Research Papers (TP)
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Item Why does an accelerate charge radiate?(Indian Academy of Sciences, 2009-05-15) Padmanabhan, T.The fact that an accelerated charge radiates energy is considered an elementary textbook result in electromagnetism. Nevertheless, this process of radiation (and its reaction on the charged particle) raises several conundrums about which technical papers are written even today. In this installment, we will try to understand why an accelerated charge radiates in a simple, yet rigorous, manner.Item Why are black holes hot?(Indian Academy of Sciences, 2008-05-11) Padmanabhan, T.One of the celebrated results in black hole physics is that black holes have a temperature and they emit a thermal spectrum of radiation. Though a rigorous derivation of this result requires quantum field theory, a flavour of the essential ideas can be provided at an elementary level as indicated here.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 Radiation from a charged particle and radiation reaction reexamined(American Physical Society, 1998-05-22) Gupta, Abhinav; Padmanabhan, T.We study the electromagnetic fields of an arbitrarily moving charged particle and the radiation reaction on the charged particle using a novel approach. We first show that the fields of an arbitrarily moving charged particle in an inertial frame can be related in a simple manner to the fields of a uniformly accelerated charged particle in its rest frame. Since the latter field is static and easily obtainable, it is possible to derive the fields of an arbitrarily moving charged particle by a coordinate transformation. More importantly, this formalism allows us to calculate the self-force on a charged particle in a remarkably simple manner. We show that the original expression for this force, obtained by Dirac, can be rederived with much less computation and in an intuitively simple manner using our formalism.Item Possible cosmological scenario with an unstable 17 Kev neutrino(Nature Publishing Group, 1985-09-26) Padmanabhan, T.; Vasanthi, M. M.