Answer
Anthropic means “relating to human beings or their existence.” Principle means “law.” The Anthropic Principle is the Law of Human Existence. It is well known that our existence in this universe depends on numerous cosmological constants and parameters whose numerical values must fall within a very narrow range of values. If even a single variable were off, even slightly, we would not exist. The extreme improbability that so many variables would align so auspiciously in our favor merely by chance has led some scientists and philosophers to propose instead that it was God who providentially engineered the universe to suit our specific needs. This is the Anthropic Principle: that the universe appears to have been fine-tuned for our existence.
Consider protons, for example. Protons are the positively charged subatomic particles which (along with neutrons) form the nucleus of an atom (around which negatively charged electrons orbit). Whether by providence or fortuitous luck (depending on your perspective), protons just happen to be 1,836 times larger than electrons. If they were a little bigger or a little smaller, we would not exist (because atoms could not form the molecules we require). So how did protons end up being 1,836 times larger than electrons? Why not 100 times larger or 100,000 times? Why not smaller? Of all the possible variables, how did protons end up being just the right size? Was it luck or contrivance?
Or how is it that protons carry a positive electrical charge equal to that of the negatively charged electrons? If protons did not balance electrons and vice versa, we would not exist. They are not comparable in size, yet they are perfectly balanced. Did nature just stumble upon such a propitious relationship, or did God ordain it for our sakes?
Here are some examples of how the Anthropic Principle directly affects the livability of our planet:
The unique properties of water. Every known life form depends on water. Thankfully, unlike every other substance known to man, water’s solid form (ice) is less dense than its liquid form. This causes ice to float. If ice did not float, our planet would experience runaway freezing. Other important properties of water include its solvency, cohesiveness, adhesiveness and other thermal properties.
Earth’s atmosphere. If there were too much of just one of the many gases which make up our atmosphere, our planet would suffer a runaway greenhouse effect. On the other hand, if there were not enough of these gases, life on this planet would be devastated by cosmic radiation.
Earth’s reflectivity or “albedo” (the total amount of light reflected off the planet versus the total amount of light absorbed). If Earth’s albedo were much greater than it is now, we would experience runaway freezing. If it were much less than it is, we would experience a runaway greenhouse effect.
Earth’s magnetic field. If it were much weaker, our planet would be devastated by cosmic radiation. If it were much stronger, we would be devastated by severe electromagnetic storms.
Earth’s place in the solar system. If we were much further from the sun, our planet’s water would freeze. If we were much closer, it would boil. This is just one of numerous examples of how our privileged place in the solar system allows for life on Earth.
Our solar system’s place in the galaxy. Once again, there are numerous examples of this. For instance, if our solar system were too close to the center of our galaxy, or to any of the spiral arms at its edge, or any cluster of stars, for that matter, our planet would be devastated by cosmic radiation.
The color of our sun. If the sun were much redder, on the one hand, or bluer, on the other, photosynthesis would be impeded. Photosynthesis is a natural biochemical process crucial to life on Earth.
The above list is by no means exhaustive. It is just a small sample of the many factors which must be just right in order for life to exist on Earth. We are very fortunate to live on a privileged planet in a privileged solar system in a privileged galaxy in a privileged universe.
The question for us now is, with so many universal constants and cosmological parameters defining our universe, and with so many possible variables for each one, how did they all just happen to fall within the extremely narrow range of values required for our existence? The general consensus is that we are either here by fortuitous luck against tremendous odds or by the purposeful design of an intelligent Agent.
Some proponents of the here-by-chance perspective have sought to level the odds against fortuitous luck by hypothesizing a scenario whereby our universe is just one among many in what has come to be termed a “multiverse.” This gives nature many more chances to “get it right,” bringing the odds against its success down significantly.
Imagine innumerable lifeless universes in which one or more of the necessary variables fail to fall within the specific range of values required for life. The idea is that nature would eventually get it right, and apparently has done so as evidenced by the fact that we exist (or so the argument goes). We are the lucky ones whose universe stumbled upon the right combination of cosmological values. The Anthropic Principle is often cited as empirical grounds for the otherwise mathematically hypothetical multiverse.
Intelligent Design theorists hail the Anthropic Principle as further evidence in support of their thesis that life was engineered by a transcendent Mastermind. Not only do biological systems bear the hallmarks of design (the information content of DNA, specified complexity, irreducible complexity, etc.), but the universe which supports and provides a context for life appears to have been designed as a means to that end.
