Time traveling is a common trope used in sci-fi and other fictional genres. Time travel has given rise to fantasy concepts such as traveling far into the future to view a new technologically evolved planet or traveling back in time to affect the future or meet a historical figure. The passage of time, however, appears to be an immovable, unchangeable force in the real world.

Newton's long-standing theory of absolute time was refuted by Einstein's theory of relativity, arguing that time is a relative concept. Depending on how observers move in space, from two events one observer may see one event happen before the other but another observer may see the other event happen first. Both observers are equally correct. General relativity argued that anything with a mass creates bends in the surface of space-time. 

Einstein’s theory of relativity demonstrated relativistic time dilation. The faster you go, the lesser time you use. This can be seen in real life with astronauts in the international space station. 6 months on the space station would lead to someone aging 0.05 seconds less than someone on the ground. Is this time-traveling? Imagine if you were traveling through the universe at close to the speed of light. You spend 10 years traveling and eventually return to earth, but 50 years have passed. But what about time travel to the past?

The only thing in general relativity stopping backward time travel is the light-speed barrier. If a line represents the singular path you take through the universe, a light cone around it represents all the possible paths you could have taken. This is the furthest you can be from a point after a certain amount of time. Unless you can go faster than the speed of light, you cannot go past this light cone. To go backward, you have to go past this cone. However, time travel may be possible with a wormhole, bending space-time to close the distance. 

What do we do about time travel paradoxes? Quantum Physics may answer this. A famous example is Schrodinger's cat where a cat in a box has a 50% chance of dying. Quantum physics suggests that for every event with multiple outcomes, all outcomes happen. Until the box is opened and the universe “chooses'' one outcome to be a reality, the cat is both alive and dead. When you travel back, you create a new time stream, logically all your events are still your past.

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