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When I was a graduate student majoring in physics, I was an atheist. To me, God was a product of ignorance, due to lack of scientific knowledge. If people understood natural laws as well as physicists, they would not believe the existence of God. At that time, I did not know that most great physicists are God's believers, such as Newton, Kepler, Planck, Heisenberg and Einstein.
Ten years after I received my Ph.D. in physics, I became a Christian - not because I found any hard scientific evidence about God. Like most believers, I felt that there must be a God who had been guiding me, otherwise some events that had happened to me would not be so coincidental.
The existence of God was so real that I had been wondering: where could God be? why can't we see Him? how come modern technology cannot detect God? Since I was trained in physics, I believed that all natural phenomena should have a physical explanation. If God really existed, these questions could be answered in terms of physical law. However, I did not know where or how to find the scientific answer. It might be impossible to know the answer at all.
In 2000, almost ten years after I became a Christian, I happened to read an article about string theory. My specialty was in biophysics, not in theoretical physics, but I had great interest in our universe - especially new concepts. The string theory just offered a completely new concept about our universe. After I read that article, I was thrilled. This must be it! God must be there! He could be only a centimeter away from us without being detected. Physicists have discovered the mechanism that prevents us from seeing the Kingdom of God!
The string theory has been developed for several decades, with a goal to become "the theory of everything", that is, to unify all four fundamental forces - gravity, electromagnetism, strong and weak nuclear forces. A revolutionary discovery of the theory is that the whole universe should have nine or ten dimensions of space, instead of three dimensions (length, width and height). Then, why do we see only three dimensions? In the earlier version of the string theory, it was assumed that those extra dimensions were too small to be observed. Thus, the whole universe is essentially the same as the 3D world. If this were true, there would be no place for God. Fortunately, a few years before 2000, researchers began to realize that those extra dimensions could be as large as the ordinary three dimensions. The reason why we cannot see extra dimensions is because all matter and electromagnetic waves (photons) are confined in a three dimensional sub-universe called "braneworld".
At present, the major messenger we use to see or detect things is the electromagnetic waves which include radio waves, microwaves, infrared, visible light, ultraviolet, and x-ray. Since the electromagnetic waves are confined in our braneworld, this immediately explains why we cannot detect God if He lives outside our braneworld. The relationship between our braneworld (three dimensional) and the whole universe (ten dimensional) is like a TV screen (two dimensional) in our house (three dimensional). Suppose there is a creature confined on the TV screen, he cannot see us even if we are only a centimeter away from him. Similarly, if God lives in the ten dimensional space but outside our braneworld, we cannot see Him even if He is only a centimeter away from us.
What is the mechanism that confines matter and photons in our braneworld?
We know that all matter in our world is composed of atoms. An atom consists of a nucleus surrounded by electrons. A nucleus contains protons and neutrons, which are made up of quarks. The electron and quarks are elementary particles which cannot be divided further. In addition to the elementary particles that constitute matter, there is another type of elementary particles that mediate the fundamental force between matter particles. For example, the electromagnetic force is mediated by photon, the gravity is mediated by graviton. You may have learned that the magnetic force is mediated by the magnetic field. Similarly, you can say that the electromagnetic force is mediated by the electromagnetic field. In quantum physics, a "field" can also be viewed ("quantized") as a particle. The photon is the particle name of the electromagnetic field. The graviton is the particle name of the gravitational field. A field may also be static (like a magnetic field) or dynamic (such as the electromagnetic waves).
According to the string theory, an elementary particle can be considered as a "string" with extremely short length. All properties (mass, energy, etc.) of an elementary particle are inherent in the string's vibrational patterns. The particle string is analogous to a violin string whose vibrations produce various music notes. Similarly, different vibrational patterns of a particle string give rise to different properties. The vibrational patterns depend on the endpoints of the string. The particle string can be either "open" (with two endpoints, like a violin string) or "closed" (no endpoints, like a ring). In contrast to a violin string whose endpoints are fixed on a wooden board, the endpoints of a particle string are fixed in a "brane" (as in membrane), which is a space with two, three, or more dimensions. Since its endpoints are fixed, the open-string particle cannot escape from the brane. However, the closed-string particle can go anywhere because it has no endpoints to be fixed. Mathematically, different endpoints have different boundary conditions that are used in solving the equations for the vibrational patterns.
The properties of elementary particles in our world are known from experiments. The vibrational patterns of a particle string can be obtained by solving the fundamental equation in the string theory with appropriate boundary conditions (corresponding to endpoints). It has been found that, in order to match particle properties with vibrational patterns, the graviton must be a closed string, while all other particles must be an open string with endpoints fixed in a three-dimensional brane. This predicts that all matter and photons must be trapped in a three-dimensional braneworld. The only thing that can escape from the braneworld is the graviton.
What about the Kingdom of God in the higher dimensional space (the "bulk universe")? Their matter is likely to be made of the elementary particles with closed string, thereby free to go anywhere. Their particles must not have any charge that interacts with photons, because the photon is an open-string particle which is unique to our braneworld. The electromagnetic waves (photons) will pass through their matter without being reflected at all. Therefore, even if their matter (including life) enters into our braneworld, it still cannot be detected by electromagnetic waves.
All our matter particles can interact with the graviton, which is a closed string. It is very likely that the matter in the Kingdom of God can also interact with the graviton. Hence, in principle, God could be detected by gravitational waves (gravitons). However, the gravitational waves are extremely weak. Physicists have not been able to generate detectable gravitational waves yet. In the future, when we have better scientific knowledge and technology, we may be able to prove the existence of God by using gravitational waves to detect Him directly.