Basically: In an infinite universe; you are never moving, anywhere: You are only ever moving in relation to some object near you. So, you can measure the speed of light against that. And if you are traveling in the opposite direction of something traveling half the speed of light, and you are...
What you suggest is not the problem. You can create conditions in which velocity does not effect matter within a defined space. If you know what you are doing.
Not this tired old argument.
You cannot `go back in time`; as time is a measuring stick (and just like the `temperature` of `something measured` is not altered by changing the numbers on the side of the gauge); and as its only as tangible as a shadow.
You would also have to reverse every atom...
You could create fuel via Solar or via magnetism from passing whatever or temperature chage. etc...
One Limitation, is that sooner or later you will bump into something; and you cannot turn mass (to miss objects such as space rocks) at very high velocity. And in an infinite universe with an...