ua nt - p h / 05 10 22 3 v 4 1 J un 2 00 7 Foundations Of Quantum Theory Revisited ∗

After giving a panoramic view of the " textbook " interpretation of the new quantum mechanics, as a sequel to the old quantum theory, the conceptual basis of quantum theory since the Copenhagen Interpretation is reviewed in the context of various proposals since Einstein and Niels Bohr, designed to throw light on possible new facets bearing on its foundations, the key issues to its inherent " incomplete-ness " being A) measurement, and B) quantum non-locality. A related item on measurement, namely Quantum Zeno (as well as anti-Zeno) effect is also reviewed briefly. The inputs for the new facets are from some key Indian experts : The issues concerning the foundations of quantum theory mainly hinge on two aspects, namely, A) quantum measurement problem, and B) quantum non-locality. To appreciate these subtleties, it is first necessary to have an overview of its " standard picture " [1] which is summarized below. After the discovery by Max Planck of the existence of discrete quanta to account for the observed black-body spectrum, Quantum Theory got a big boost at the hands of Ein-stein himself who found a natural explanation of some crucial experimental observations on the photoelectric effect in terms of the particle nature of electromagnetic radiation, thus establishing its dual character. What was now needed was a viable atomic model to account for the discrete values of several measurable parameters of atomic systems, especially i) the Ritz classification of spectral lines in terms of the Rydberg-Ritz combination principle; and ii) Franck-Hertz experiment on the discrete energy losses of electrons on collision with atoms. To that end, the right background was provided by Rutherford's discovery of the atomic structure, and a crucial step was taken by Niels Bohr who postulated that i) atomic systems can only exist in certain quantized states, each corresponding to a well-defined energy, so that transitions between them are accompanied by radiation whose energy (E) equals their energy difference ; and ii) the frequency of the radiation quantum is equal to E/¯ h. These two postulates sufficed for an understanding of both the Rydburg-Ritz combination principle, and the Franck-Hertz experiment. Bohr's postulates received a further boost through a quantization rule discovered by Wilson and