One of the most famous aspects of quantum theory is its understanding of complementarity, and the relationship of seemingly conflicting things in a coherent reality. The famous double slit experiment demonstrates the wave character and the particle character of quantum entities, such as electrons. Classically, something is either a wave or a particle. Quantum theory, and specifically quantum field theory, relates wave and particle so that both are held together coherently. This is done through the understanding of �superposition,� that an electron, for instance, is not understood to have a location until it is experimentally determined, instead containing a superposition. In the double slit experiment, this means that an individual electron acts like it goes through both slits, and not one or the other. Though this may be counter-intuitive, it is a well-documented and understood phenomenon of quantum physics. As Polkinghorne emphasizes, the apparent paradox of wave/particle duality has been perfectly understood in Dirac�s quantum field theory. �It is the case that we have a theory that combines wave and particle models without taint of paradox and which is open to our rational inspection.�
Another important insight of quantum theory that merits mentioning is the Heisenberg uncertainty principle. This principle states that (for example) an electron�s position and momentum cannot both be known. It is possible to determine by experiment either where an electron is (position), or what it is doing (momentum), but it is impossible to determine both where it is and what it is doing. This principle gives insight into the �fuzziness� of the quantum world.
In briefly outlining these two aspects of quantum theory, we have begun to give an introduction into the type of world that is understood to exist in quantum theory. It must be acknowledged that many important aspects of the quantum world have been neglected in this introduction, such as non-locality. The important point to take away from this very brief sketch of quantum theory is that it doesn�t work like Newtonian physics. The quantum world cannot be easily understood through diagrams, and does not act like billiard balls on a pool table.
