certain extent, philosophers are the conjurors of proto-scientific concepts — musings that should in turn be proven or disproven through the application of the scientific method.
Moreover, this doesn't imply or prove that God or some other Prime Mover exists, though many have taken it to that extreme. All the AP does in this regard is tell us that the laws of the universe should be understood through the context of the presence of observers.
Interestingly, Carter later regretted using the word ‘anthropic.' It has misled some into thinking that he was referring to Homo sapiens specifically (or that observers were limited to carbon based life). But his principle applies to any observer anywhere in the universe.
For example, a dolphin, which is a conscious being, can be considered an observer. Same goes for a self-aware robot on the other side of the universe. Or more conceptually, imagine a universe in which only evolving streams of information can exist. Eventually, a self-aware algorithm could emerge that's capable of assessing its surroundings. This would be an observer, too, but one far removed from our own experience.
Image at left: "Wonder - Zena Gazing at the Moon" by Alex Grey (1996) .
Since Carter's original elucidation, the AP has literally been re-interpreted and re-defined hundreds of times. Other proposed names include "self-locating belief" and "indexical information" (not difficult to see why these didn't catch on). The "fine-tuning argument", however, has gained traction as a kind of substitute term, or correlated area of inquiry.
One of the more interesting re-evaluations of Carter's original idea comes from the mathematician John Barrow and physicist Frank Tipler. They devised a third principle, the Final Anthropic Principle, which states that intelligent information processing must come into evidence in the universe, and, once it comes into existence, it will never die out.
If this is true, not only is the universe here for us, but its configuration is such that we will become its permanent residents (in some form or another).
Welcome to the multiverse
As noted, many scientists hate the AP — and often with a passion. Critics contend that it's a product of cyclical thinking, and that's its self-evident — or that life should be simply be thought of as mere epiphenomenon (our presence in the universe is merely a side-effect, or coincidence).
Others, like physicist Lee Smolin, argue that the characteristics of the universe can be explained in other ways, such as his theory of cosmological natural selection. As Smolin told io9, "The Anthropic Principle is simply incapable of making a falsifiable prediction for any kind of testable experiment."