First spoken by physicist Brandon Carter in 1973, stating that the universe as we observe and explain it (laws, constants, structure…) is founded on the premise that we, as observers and life form, are able to exist within these conditions. As such, the features of the universe as we know them and if they were to be altered only slightly, no life form could exist or evolve in the universe.
However, although we know the facts, we still do not know the reasons, and the question of “why these values?” still persists. Two different declinations of the anthropic principle, the weak and the strong, attempt to answer three rather existential questions: why does the Universe take on this particular structure? How did allow intelligent life to rise within it? And finally, how are we, intelligent life forms, able to understand this structure (the constants and laws which govern interactions within our Universe)?
In direct confrontation with the Copernican Principle, stating that no scientific theory should grant a special place to humans, nor take into account their influence, the Anthropic Principle accounts for a Universe tailored to the existence of intelligent life. Our Universe is as such, seen by some to be, fine-tuned.
Weak Anthropic Principle
The version, initially proposed by Brandon Carter at the Symposium celebrating Copernicus’s 500th birthday, also abbreviated as the WAP, states that our existence, or more generally the existence of carbon-based intelligent life forms, imposes a restriction on the Universe. For had it not agreed with our description of it, we wouldn’t even have been here to investigate the parameters of the Universe in the first place. In other words, we live in a Universe complex and old enough for intelligent life to have evolved. It is referred to as “weak” due to the fact that our existence is not an end goal or a “must”, but rather a product of just the right conditions.
This theory is generally widely accepted and seems to stem from shared common sense as there’s no sense in asking why. For if our universe weren’t hospitable to life, then we wouldn’t even be here to wonder about it.
Strong Anthropic Principle
The more speculative and less founded versions of the weak anthropic principle, known as the strong anthropic principle, and also proposed by Carter, vaguely states that “the Universe must have those properties which allow life to develop within it at some stage in its history”. It is referred to as “strong” because it implies human life to be the desired product and that our existence is a “must”, we are special. This theory is frowned up, seen as rather reductive as it claims that only life-supporting universes are possible since we live in one capable of supporting life.
In addition, scientists have developed the speculate of a Participatory Anthropic Principle and a Final Anthropic Principle. The Participatory explanation states that it takes an intelligent observer such as humans in order to shift the Universe’s probability from superposition to reality. In other words, that universes only become real once they are observed. On the other hand, the final explanation states that life is a property that will necessarily arise in a Universe and that once it has developed, it will never die out. However, both theories remain thoroughly speculative.
To conclude, supporters of the Anthropic Principle rightfully claim that if true, it would explain why our Universe has the properties and age that we observe it to have, for otherwise, we would not even be here. However, it still leaves open the question “why”? as it is not a satisfactory physical explanation but rather a philosophical one.