The anthropic principle is the belief that, if we take human life as a given condition of the universe, scientists may use this as the starting point to derive expected properties of the universe as being consistent with creating human life. It is a principle which has an important role in cosmology, specifically in trying to deal with the apparent fine-tuning of the universe.
Origin of the Anthropic Principle
The phrase "anthropic principle" was first proposed in 1973 by Australian physicist Brandon Carter. He proposed this on the 500th anniversary of the birth of Nicolaus Copernicus, as a contrast to the Copernican principle that is viewed as having demoted humanity from any sort of privileged position within the universe.
Now, it's not that Carter thought humans had a central position in the universe. The Copernican principle was still basically intact. (In this way, the term "anthropic," which means "relating to mankind or the period of man's existence," is somewhat unfortunate, as one of the quotes below indicates.) Instead, what Carter had in mind was merely that the fact of human life is one piece of evidence which cannot, in and of itself, be completely discounted. As he said, "Although our situation is not necessarily central, it is inevitably privileged to some extent." By doing this, Carter really called into question an unfounded consequence of the Copernican principle.
Prior to Copernicus, the standard viewpoint was that the Earth was a special place, obeying fundamentally different physical laws than all the rest of the universe - the heavens, the stars, the other planets, etc. With the decision that the Earth was not fundamentally different, it was very natural to assume the opposite: All regions of the universe are identical.
We could, of course, imagine a lot of universes that have physical properties that don't allow for human existence. For example, perhaps the universe could have formed so that the electromagnetic repulsion was stronger than the attraction of the strong nuclear interaction? In this case, protons would push each other apart instead of bonding together into an atomic nucleus. Atoms, as we know them, would never form ... and thus no life! (At least as we know it.)
How can science explain that our universe isn't like this? Well, according to Carter, the very fact that we can ask the question means that we obviously cannot be in this universe ... or any other universe that makes it impossible for us to exist. Those other universes could have formed, but we wouldn't be there to ask the question.
Variants of the Anthropic PrincipleCarter presented two variants of the anthropic principle, which have been refined and modified much over the years. The wording of the two principles below is my own, but I think captures the key elements of the main formulations:
- Weak Anthropic Principle (WAP): Observed scientific values must be able to allow there to exist at least one region of the universe that has physical properties allowing humans to exist, and we exist within that region.
- Strong Anthropic Principle (WAP): The universe must have properties that allow life to exist within it at some point.
The Strong Anthropic Principle is highly controversial. In some ways, since we do exist, this becomes nothing more than a truism. However, in their controversial 1986 book The Cosmological Anthropic Principle, physicists John Barrow and Frank Tipler claim that the "must" isn't just a fact based on observation in our universe, but rather a fundamental requirement for any universe to exist. They base this controversial argument largely on quantum physics and the Participatory Anthropic Principle (PAP) proposed by physicist John Archibald Wheeler.
A Controversial Interlude - Final Anthropic Principle
If you think that they couldn't get more controversial than this, Barrow and Tipler go much further than Carter (or even Wheeler), making a claim which holds very little credibility in the scientific community as a fundamental condition of the universe:
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.
There is really no scientific justification for believing that the Final Anthropic Principle holds any scientific significance. Most believe it is little more of a theological claim dressed up in vaguely scientific clothing. Still, as an "intelligent information-processing" species, I suppose it might not hurt to keep our fingers crossed on this one ... at least until we develop intelligent machines, and then I suppose even the FAP might allow for a robot apocalypse.
Justifying the Anthropic Principle
As stated above, the weak and strong versions of the anthropic principle are, in some sense, really truisms about our position in the universe. Since we know that we exist, we can make certain specific claims about the universe (or at least our region of the universe) based upon that knowledge. I think the following quote well sums up the justification for this stance:
"Obviously, when the beings on a planet that supports life examine the world around them, they are bound to find that their environment satisfies the conditions they require to exist.
It is possible to turn that last statement into a scientific principle: Our very existence imposes rules determining from where and at what time it is possible for us to observe the universe. That is, the fact of our being restricts the characteristics of the kind of environment in which we find ourselves. That principle is called the weak anthropic principle.... A better term than "anthropic principle" would have been "selection principle," because the principle refers to how our own knowledge of our existence imposes rules that select, out of all the possible environment, only those environments with the characteristics that allow life." -- Stephen Hawking & Leonard Mlodinow, The Grand Design
The Anthropic Principle in Action
The key role of the anthropic principle in cosmology is in helping to provide an explanation for why our universe has the properties it does. It used to be that cosmologists really believed they would discover some sort of fundamental property that set the unique values we observe in our universe ... but this has not happened. Instead, it turns out that there are a variety of values in the universe that seem to require a very narrow, specific range for our universe to function the way it does. This has become known as the fine-tuning problem, in that it is a problem to explain how these values are so finely-tuned for human life.
Carter's anthropic principle allows for a wide range of theoretically possible universes, each containing different physical properties, and ours belongs to the (relatively) small set of them that would allow for human life. This is the fundamental reason that physicists believe there are probably multiple universes. (See our article: "Why Are There Multiple Universes?")
This reasoning has become very popular among not only cosmologists, but also the physicists involved in string theory. Physicists have found that there are so many possible variants of string theory (perhaps as many as 10500, which really boggles the mind ... even the minds of string theorists!) that some, notably Leonard Susskind, have begun to adopt the viewpoint that there is a vast string theory landscape, which leads to multiple universes and anthropic reasoning should be applied in evaluating scientific theories related to our place in this landscape.
One of the best examples of anthropic reasoning came when Stephen Weinberg used it to predict the expected value of the cosmological constant and got a result that predicted a small but positive value, which didn't fit with the expectations of the day. Nearly a decade later, when physicists discovered the expansion of the universe was accelerating, Weinberg realized that his earlier anthropic reasoning had been spot on:
"... Shortly after the discovery of our accelerating universe, physicist Stephen Weinberg proposed, based on an argument he had developed more than a decade earlier--before the discovery of dark energy--that ... perhaps the value of the cosmological constant that we measure today were somehow "anthropically" selected. That is, if somehow there were many universes, and in each universe the value of the energy of empty space took a randomly chosen value based on some probability distribution among all possible energies, then only in those universes in which the value is not that different from what we measure would life as we know it be able to evolve.... Put another way, it is not too surprising to find that we live in a universe in which we can live!" -- Lawrence M. Krauss, A Universe From Nothing
Criticisms of the Anthropic Principle
There's really no shortage of critics of the anthropic principle. In two very popular critiques of string theory, Lee Smolin's The Trouble With Physics and Peter Woit's Not Even Wrong, the anthropic principle is cited as one of the major points of contention.
The critics do make a valid point that the anthropic principle is something of a dodge, because it reframes the question that science normally asks. Instead of looking for specific values and the reason why those values are what they are, it instead allows for an entire range of values as long as they're consistent with an already-known end result. There is something fundamentally unsettling about this approach.