The force that the ship moves and you don't move as fast. You would smash against the walls of the ship.
Einstein proved along with many other scientists that it is physically impossible for something with mass to travel at the speed of light.
Creating a machine, that could resist the gravitational pull.
Creating a machine that makes it so you don't have to move (fast) to time travel.
There are many more possibilities, but I cannot name them all.
The time is