Principle of Relativity

Time and distance are not fixed. They are relative. The speed of light is always at 186,000 miles per second. Einstein proved that you can only move through time in one direction and that is because you can't go in two opposite directions at the same time. Maybe it is because time is moving forwards, not backwards.

The motion of the object is called a uniform translation (“uniform” due to constant velocity and direction, “translation” because although the object changes its position relative to a surrounding object yet it does not rotate doing so). We can observe from fixed co-ordinates that the object is uniform and in a straight line. If we were to observe the same object from a moving co-ordinate, we should find that the motion of the object would be different in velocity and direction, but that it would still be uniform and in a straight line. The co-ordinate system is based on position in relation to space and time. If an object is moving uniformly with respect to a co-ordinate system K, then it will also be moving uniformly relative to a second co-ordinate system K', provided that the second co-ordinate system is executing the same motion with respect to the first co-ordinate.	1
Basically, if the co-ordinate is a co-ordinate system, the mechanical laws of that system work with each and every co-ordinate in the system. The mechanical laws stay the same throughout.	2
This is called the principle of relativity.	3
One thing I found interesting reading this stuff is that velocities can be added to create a new velocity based on the co-ordinates, like speed and distance. So if a guy on a train that is going 50 mph throws a ball that leaves his hand at 20 mph, the ball is now going at 70 mph. That's about all I can understand when it comes to that theory. I understand the principle of relativity itself and the addition of velocities, but everything else is too complex for me. It's all scientific formulas represented by variables. Even if I was shown the formula that proves time travel is possible, I wouldn't understand it.