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 = 2 M will emit approximately thermal radiation for a period of time .
In particular , a shell which collapses from some initial radius to a final radius 2 M ( 1 - \epsilon ^ { 2 } ) ^ { -1 } ( where \epsilon \ll 1 ) without forming a black hole , will emit thermal radiation during the period M \lesssim t \lesssim M \ln ( 1 / \epsilon ^ { 2 } ) .
Later on ( t \gg M \ln ( 1 / \epsilon ^ { 2 } ) ) , 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 not 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 .