This has been a necessarily abbreviated and condensed foray into the wonderful, and sometimes bizarre, world of quantum mechanics. If a couple of fundamental principles of quantum physics were to be singled out from all of the above, they would probably be the dual wave-like and particle-like behaviour of matter and radiation, and the prediction of probabilities in situations where classical physics predicts certainties.
For many, even those in the scientific community, these are difficult concepts to come to terms with, and even Albert Einstein had serious philosophical problems with a universe which behaves in an apparently totally random manner at the sub-atomic level, repeatedly claiming that “God does not play dice” (although Einstein is widely considered to have “lost” the extensive public debates he carried on with Niels Bohr on the subject). For a better or more comprehensive understanding of this complex and confusing subject, there is a copious amount of literature on the subject, both for the beginner and the expert alike, a few of which are mentioned on the Sources page.
Despite its difficulties, however, quantum theory remains an essential part of the bedrock of modern physics. It is arguably one of the most successful theories in all of science, and, despite its seemingly esoteric nature, it is primarily a practical branch of physics, paving the way for applications such as the laser, the electron microscope, the transistor, the superconductor and nuclear power, as well as explaining at a stroke important physical phenomena such as chemical bonding, the structure of the atom, the conduction of electricity, the mechanical and thermal properties of solids and the density of collapsed stars.
However, successful as it is in predicting and describing the world around us, quantum theory only successfully explains three of the four fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force. It does not explain the workings of gravity.
As has been mentioned in other sections (see here, for example), the way forward for physics seems now to rest with attempts to combine quantum theory with the General Theory of Relativity in a unified theory of quantum gravity (or quantum theory of gravity), the so-called “theory of everything”, which it is hoped will make sense of the entire universe. Candidates like superstring theory and loop quantum gravity, however, still need to overcome major formal and conceptual problems before such a claim can be made.
In parting, let me just mention one other interesting field of speculation related to quantum theory. It has been suggested that, if the whole universe (space, time, energy and everything else) is quantized and consists of indivisible fundamamental particles, then it has a finite number of components and a finite number of states, like the bits and pixels of a computer program. In theory, this makes the universe "computable", and has led some to hypothesize that perhaps all of reality as we perceive it might actually be part of a huge Matrix-like computer simulation, individual parts of which only assume definite form when observed. Speculation only, perhaps, but an intriguing one nonetheless, and one which seems increasingly difficult to disprove as the details of quantum theory are ironed out.