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Recently, the gravitational collapse of an infinite cylindrical thin shell ofmatter in an otherwise empty spacetime with two hypersurface orthogonal Killingvectors was studied by Gon\c{c}alves [Phys. Rev. {\bf D65}, 084045 (2002).]. Byusing three "alternative" criteria for trapped surfaces, the author claimed tohave shown that {\em they can never form either outside or on the shell,regardingless of the matter content for the shell, except at asymptoticalfuture null infinite}.  Following Penrose's original idea, we first define trapped surfaces incylindrical spacetimes in terms of the expansions of null directions orthogonalto the surfaces, and then show that the first criterion used by Gon\c{c}alvesis incorrect. We also show that his analysis of non-existence of trappedsurfaces in vacuum is incomplete. To confirm our claim, we present an examplethat is a solution to the vacuum Einstein field equations and satisfies all theregular conditions imposed by Gon\c{c}alves. After extending the solution tothe whole spacetime, we show explicitly that trapped surfaces exist in theextended region.Comment: latex, 2 figures, the last version to appear in Phys. Rev.

Comment on “Absence of trapped surfaces and singularities in cylindrical collapse”