What is the Anthropic Principle?

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 favour merely by chance has led some scientists and philosophers to propose instead the universe has been engineered to suit our specific needs. This is the Anthropic Principle: that the universe appears to have been fine-tuned for our existence.

Does intelligent life cause the Universe ?

This philosophical principle has led, via a complicated chain of reasoning among some scientists, ultimately to the hypothesis that the universe may only exist at all once it has conscious observers. Before that point, some presume, it was merely a mathematical abstraction. In other words “Does intelligent life cause the Universe ?”. Bear with me we’ll get to that in due course.

Origins of this Principle

It was anticipated more than a hundred years ago by Alfred Russel Wallace, who 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.”

More recently, scientists have come to learn how improbable the chain of events is that led to the origin of life and eventually of humans, the only organisms capable of reasoning about their own origins. They point to the fact that certain physical characteristics of the universe all have the values they should have if intelligent life is to arise in the universe – its age, its expansion rate, the number of electrons and protons, even something as seemingly inevitable as the number of dimensions, three of space and one of time. But it’s a slippery kind of improbability. Sometimes it can seem like the statement: “How likely is it that at exactly 12:30 today, the postman will deliver a letter to me with a green envelope from my mother with a stamp on it at exactly 45 degrees ?” In fact, if you ask that question before the event, with no foreknowledge, the answer is that it is extremely unlikely. But to ask it afterwards is trivial – the answer is that the event happened and therefore it was certain. Clearly, we are asking the question about us and the universe in a context where we know we exist – as Descartes pointed out – and therefore it doesn’t seem to make much sense to raise questions about the probability or improbability of the steps that led to us being here. If the numbers had been slightly different there would have been a different universe, without us, and there would have been no one to worry about it.

So how narrow are the parameters needed for our existence ?

Here are some examples of how attributes of our universe directly affacts the Anthropic Principle :

Size of protons 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?

Particle charges 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?

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.

The colour 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.

Other theories

One hypothesis that avoids the need for this very special chain of events to have occurred in the only universe we know is that there exist parallel universes, each with a different set of laws, dimensions and constants. Those in which no form of intelligent life could arise, the vast majority, will clearly not have inhabitants who wonder about their existence, and a small number equal to or greater than one will have intelligent life, us.

Variants on the Anthropic Principle

There is a Weak Anthropic Principle (WAP), which says the various physical characteristics of the universe had to be the way they are in order to allow carbon-based lifeforms (the only type we know) to arise and for the universe to last long enough for this to occur.

There is also a Strong Anthropic Principle (SAP), which says that the purpose of the universe is to give rise to intelligent life. This obviously hints of the intelligent design argument pushed by creationists in the United States. There’s also the Participatory Anthropic Principle (PAP), put forward by the physicist John Wheeler, who believes that no universe can exist unless it contains conscious observers. By observing the universe we bring it into existence.

Finally, there is FAP, the Final Anthropic Principle, following on from PAP, which says that now that life has begun it will be impossible to destroy, otherwise the universe would lose all its observers and disappear.