All Conformal Effective String Theories are Isospectral to Nambu-Goto Theory

It is shown that all Polchinski-Strominger effective string theories are \emph{isospectral} to Nambu-Goto theory. The relevance of these results to QCD-Strings is discussed.Comment: 4 pages in REVTEX. Various typos fixed, the abstract and discussions modestly enlarged and presentation improved in v

Does the first part of the second law also imply its second part?

Sommerfeld called the first part of the second law to be the entropy axiom, which is about the existence of the state function entropy. It was usually thought that the second part of the second law, which is about the non-decreasing nature of entropy of thermally isolated systems, did not follow from the first part. In this note, we point out the surprise that the first part in fact implies the second part.Comment: 7 pages, 4 figures, prepared in JHEP styl

Three results on weak measurements

Three recent results on weak measurements are presented. They are: i) repeated measurements on a single copy can not provide any information on it and further, that in the limit of very large such measurements, weak measurements have exactly the same characterstics as strong measurements, ii) the apparent non-invasiveness of weak measurements is \emph{illusory} and they are no more advantageous than strong measurements even in the specific context of establishing Leggett-Garg inequalities, when errors are properly taken into account, and, finally, iii) weak value measurements are optimal, in the precise sense of Wootters and Fields, when the post-selected states are mutually unbiased with respect to the eigenstates of the observable whose weak values are being measured. Notion of weak value coordinates for state spaces are introduced and elaborated.Comment: 7 pages in Revtex, 2 figures, to appear in {\it Quantum Measurements} , Current Scienc

On Finite Size Effects in d=2d=2 Quantum Gravity

A systematic investigation is given of finite size effects in $d=2$ quantum gravity or equivalently the theory of dynamically triangulated random surfaces. For Ising models coupled to random surfaces, finite size effects are studied on the one hand by numerical generation of the partition function to arbitrary accuracy by a deterministic calculus, and on the other hand by an analytic theory based on the singularity analysis of the explicit parametric form of the free energy of the corresponding matrix model. Both these reveal that the form of the finite size corrections, not surprisingly, depend on the string susceptibility. For the general case where the parametric form of the matrix model free energy is not explicitly known, it is shown how to perform the singularity analysis. All these considerations also apply to other observables like susceptibility etc. In the case of the Ising model it is shown that the standard Fisher-scaling laws are reproduced.Comment: 9 pages, Late

On string momentum in effective string theories

Centre of mass momentum for strings is an important ingredient in the calculation of the spectrum of string theories. It is calculated by the N\"other prescription for two types of conformally invariant effective string theories of the Polchinski-Strominger type. One is the so called Polyakov-Liouville theory \cite{liouville} and the other an extension of the original Polchinski-Strominger action analysed upto order $R^{-3}$ where $2\pi R$ is the length of the string. In the first case analysis is carried out to order $R^{-2}$ and in the second case to order $R^{-3}$. In both cases the correction to the free bosonic theory result is shown to be of the {\em improvement} type so that the correction to the total string momentum, obtained by integrating over the spatial coordinate of the string world sheet, vanishes.Comment: 10 pages in LaTeX2e prepared in JHEP styl

The Superposition Principle in Quantum Mechanics - did the rock enter the foundation surreptitiously?

The superposition principle forms the very backbone of quantum theory. The resulting linear structure of quantum theory is structurally so rigid that tampering with it may have serious, seemingly unphysical, consequences. This principle has been succesful at even the highest available accelerator energies. Is this aspect of quantum theory forever then? The present work is an attempt to understand the attitude of the founding fathers, particularly of Bohr and Dirac, towards this principle. The Heisenberg matrix mechanics on the one hand, and the Schrodinger wave mechanics on the other, are critically examined to shed light as to how this principle entered the very foundations of quantum theory.Comment: 6 pages in LaTeX, will appear in proceedings of the conference '100 years of the Bohr Atom 1913-2013

On repeated (continuous) weak measurements of a single copy of an unknown quantum state

In this paper we investigate repeated weak measurements,without post-selection, on a \emph{single copy} of an \emph{unknown} quantum state. The resulting random walk in state space is precisely characterised in terms of joint probabilities for outcomes. We conclusively answer, in the negative, the very important question whether the statistics of such repeated measurements can determine the unknown state. We quantify the notion of error in this context as the departure of a suitably averaged density matrix from the initial state. When the number of weak measurements is small the original state is preserved to a great degree, but only an ensemble of such measurements, of a complete set of observables, can determine the unknown state. By a careful analysis of errors, it is shown that there is a precise tradeoff between errors and \emph{invasiveness}. Lower the errors, greater the invasiveness. Though the outcomes are not independently distributed, an analytical expression is obtained for how averages are distributed, which is shown to be the way outcomes are distributed in a \emph{strong measurement}. An \emph{error-disturbance} relation, though not of the Ozawa-type, is also derived. In the limit of vanishing errors, the invasiveness approaches what would obtain from strong measurements.Comment: 5 pages in RevTeX 4; in this latest version, the title has been modified a bit, abstract cleaned up and a note added about a work by Tamir, Cohen and Priel that appeared subsequent to my work addressing related issue

A critique of Sadi Carnot's work and a mathematical theory of the caloric

In this work, Sadi Carnot;s fundamental work is critically examined, and contrasted with modern thermodynamics. A mathematical theory of his work is given on the basis of the observation that in caloric theory dQ is a perfect differential.Comment: 17 pages, 8 figures, prepared in JHEP styl

Unknown single oscillator coherent states do have statistical significance

It is shown, contrary to popular belief, that {\it single unknown} oscillator coherent states can be endowed with a {\em measurable statistical significance}.Comment: 4 pages in Revte

Killing vectors of FLRW metric (in comoving coordinates) and zero modes of the scalar Laplacian

Based on an examination of the solutions to the Killing Vector equations for the FLRW-metric in co moving coordinates , it is conjectured and proved that the components(in these coordinates) of Killing Vectors, when suitably scaled by functions, are \emph{zero modes} of the corresponding \emph{scalar} Laplacian. The complete such set of zero modes(infinitely many) are explicitly constructed for the two-sphere. They are parametrised by an integer n. For $n\,\ge\,2$, all the solutions are \emph{irregular} (in the sense that they are neither well defined everywhere nor are \emph{square-integrable}). The associated 2-d vectors are also \emph{not normalisable}. The $n=0$ solutions being constants (these correspond to the zero angular momentum solutions) are regular and normalizable. Not all of the $n=1$ solutions are regular but the associated vectors are normalizable. Of course, the action of scalar Laplacian coordinate independent significance only when acting on scalars. However, our conclusions have an unambiguous meaning as long as one works in this coordinate system. As an intermediate step, the covariant Laplacians(vector Laplacians) of Killing vectors are worked out for four-manifolds in two different ways, both of which have the novelty of not explicitly needing the connections. It is further shown that for certain maximally symmetric sub-manifolds(hypersurfaces of one or more constant comoving coordinates) of the FLRW-spaces also, the scaled Killing vector components are zero modes of their corresponding scalar Laplacians. The Killing vectors for the maximally symmetric four-manifolds are worked out using the elegant embedding formalism originally due to Schr\"odinger . Some consequences of our results are worked out. Relevance to some very recent works on zero modes in AdS/CFT correspondences , as well as on braneworld scenarios is briefly commented upon.Comment: 30 pages in JHEP style. In this third revision we have revised the title, abstract, and the body, to stress the coordinate system used, as well as remove the confusion between components and vectors in the earlier version. The zero modes in greater detail using the notions of well-definedness, square integrability and normalizabilit