S. Sengupta’s research while affiliated with Jadavpur University and other places

It is argued that two distinct types of complementarity are implied in Bohr's complementarity principle. While in the case of complementary variables it is the quantum mechanical uncertainty relation which is at work, the collapse hypothesis ensures this exclusiveness in the so-called wave-particle complementarity experiments. In particular it is shown that the conventional analysis of the double slit experiment which invokes the uncertainty principle to explain the absence of the simultaneous knowledge of the which-slit information and the interference pattern is incorrect and implies consequences that are quantum mechanically inconsistent.

The complex quantum-classical relationship is reviewed and the inadequacy of quantum mechanical wavefunction description for the centre of mass motion of a macroscopic system is discussed. The emergence of manifest classical reality in this case is analyzed and we interpret the unphysical infinitely rapid oscillations of the wavefunction near the classical regime observed in our earlier studies as the breakdown of wavefunction description for the normal macroscopic mass domain above 10−15 g. It is contended that, production of quantum interference with large macromolecules like viruses (m ∼ 10−12 g), as proposed by some authors, is impossible. Another testable prediction asserts that particle tracks, whenever observed, will obey classical law and offers an interesting experimental verification.

The authors discuss the nature of deformation of the electron charge cloud of an ion implied in the deformable shell model and considers an application of the model to the case of LiF crystals. It is found that the properties of the crystal are fairly well reproduced by the present model and certain discrepancies noted by earlier authors are removed. The results of other theoretical investigations are also included for comparison.

The significance of Ehrenfest Theorem in quantum-classical relationship is discussed in terms of the general formulation of the theorem. With the ensemble interpretation of the quantum mechanical $\Psi$-function the Generalised Ehrenfest Theorem reveals some interesting exact relationships between quantum- and classical expectation values. These general results seem to imply a deep rooted unity (inspite of apparent radical differences in conceptual structures) between classical and quantum mechanics. Some significant consequences and important physical insights which follow from the general formulation are discussed with examples. Most important is that it offers, under reasonable approximations, a pure quantum mechanical description of the Stern-Gerlach experiment with realistic inhomogeneous magnetic field $(\nabla.{\bf B} = 0)$.

According to Bohr’s complementarity principle, two distinct types of complementarity exist-one of complementary variables and other in the so-called wave-particle complementarity experiments. Some authors have claimed that mutual exclusiveness (ME) in both the cases arise due to uncertainty principle and have analysed the second type in terms of Fourier space analysis and consequent putative “momentum kick” distribution. Some others, on the other hand, have identified the collapse hypothesis as the actual quantum mechanical principle responsible for ME in the interferometry experiments. In this paper the momentum space analysis is thoroughly examined vis-a-vis the general quantum mechanical description in terms of the changes in the wave function. It is argued that such alternative explanations are not in full conformity with the strict quantum mechanical description.

The classical and quantum physics seem to divide nature into two domains macroscopic and microscopic. It is also certain that they accurately predict experimental results in their respective regions. However, the reduction theory, namely, the general derivation of classical results from the quantum mechanics is still a far cry. The outcome of some recent investigations suggests that there possibly does not exist any universal method for obtaining classical results from quantum mechanics. In the present work we intend to investigate the problem phenomenonwise and address specifically the phenomenon of scattering. We suggest a general approach to obtain the classical limit formula from the phase shiftδ l, in the limiting value of a suitable parameter on whichδ l depends. The classical result has been derived for three different potential fields in which the phase shifts are exactly known. Unlike the current wisdom that the classical limit can be reached only in the high energy regime it is found that the classical limit parameter in addition to other factors depends on the details of the potential fields. In the last section we have discussed the implications of the results obtained.

Conditions are examined under which the quantum-mechanical travel time across a potential barrier goes over to the classical result. It is found that the limiting process is quite complex and sensitive to many details such as the nature of the incident state vector and the analytic properties of the potential barrier. An interesting result is that in the present problem the classical limit is reached for a wave packet that is sharply localised in momentum space only and is insensitive to the spatial extension. This is in sharp contrast to the results we obtained for scattering. Another significant conclusion is that the classical limit may converge either to that of a single classical particle or of a classical statistical ensemble depending on the nature of the initial wave packet.

The passage to the classical result from the quantum-mechanical theory of scattering is investigated. Two concrete cited examples clearly indicate that classical physics is not retrieved in the high-energy limit. It is contended that the approach to this limit must envisage a modification of the incident wave function and not the energy. Then explicit analytical results which show the dependence of the scattering cross-sections on the nature of the incoming wave packets are derived. The first-order corrections arising out of the finite width of the two types of incident wave packets considered in the present work contain interesting new physics. The insight gained from these results together with the inclusion of the effect of the spreading of the wave packet suggests a more comprehensive formulation of Bohm’s criteria for the validity of the classical approximation in scattering. The augmented criteria provide significant clarification and revision of our present understanding of the classical limit situation of the scattering phenomena. The implications and possible applications of the new results are discussed.

A Green's function calculation is presented for dilute concentration isovalent substitutional defects in alkali halides. A simple model is suggested to extend this calculation to the high concentration range. Comparing the values of lattice relaxation obtained from this estimation to EXAFS measurement, a very good agreement is noticed over the entire composition range. Es wird eine Berechnung mittels Greenscher Funktionen für verdünnte isovalente Substitutionsdefekte in Alkalihalogeniden angegeben. Ein einfaches Modell wird vorgeschlagen, das diese Berechung auf den Bereich hoher Konzentrationen erweitert. Durch Vergleich der aus dieser Berechung erhaltenen Werte der Gitterrelaxation mit EXAFS-Messungen wird gute Übereinstimmung über den gesamten Zusammensetzungsbereich gefunden.