Events joined by a space-like interval can never influence each other, since that would imply a flow of information at speeds faster than the speed of light. Space-time diagrams can also be used to represent world lines that travel into the past instead of the future. As shown in the illustration to the right a world line can loop back on itself. In this example the world line curves back and comes close to itself. On a world line it is important to remember that the direction of the world line is in the direction of the local future of the particle.
Events happen in a sequence and direction of the world line.
If the particle is a living person then memories would also be formed in the direction of the world line. As soon as the loop bends more than 45 degrees away from the time axis the particle must be traveling at a faster than light speed. By understanding and using curved space-time. There are two important points here that we will use later to show how reverse time travel is indeed possible at sub-light speed. First, general relativity shows that space-time can be curved by heavy gravity.