What are the planetary conditions for humanity to exist?

The existence of humanity requires a number of challenging conditions, and the concepts of the weak and strong anthropic principle shed light on scientific inquiry and its limitations.

 

The conditions for intelligent life, such as humans, to arise on a planet are very demanding. The planet must be located in a narrow zone where water can exist as a liquid, and the water must be rich in organic matter. It must also have just the right amount of gravity to hold an atmosphere steady, and that atmosphere must contain enough oxygen to form an ozone layer that blocks ultraviolet radiation. The planet’s core must be made of liquid metal because it needs a magnetic field to keep stellar winds at bay, and it must have a rotation rate that keeps the core fluid. Even if all of these conditions are met, the planet would still have to be lucky enough to escape external threats (interplanetary collisions, orbital deviations, etc.) that would damage it for 4.5 billion years before humanity could appear. No wonder we call Earth a miracle planet.
But if we change the order of our thoughts, things change. Since it’s clear that humanity exists, we can conclude that there must be a planet that fulfills all the conditions for human existence, and it’s not surprising that we live on it. This same logic is extended beyond Earth to the entire universe and is known as the Weak Anthropic Principle. The essence of this logic is that the universe we observe is capable of producing intelligent life, humans, so it’s not surprising that it exists in the way it does. Applying this principle, it follows that even though the probability of a planet having the conditions to support life is very low, it is not surprising that the mother planet of intelligent life is a rare planet. This principle is not a logical leap, and it can be useful. When designing an experiment to measure some unknown constant, the weak anthropic principle can be applied to predict the likely value of the constant and then design the experiment accordingly, resulting in a more efficient experiment.
The strong anthropic principle, on the other hand, is based on the logic that the universe that is observable to humans must have been structured as it is because it requires intelligent life. What is important in this logic is that certain physical phenomena become natural and inevitable through the existence of humanity. The weak anthropic principle does not answer the question: why is the proton 1836 times the mass of the electron? It can only say that the chemical reactions necessary for the process of anthropogenesis are possible when the proton has 1836 times the mass of the electron, so it is not strange that this value is observed. On the other hand, the strong anthropic principle might answer the same question by saying that if the mass of the proton were different, humanity would not exist, so it is natural for the proton to be 1836 times the mass of the electron. This principle may seem appealing at first glance, but it uses dangerous logic that ignores basic causality. The probability of a planet having the conditions for humanity to arise is very low, but there are tons of planets in the universe, so it wouldn’t be surprising if a few of them did. And on one of those planets, humanity could indeed have been born. It is not correct to use this fact to argue that Earth must be special because it is a habitable planet. The Strong Anthropic Principle requires us to accept the logical fallacy that the Earth was tailored to be habitable from the time it first formed 4.5 billion years ago.
Physics always struggles with the question of why? Because any theory always contains assumptions, and why? is a question that attacks those assumptions and cannot be answered by contemporary science. For example, since the speed of light is determined by the permittivity and permeability of the vacuum, and these values are constants independent of the state of motion, we can deduce that the speed of light is always constant regardless of the state of motion. However, we still don’t know why its speed is 300,000 kilometers per second. Until we have a deeper understanding of physics, we can only accept the experimental measurements. The same problem applies to important assumptions that form the basic framework of physics. Lorentz symmetry states that the laws of physics are always constant, regardless of the position or state of motion of the experimenter, but why this holds is not something that science can answer today.
Physicists had hoped that as physics progressed, the number of assumptions would decrease, but a deeper understanding of nature did not mean fewer assumptions were accepted. In classical electromagnetism, only four of Maxwell’s equations, the speed of light, and the dielectric constant of the vacuum were needed to create a theory, whereas the modern Standard Model requires 25 constants. Baez, J. (2011) This uncomfortable situation is exacerbated by the need to fine-tune the constants. Fine-tuning refers to the act of fitting the values of constants in “unnatural” situations where the range of constants for which a theory can function properly is very small, with the cosmological constant being a prime example. This constant determines how much the universe is expanding or contracting, and is known to be around 10^-122. Situations in which the fit of a theory depends heavily on the fine-tuning of a constant indicate that the theory is not perfect, and some physicists, uncomfortable with the fact that our deepest understanding of nature is so incomplete, have sought to address this situation with a strong anthropic principle.
Physicists who embrace the anthropic principle always answer the same question: why? The reason the cosmological constant is so small is because of the existence of humans in the universe, and the values of the 25 constants in the Standard Model were chosen because of the need for humans to live in the universe. There are more than 10^500 different types of universes predicted by superstring theory, but Douglas, M. R. (2003) argues that there is nothing to worry about because ours is the only one in which humans could have been born. This argument is used to explain away unacceptable assumptions, and it can be seen to stem from a fear of the unknown. No matter how far science advances, there will always be questions about assumptions, and even if another theory emerges that answers them, it will contain other unexplained assumptions. The anthropic principle, which could break this chain of ignorance, may have seemed an attractive enough weapon to them.
On the other hand, the argument that a universe in which intelligent life is not observable cannot exist suggests that the anthropic principle puts observability and the laws of physics on an equal footing. This kind of thinking is not conducive to the advancement of science. It prohibits questioning. If we accept that all observable physical phenomena are a natural consequence of the existence of humanity, then what is there to study? The activity of questioning and creating new theories becomes impossible because all questions can already be answered. Another problem with the anthropic principle is that it cannot be disproved or verified. Disproving the anthropic principle would put us in the contradictory situation of observing a universe in which humans could not possibly exist, and verifying it is completely impossible due to the nature of the claim that the laws of physics are tailored to allow for the existence of humans. We can hardly call this science.
Science is imperfect. As mentioned above, even with more advanced theories, there are always going to be assumptions that fail to explain things. However, the goal of science is not to create a perfect theory with all questions answered. If we understand that the process of science is an endless search for deeper understanding and explanations than current theories, and that the purpose of science is to increase the breadth and depth of our understanding of nature through this process, then the anthropic principle will lose its place.