Anthropic principle – Definition and Explanations

The anthropic principle (from the Greek anthropos , man) is a metaphysical principle which states that if we observe the universe as we know it, it is above all else because … we are there! Because if we weren’t there, we wouldn’t be there to see it. It was formulated and developed by Brandon Carter of the observatory of Meudon but one finds the principle of it in the first book of the World like will and like representation of Schopenhauer where it shows on the one hand the impossibility for any science to reach a reality in itself, on the other hand the confusion that men make between the conceived universe and a supposed objective universe independently of the subject who conceives it.

This implies that any theory which includes our existence and it will certainly be those on the world as we understand it, must necessarily be consistent with our own existence. Thus, according to Schopenhauer, any conception of the universe is a petition of principle because the subject who conceives the theory is posited as the finality of this same theory and is therefore introduced at the very origin of this theory to which the development of the universe must be ordered to explain its own appearance.

There are two main versions of the anthropic principle: the weak and strong principles .

Weak anthropic principle

The weak anthropic principle does not rule on the question of knowing whether our presence is the result of a particularly improbable chance or a deterministic process. He expresses that if the universe had evolved in a way that did not allow conscious entities to appear in it, no conscious entity would have been there to notice it, and therefore that there would be no knowledge of this universe; in other words, such a universe would not exist. And therefore, that from our point of view(of conscious entity in the universe) even if our universe is only one of the multiple universes which could have existed there is nothing improbable a posteriori. Thus, the probabilities that we had to appear (individually or collectively) are so low a priori that we are tempted to say to ourselves “what luck!”. In reality, if it had been otherwise, we could not have complained of our bad luck since we would never have existed!

Strong anthropic principle

However, a more religiously connoted variant of the anthropic principle sets out the idea of ​​a will or a necessity intervening in the evolution of our universe: this universe was specially designed so that we were placed there. We speak in this case of a strong anthropic principle . In this version, we try to reintroduce the principle of a universal purpose which would be the man himself and therefore to think of the initial conditions of the universe according to this finality. The strong anthropic principle is only a variation on the traditional religious theme of “cosmic design”, criticized by Russell in Science and religion.

Conditional probabilities

The anthropic principle is one illustration of all the difference that can exist between an a priori probability (for example: probability of drawing 3 “six” with 3 dice: 1 in 216) and conditional probabilities (example: probability of having drawn 3 “six” with 3 dice knowing that the total is greater than or equal to 17: 1 in 4).

One of the most illuminating metaphors to assimilate the idea is as follows:

“Don’t get too excited about the fact that you see an arrow planted right in the center of a painted target. Are you sure that the target was not painted after the arrival of the arrow?”

So even if we are (individually or collectively) the fruit most unlikely chance of a priori , our very existence is proof that the event occurred. But this sentence actually says that we paint a posteriori to explain from there the fact that we … paint.

Importance of the anthropic principle

This principle applies to all events regardless of any causal research : at the deepest cosmological level (nature of physical laws , values ​​of cosmological constants, etc.), at the level of the species (if we do not see d ‘extraterrestrials, it is perhaps that we are the first intelligent species of the galaxy , as improbable as it may seem a priori ), and at the individual level (as we can rave about being from this particular sperm , among millions).

All of this may seem tautological, and in fact it is. The approach is similar to that of I think, so I’m from Descartes. It’s about saying, “I see myself, therefore I am possible.” But we used to observe the world from an outside and objectiveeye for so longthat we ended up neglecting that we are inside. This is really unimportant in the fall of bodies, but it is different for two areas where neglecting this existence would make us neglect information  :

  • cosmology;
  • quantum mechanics.

The fact that we exist allows us to observe by construction only one instance of the universes that could have existed.

For example, if the gravitational pull had been weaker, the protostellar agglomeration process might not have happened and in this case the universe would remain a large cloud of hydrogen . Had it been stronger, nuclear reactions could have been packaged, producing only heavy elements like uranium: the chemistry of carbon , essential for life , would then not have been possible. Furthermore, if the stars had burned their hydrogen in their nuclear furnace too quickly, they would not have given life time to develop.

We can also invoke the anthropic principle to explain why I am as I am: if I had been different , I would not have existed, it would not have been me! For all my life, it was necessary that I be as I am. And likewise, it was necessary that all my ancestors all had the chance to reproduce, that is to say in particular not to die at a young age at times when this was most frequent. A priori, what luck, but a posteriori, nothing very surprising.


The anthropic principle in subatomic physics

The anthropic principle can also explain the incredible stability of the proton: if it is difficult to observe the disintegration of this particle in nature, it is because our own existence is linked to the opposite phenomenon: matter is created by the production of protons, at a time when the universe contained an indistinct number of quarks and antiquarks. A priori , the universe had as many probabilities of producing more antimatter than matter, than the reverse: the violation of the CP symmetry occurred in one direction (the neutral kaon K 0disintegrating more easily into quarks than antiquarks), it could just as easily have happened in the opposite direction. And a priori we can think that there was even more probabilities that the CP symmetry would be preserved, in which case the universe, producing as much matter as antimatter, would be practically empty .

The geometry of the universe

It is also by the anthropic principle that we explain that the geometry of the universe is in three spatial dimensions : in two dimensions, life is doomed to failure (the digestive system would cut us in half, for example) ; in four dimensions, the world would be very unstable, because both the electromagnetic forces and the gravitational forces would have varied in inverse function of the cube of the distance (instead of the square): the stars would have followed spiral trajectories before crashing in the center or escape into space, and the waves would propagate without speed determined, resulting in a total inconsistency of signals, from the microscopic level to the scale of the universe.

Regarding a universe where space has three dimensions, there are three possible scenarios.

  1. The universe is closed. Its density is very high. In this case, its lifespan will be very short. After a brief expansion, it will collapse on its own. The stars will not have time to form there. There will therefore be no nucleosynthesis , no heavy atoms , no planets, no life and therefore no observer.
  2. The universe is open. Its density is very low, the expansion therefore meets no brake . Its lifespan is in principle infinite. But no condensation can start, so no star can form there either.
  3. The universe has a density equal to or close to the critical density. In other words, it is flat or almost flat. On the one hand, it is empty enough for the entropy to be able to evacuate and dilute in the expanding space and on the other hand dense enough for the nucleosynthesis to start and durable enough for it to continue several billion years. It’s our universe. It alone seems to have the possibility of generating a stellar , nuclear, chemical and biological evolution .

In other words, a fertile universe is a universe whose geometry must be plane and whose density coincides with the critical density. Such must be the universe so that it can generate an observer. It supposes a type of coincidence that is currently inexpicable (see Hubert Reeves, The Hour of Getting Drunk , ch. 8).

Magic and faith

Warriors, more generally human beings who engage in dangerous activities, often believe in the effectiveness of lucky luck (or bad luck) in infinite diversity, or consult soothsayers with various methods (such as surveys) or esoteric , like astrologers. This is a new illustration of the anthropic principle: the surviving warriors, or the victorious sportsmen or entrepreneurs, or the recognized creatives, by definition survived. They therefore have every reason to believe in the effectiveness of their protections or the methods of their advice, however esoteric they may be. Conversely, the dead (physical or economic) are no longer there to be surprised at the ineffectiveness of their own.


A slightly different way of understanding the anthropic principle is the concept of a statistical filter . Again a short example does better than a long speech: how big is the smallest fish in the pond? You catch 100 fish, all larger than 3 cm. Does this measure confirm the hypothesis that no fish is much smaller than 3 cm in this pond? Not if the net cannot catch smaller fish. Knowing the limits of our procedure for acquisition of data affects the conclusions that can be drawn from our data.


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