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### Traveling Through Time.
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When I was a kid I watched a lot shows that had time travel in it, particular Dr Who, and thought that it would be pretty nifty to have a time machine. But when I started to think about this a little further, I came to the realization that it was rather impossible. One, if we have developed a time machine at any point in the future we would already be aware of it because they would have visited by now. And two, due to the nature of time or at least my understanding of it at that point, it would break some things.

But lets suspend our disbelief for a moment and assume we have a working time machine chilling in the back yard. What are some of the issues we might run into when journeying through time.
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### Predestination Paradox
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A Predestination Paradox happens when the actions of the person traveling back becomes part of past events in their timeline, and may influence the event they are trying to prevent and cause it anyway. This results in a loop in which Event 1 in the past influences Event 2 in the future (the time travel) which then causes Event 1 to occur. With this circular loop of events ensuring that history is never altered by the individual. That any attempts to stop something from happening in the past will simply lead to the cause itself rather than stopping it. This paradox suggests that things are always destined to turn out the same way (pre-destiny), and that whatever has happened must happen.


Sounds convoluted? Well here are a few examples. Say your friend dies in a car accident and you travel back in time to save them from that fate. Only to find out on the way to the accident that the interactions you had going there influenced them to step in front of the bus you were riding on. Your attempt to change that outcome directly influenced the events and lead to a predestination paradox. One way of dealing with this type of paradox is assuming that the version of events you have experienced already are leading up to the choice of going back into the past. And that by doing so you will only end up fulfilling your role in creating an event in history instead of changing it.

This is summed up quite nicely a quote from the movie "The Time Machine".

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you.”
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### Bootstrap Paradox
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A Bootstrap Paradox is slightly more complicated, and entails a person, object, or piece of information sent back in time resulting in an infinite loop where the object has no identifiable origin, and exists without ever being created. For example a person traveling back in time and giving themselves the script for a movie which they go on to create and earn great fame would create a bootstrap paradox involving information. As the script has no true point of creation or origin.

A paradox event involving this particular one could happen if a 20 year old male traveler goes back 21 years, meets a woman, has an affair and travels home without knowing the woman was pregnant. Her child grows up to be the 20 year old time traveler who travels back 21 years, meets the same woman, and so on.

These bootstrap paradoxes imply that the future, past and present are not defined. Giving scientists an obvious problem on how to pinpoint the 'origin point' of anything. A word generally used to refer to the past, but is now rendered meaningless. This leads us to question how the object or data was created, and by who. 
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### Grandfather Paradox
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This time paradox would be caused if time was linear. Say if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past. So you decided to travel back to wipe out your own grandfather before he sired your father who he went on a murderous rampage. Figuring that if you remove him from the timeline before he meets your grandmother the whole family line will vanish and the world will be a better place. According to theoretical physicists, the situation could play like this:

Time line protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! BANG! The bullet mysteriously ricochets off a speck of dust in the air, rebounds off a wall and hits you instead. Game over man, Game over.

Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present” but you never existed here. Everything about you has been erased. You’ve entered a timeline where you never existed. Either you have created a branching timeline where you have just popped into existence or slipped over to some kind of parallel universe. 
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### Polchinski’s Paradox - Of Billiard Balls
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American theoretical physicist Joseph Polchinski proposed a paradox in which a Billiard ball enters a wormhole. Then emerges out the other end in the past just in time to collide with its younger version and stop it going into the wormhole in the first place. This particular paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel. It has the advantage of being based upon the laws of motion, without having to refer to the concept of free will and so presents a better research method for scientists to think about the paradox.

When Polchinski proposed this paradox, he had the Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden. However, a number of solutions have been formulated which involves the ball interacting in a way that changes its younger version’s course, but not enough to stop it entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.
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### Are Time Paradoxes Inevitable?
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The Butterfly Effect is a reference to Chaos Theory where trivial changes can have huge cascade reactions over long periods of time. Consequently, the timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

For example just interacting with the way the wind blows in the past might cause a cascade event where a hurricane takes place in your home town thousands of years into the future, wiping out your entire family on your return to the present.
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### Solutions
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Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

- Time travel is impossible because of the very paradox it creates.

- Successfully altering events in the past will set off another set of events which will cause the present to remain the same.

- An alternate parallel universe or timeline is created each time an event is altered in the past.

- A person traveling to the past would exist in the new timeline, but have their own timeline erased.

I know this was a big one, and I thank you for your patience in getting through it. Hopefully you had as much fun reading it as I did writing it up.

Much love dearest readers, hope you all have a great day. <3
source and more content <a href="http://www.astronomytrek.com/5-bizarre-paradoxes-of-time-travel-explained/">here</a>.

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