Professor T. Padmanabhan
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Item Radiation from collapsing shells, semiclassical backreaction, and black hole formation(American Physical Society, 2009-08-14) Paranjape, Aseem; Padmanabhan, T.We provide a detailed analysis of quantum field theory around a collapsing shell and discuss several conceptual issues related to the emission of radiation flux and formation of black holes. Explicit calculations are performed using a model for a collapsing shell which turns out to be analytically solvable. We use the insights gained in this model to draw reliable conclusions regarding more realistic models. We first show that any shell of mass M which collapses to a radius close to r=2M will emit approximately thermal radiation for a period of time. In particular, a shell which collapses from some initial radius to a final radius 2M(1-ε²)-¹ (where ε łl 1) without forming a black hole, will emit thermal radiation during the period M≲ t ≲ Młn (1/ε²). Later on (tgg M łn(1/ε²)), the flux from such a shell will decay to zero exponentially. We next study the effect of backreaction computed using the vacuum expectation value of the stress tensor on the collapse. We find that, in any realistic collapse scenario, the backreaction effects do emphnot prevent the formation of the event horizon. The time at which the event horizon is formed is, of course, delayed due to the radiated flux -- which decreases the mass of the shell -- but this effect is not sufficient to prevent horizon formation. We also clarify several conceptual issues and provide pedagogical details of the calculations in the Appendices to the paper.Item Semiclassical approximations for gravity and the issue of backreaction(IOP Publishing, 1988-08-28) Padmanabhan, T.Semiclassical approximations, which are useful in the study of a quantum system interacting with a classical system, are studied and compared. A toy quantum mechanical model with two degrees of freedom (which mimics the features of gravity interacting with quantum fields) is used for illustration. In particular, we consider the Born-Oppenheimer approximation (BOA) (corresponding to G-O at fixed h), the effective action approach (h - 0 at fixed G) and their combinations. We show that in the strict BOA limit there is no backreaction on gravity. Gravity is described by classical equations and the fields are quantised in that background. In the effective action approach one can obtain a semiclassical description for gravity, if certain stringent requirements are satisfied. In most situations of interest these conditions will not be met and the O(h) contribution from gravitons will be comparable to that from quantum fields. We study the system using both the Schrodinger equation and path integrals and indicate the correspondence.