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.
gravitational constant: Determines strength of gravity. If lower than stars would have insufficient pressure to overcome the Coulomb barrier to start thermonuclear fusion (i.e. stars would not shine). If higher, stars burn too fast, use up fuel before life has a chance to evolve.
strong force coupling constant: Holds particles together in the nucleus of atoms. If weaker than multi-proton particles would not hold together, hydrogen would be the only element in the Universe. If stronger, all elements lighter than iron would be rare. Also radioactive decay would be less, which heats the core of the Earth.
electromagnetic coupling constant: Determines strength of electromagnetic force that couples electrons to the nucleus. If less, than no electrons would be held in orbit. If stronger, electrons will not bond with other atoms. Either way, no molecules.
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 equally 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 worse 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 1, which is strange since 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).
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 is the infamous many-worlds hypothesis used to explain how the position of an electron can be fuzzy or uncertain. 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.
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 (Grand Unification Theory) unification point, space-time 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.
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'.
Another recent attempt to form a TOE (Theory of Everything) is through M (for membrane) or string theory. String theory is actually a high order theory where other models, such as supergravity and quantum gravity, appear as approximations. The basic premise to string theory is that subatomic entities, such as quarks and forces, are actually tiny loops, strings and membranes that behave as particles at high energies.
One of the problems in particle physics is the bewildering number of elementary particles (muons and pions and mesons etc.). String theory answers this problem by proposing that small loops, about 100 billion billion times smaller than the proton, are vibrating below the subatomic level and each mode of vibration represents a distinct resonance which corresponds to a particular particle. Thus, if we could magnify a quantum particle we would see a tiny vibrating string or loop.
The fantastic aspect to string theory, that makes it such an attractive candidate for a TOE, is that it not only explains the nature of quantum particles but it also explains spacetime as well. Strings can break into smaller strings or combine to form larger strings. This complicated set of motions must obey self-consistent rules and the constraint caused by these rules results in the same relations described by relativity theory.
Another aspect of string theory that differs from other TOE candidates is its high aesthetic beauty. For string theory is a geometric theory, one that, like general relativity, describes objects and interactions through the use of geometry and does not suffer from infinities or what is called normalization problems such as quantum mechanics. It may be impossible to test the predictions of string theory since it would require temperatures and energies similar to those at the beginning of the Universe. Thus, we resort to judging the merit of this theory on its elegance and internal consistence rather than experimental data.
The latest compulation of string theory and supergravity that brings together a multi-dimensional world view with the Standard Model is called brane world. In the brane world scenario, the entire Universe is an eleven dimensional 'bulk' composed of an infinite number of ten dimensional branes. Each brane is consists of a macroscopic 4D spacetime and a compacted, microscopic 6D quantum world (a Calabi-Yau space). The Calabi-Yau sector is twisted beyond all possible detection and houses all the symmetries of the Standard model.
The attractive aspect to the brane world scenario is that three of the four fundamental forces (strong, weak, electromagnetism and their assoicated particles) are represented by open strings. Gravity (and gravitons) are represented by closed strings (loops). Open strings are attached to their respective branes, but closed strings (gravity) are free to move between branes. This explains why gravity is so much weaker than the other forces, and gravity can be used to commiuncate between the branes (gravity phones). This also leads to a natural explanation for dark matter.
All the branes are embedded in a higher dimension all the 'bulk', a pile of parallel universes. Thus, the total Universe is eleven dimensional, 4D spacetime + 6D quantum space + 1D bulk. An infinite number of parallel universes is literally just a millimeter away, but outside your 4D vision.
Even a GUTS is incomplete because it would not include spacetime and therefore gravity. It is hypothesized that a ``Theory of Everything'' (TOE) will bring together all the fundamental forces, matter and curved spacetime under one unifying picture. For cosmology, this will be the single force that controlled the Universe at the time of formation. The current approach to the search for a TOE is to attempt to uncover some fundamental symmetry, perhaps a symmetry of symmetries. There should be predictions from a TOE, such as the existence of the Higgs particle, the origin of mass in the Universe.
One example of a attempt to formula a TOE is supergravity, a quantum theory that unities particle types through the use of ten dimensional spacetime (see diagram below). Spacetime (4D construct) was successful at explaining gravity. What if the subatomic world is also a geometric phenomenon.
The idea that gravity is different from the other forces. While it is extremely long ranged, gravity is extremely weak (notice that a small magnet on the refrigerator can overcome the entire gravitational force of the Earth). Perhaps gravity is weak because it is 'spread' out over many dimensions. This type of theory is called brane or membrane theory and hypothesizes that the 3D world is just one splice of a multi-dimensional space where gravity propagates between the branes (slices).
In fact, the brane theory doesn't necessary have to use stacks of other, 'nearby' dimensions, the other dimensions could be small and 'curled' into the microscopic world. Many more dimensions of time and space could lie buried at the quantum level, outside our normal experience, only having an impact on the microscopic world of elementary particles.
It is entirely possible that beneath the quantum domain is a world of pure chaos, without any fixed laws or symmetries. One thing is obvious, that the more our efforts reach into the realm of fundamental laws, the more removed from experience are the results.