[QuadmasterXLII]

observers moving relative to each other agree on what “the speed of light” is, but disagree about what “2 x the speed of light” is. Specifically, they disagree about which superluminal trajectories are forward in time vs backwards in time. Therefore, if you have a device that accelerates you to 2x the speed of light, it must accelerate you to 2x the speed of light in your own reference frame. As a result, you can accelerate to .99 c relative to earth using conventional means, activate the device, and then be travelling at 2x the speed of light in the .99c frame, which is backwards in time in the earth frame. You could then stop, turn around, and repeat the procedure to arrive at earth before you left.

The Alcubierre solution avoids this by being symmetrical, so that observers don’t have to agree on which direction is the destination and which is the origin