# A simple example of how “relative time” implies that time travel is effectively impossible.

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As an example let us consider an event involving 2 particles A and B, moving away from a point or planet P, perhaps after an explosion. Let us consider it from the perspective of A moving relative to P, from which we observe that B is also moving away from P.

The notion of “universal time” suggests the idea that if time were to be “reversed”, then every process involving time would be reversed. A would move back toward P, as would B, and they would do so consistently along a single “time line”.

However, we know that universal time is not real, and that time is in fact relative. The aspect of that which is important in this example is that time according to A is not the same as time according to B.

Suppose that A did in fact reverse direction and began moving back toward P. Suppose that it's possible to consider this in a way where we can't distinguish between the reversal of time between A and P, vs a simple reversal of direction of travel of A relative to P. For all intents and purposes, a simple enough particle A moving back toward a simple enough particle P might be considered time travel backwards.

However, the time defined by A and P is independent of the time between B and P. What is done to affect the former does not necessarily affect the latter. So while time can be considered going in reverse for A and P, particle B is continuing to move away from P, which we would call “forward in time”.

The same applies to any particles C, D, etc. So suppose we define a clock at P between particles P and C (or any set of particles that we wish). Manipulation of the relative time between A and P would not affect the relative time measured by P and C etc. So while A can be considered moving back in time toward P, that doesn't affect the time measured by the clock at P. A can move back in time and return to a former state of P relative to A, yet it cannot return to a former state of P relative to B, C, etc. According to A, P has continued moving forward in time according to everything else, including its own clocks.

Thus the effect of any sort of time travel involving A and P will have no noticeable effect in a complex enough system involving multiple particles, or particles with their own internal time-related processes.

In conclusion, I submit that effective time travel would not involve manipulation of a single variable called “time”; it would require manipulation of countless variables of time defined between all of the particles involved. In other words, time travel is possible, but only relatively, not universally.

To get more complicated, we might say that time is related to entropy in this way: When you have any 2 particles split from a single location, you introduce distance between them, which effectively defines a measure of time between them. The greater number of independent locations of particles relative to each other that you have, the harder it is to get everything back to the way it was previously.

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