The success of science in understanding the macroscopic, microscopic and cosmological worlds has led to the strong belief that it is possible to form a fully scientific explanation of any feature of the Universe. However, in the past 20 years our understanding of physics and biology has noted a peculiar specialness to our Universe, a specialness with regard to the existence of intelligent life. This sends up warning signs from the Copernican Principle, the idea that no scientific theory should invoke a special place or aspect to humans.
All the laws of Nature have particular constants associated with them, the gravitational constant, the speed of light, the electric charge, the mass of the electron, Planck's constant from quantum mechanics. Some are derived from physical laws (the speed of light, for example, comes from Maxwell's equations). However, for most, their values are arbitrary. The laws would still operate if the constants had different values, although the resulting interactions would be radically different.
All the above constants are critical to the formation of the basic building blocks of life. And, the range of possible values for these constants is very narrow, only about 1 to 5% for the combination of constants. Outside this range, and life (in particular, intelligent life) would be impossible.
It is therefore possible to imagine whole different kinds of universes with different constants, all equal valid within the laws of Nature. For example, a universe with a lower gravitational constant would have a weaker force of gravity, where stars and planets might not form. Or a universe with a high strong force which would inhibit thermonuclear fusion, which would make the luminosity of stars be much lower, a darker universe, and life would have to evolve without sunlight. Why don't those Universes exist? Why does our Universe, with its special value exist rather than another? Is there something fundamental to our physics that makes the present values for physical constants expected?
The situation became worst with the cosmological discoveries of the 1980's. The two key cosmological parameters are the cosmic expansion rate (Hubble's constant, which determines the age of the Universe) and the cosmic density parameter (), which determines the acceleration of the Universe and its geometry).
The cosmic density parameter determines the three possible shapes to the Universe; a flat Universe (Euclidean or zero curvature), a spherical or closed Universe (positive curvature) or a hyperbolic or open Universe (negative curvature). Note that this curvature is similar to spacetime curvature due to stellar masses except that the entire mass of the Universe determines the curvature.
The description of the various geometries of the Universe (open, closed, flat) also relate to their futures. There are two possible futures for our Universe, continual expansion (open and flat) or turn-around and collapse (closed). Note that flat is the specific case of expansion to zero velocity.
Current values for the critical density range from 0.1 to 1, which produces a new dilemma from modern cosmology, the flatness problem.
The flatness problem relates to the density parameter of the Universe, . Values for can take on any number, but it has to be between 0.01 and 5. If is more than 0.01 the Universe is expanding so fast that the Solar System flys apart. And has to be less than 5 or the Universe is younger than the oldest rocks. The measured value is near 0.2. This is close to an of 1, which is strange because of 1 is an unstable critical point for the geometry of the Universe.
Values slightly below or above 1 in the early Universe rapidly grow to much less than 1 or much larger than 1 (like a ball at the top of a hill). So the fact that the measured value of 0.2 is so close to 1 that we expect to find in the future that our measured value is too low and that the Universe has a value of exactly equal to 1 for stability.
And therefore, the flatness problem is that some mechanism is needed to get a value for to be very, very close to one (within one part in a billion billion).
So the philosophical dilemma is that the constants of the Universe on a microscopic (atomic constants), macroscopic (electromagnetic forces) and cosmological levels all appear to be extremely fine-tuned in order for life and intelligence to evolve.
This concern of how conscious creatures, such as ourselves, came to be in the Universe is called the anthropic principle, and has three forms; weak, strong and final.
Weak Anthropic Principle: The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe be old enough for it to have already done so.
Strong Anthropic Principle: The Universe must have those properties which allow life to develop within it at some stage in its history. Because:
Final Anthropic Principle: Intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out.
The weak version of the anthropic principle just says that our existence allows us to infer values of certain fundamental constants. Our existence is an indicator of what values these constants have. But the strong version claims not just that our existence allows us to infer the values of the constants, but that it is moreover the explanation of why they have just the values that they do.
Anthropic Principle and Circular Reasoning :
The usual criticism of any form of the anthropic principle is that it is guilty of a tautology or circular reasoning.
With the respect to our existence and the Universe, the error in reasoning is that because we are here, it must be possible that we can be here. In other words, we exist to ask the question of the anthropic principle. If we didn't exist then the question could not be asked. So there is nothing special to the anthropic principle, it simply states we exist to ask questions about the Universe.
An example of this style of question is whether life is unique to the Earth. There are many special qualities to the Earth (proper mass, distance from Sun for liquid water, position in Galaxy for heavy elements from nearby supernova explosion). But, none of these characteristics are unique to the Earth. There may exists hundreds to thousands of solar systems with similar characteristics where life would be possible, if not inevitable. We simply live on one of them, and we would not be capable of living on any other world.
This solution is mildly unsatisfying with respect to physical constants since it implies some sort-of lottery system for the existence of life, and we have no evidence of previous Universes.
Anthropic Principle and Many-Worlds Hypothesis:
Another solution to the anthropic principle is that all possible universes that can be imagined under the current laws of Nature are possible, and do have an existence as superpositions.
This is the infamous many-worlds hypothesis used to explain how the position of an electron can be fuzzy or uncertainty. Its not uncertain, it actual exists in all possible positions, each one having its own separate and unique universe. Quantum reality is explained by the using of infinite numbers of universes where every possible realization of position and energy of every particle actually exists.
With respect to the anthropic principle, we simply exist in one of the many universes where intelligent life is possible and did evolve. There are many other universes where this is not the case, existing side by side with us in some super-reality of the many-worlds. Since the many-worlds hypothesis lacks the ability to test the existence of these other universes, it is not falsifiable and, therefore, borders on pseudo-science.
Anthropic Principle and Inflation :
The solution to the anthropic principle appears to lie in the very early Universe, moments after the Big Bang, the inflation era. Our old view of the Universe was one of newtonian expansion, at less than the speed of light.
However, now we know that, because of symmetry breaking at the GUT unification point, spacetime and matter separated and a tremendous amount of energy was released. This energy produced an overpressure that was applied not to the particles of matter, but to spacetime itself. Basically, the particles stood still as the space between them expanded at an exponential rate.
During inflation, the Universe expanded a factor of 1054, so that our horizon now only sees a small piece of what was the total Universe from the Big Bang.
Our visible Universe, the part of the Big Bang within our horizon, is effectively a `bubble' on the larger Universe. However, those other bubbles are not physically real since they are outside our horizon. We can only relate to them in an imaginary, theoretical sense. They are outside our horizon and we will never be able to communicate with those other bubble universes.
Inflation's answer to the anthropic principle of any form is that many bubble universes were created from the Big Bang. Our Universe had the appropriate physical constants that lead to the evolution of intelligent life. However, that evolution was not determined or required. There may exist many other universes with similar conditions, but where the emergent property of life or intelligence did not develop.
Hopefully a complete Theory of Everything will resolve the `how' questions on the origin of physical constants. But a complete physical theory may be lacking the answers to `why' questions, which is one of the reasons that modern science is in a crisis phase of development, our ability to understand `how' has outpaced our ability to answer if we `should'.
Origin of Life:
The Earth's crust became stable about 3.9 billion years ago. Life appeared around 3.6 to 3.9 billion years ago, which is quite fast in astronomical terms. Microfossils found in ancient rocks from Australia and South Africa demonstrate that terrestrial life flourished by 3.5 billion years ago. Older rocks from Greenland, 3.9 billion years old, contain isotopic carbon, carbon that could only have belonged to a living organism. The early atmosphere of the Earth was a secondary atmosphere from volcanic outgassing, very CO2-rich with little free O2.
Liquid water provides a universal solvent and warm environment for chemical evolution. It is a vehicle for dissolved substances (it circulates). And it provides the raw material for protein construction.
Amino acids are small, highly reactive molecules composed of 20 to 30 HCNO atoms. When amino acids link together in strings they form proteins. Proteins govern chemical reaction rates and form the structural material for cell parts.
With the construction of large macromolecules, such as proteins and nucleic acids, the Earth is poised for the next stage of biochemical evolution. Living organisms are the supreme example of active matter. They represent the most developed form of organized matter and energy that we know. They exemplify growth, adaptation, complexity, unfolding form variety and unpredictability. Almost appearing to be a class apart from matter and energy, defying the laws that enslave normal matter and energy.
Every organism is unique, both in form and development. Unlike physics where one studies classes of identical objects (e.g. electrons, photons), organisms are all individuals. Moreover, collections of organisms are unique, species are unique, the evolutionary history of the Earth is unique, the entire biosphere is unique. On the other hand, a cat is a cat, a cell is a cell, there are definite regularities and distinguishing features that permit organisms to be classified.
Each level of biology has new and unexpected qualities, qualities which cannot be reduced to the properties of the component parts, this is known as holism. A living organism consists of a large range of components differing greatly in structure and function (heart, liver, hair). Yet, the components are arranged and behave in a coherent and cooperative fashion as though to a common agreed plan. This endows the organism with a discrete identity, makes a worm a worm, a dog a dog.
No living thing exits in isolation. All organisms are strongly coupled to their inanimate environment and require a continual throughput of matter and energy as well as the ability to export entropy. From a physical and chemical point of view, every organism is strongly out of equilibrium with its environment. In addition, life on Earth is an intricate network of mutually interdependent organisms held in a state of dynamic balance. Then concept of life is fully meaningful only in the context of the entire biosphere.
A large number of complex chemical reactions is the underlying process that we call life. The ingredients for life are:
RNA and DNA are molecular codes for the production of proteins. They have the unique property of being self-replicating (when an RNA molecule splits, amino acids connect to the endpoints producing an exact copy of the original chain). The beginning of biochemical evolution was when RNA and DNA evolved to coat themselves in protein shells. These coated RNA and DNA packages are called a virus. A virus is halfway between life and non-life, being non-living when in isolation, but adapting living characteristics in interaction with other virus' or cells.
The next stage in biochemical evolution was for various virus' to take on specialized tasks (energy production, protein production, etc). These individual elements would combine to form the first cell. Our earliest evidence of cellular life comes from fossil bacteria.
With the development of cells, life took on an explosive evolution into more diverse forms, invading new environments (sea, lakes, land).
Oxygen is a very small component to outgassing on the Earth, yet O2 is a significant fraction of our current atmosphere (thank goodness). Also note that O2 is highly reactive and combines quickly with rock and soil to form oxides (rust). Thus, the current amount O2 requires a constant process of replenishment. That process is photosynthesis.