The Krasnikov Tube: Energy Cost of a Permanent Faster-Than-Light Corridor

A Krasnikov tube is a corridor of modified spacetime, laid down by a traveller moving near light speed, along which the return journey can be made arbitrarily fast. Integrating the wall energy density over a cylindrical shell for a tube one light-year long gives 2.86e+58 joules for a one-metre radius, 2.86e+58 joules for ten metres, the latter equal to 1.60e+11 solar mass-energies, with a wall density of 4.82e+40 joules per cubic metre at ten metres. Unlike a warp bubble the tube is built once and reused, which amortises the cost over every later crossing, its single genuine advantage. The verdict remains negative on two grounds: the construction energy is stellar in scale and negative in sign, and causality analysis shows the tube cannot be used to travel into its own past, removing the time-machine application sometimes claimed for it. Computed schematically, not measured.