Interstellar space is not empty It is fulfilled with photons and particle dusts and a ship at superluminal speeds would impact these obstacles in highly energetic collisions disrupting the warp field and placing the astronauts in danger.
This was pointed out by a great number of authors like Clark,Hiscock,Larson,McMonigal,Lewis,O’Byrne, Barcelo,Finazzi and Liberati.
In order to travel significant interstellar distances in reasonable amounts of time a ship would need to attain 200 times the speed of light but according to Clark,Hiscock and Larson the impact between the ship and a single photon of Cosmic Background Radiation(COBE)would release an amount of energy equal to the photosphere of a star like the Sun.And how many photons of COBE we have per cubic centimeter of space?This serious problem seems to have no solution at first sight.
However some years ago Harold White from NASA appeared with an idea that may well solve this problem:
According to him the ship never surpass the speed of light but the warp field generates a Lorentz Boost resulting in an apparent superluminal speed as seen by the astronauts on-board the ship and on the Earth while the warp bubble is always below the light speed with the ability to manoeuvre against these obstacles avoiding the lethal collisions.
In this work we examine the feasibility of the White idea using clear mathematical arguments and we arrived at the conclusion that Harold White is correct