American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and 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.Solutions have been formulated to avoid the inconsistencies Polchinski suggested, essentially involving the billiard ball delivering a blow which changes its younger version’s course, but not enough to stop it entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’

5: Polchinski’s Paradox

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence.

The Story of The Paleontologist 

Similar to the Grandfather Paradox which paradoxically prevents your birth, the Killing Hitler paradox erases your reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect”, killing Hitler would have wider consequences for everyone in the world, even just studying him in school. The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

4: Let’s Kill Hitler Paradox

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect. Say, for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions.

Are Self-fulfilling Prophecies Paradoxes?

Scientists have suggested ingenious ways to avoid time travel paradoxes, including–The Solution: time travel is impossible due to the very paradox it creates–Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same–The Multiverse /“many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past–Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased

Solutions

1) Closed Causal Loops, such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.2) Consistency Paradoxes, such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

2 Categories Of Time Travel Paradoxes 

A <a href="http://www.astronomytrek.com/time-travel-the-predestination-paradox-explained/">Predestination Paradox </a>occurs when the actions of a person traveling back in time becomes part of past events, and may ultimately causes the event he is trying to prevent to take place.Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. Any attempts to stop something from happening in the past will simply lead to the cause itself.

1: Predestination Paradox

A <a href="http://www.astronomytrek.com/the-bootstrap-paradox-explained/">Bootstrap Paradox </a>is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. A bootstrap paradox involving a person: a 20 year old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20 year old time traveler, who travels back 21 years through time, meets a woman, and so on.

2: Bootstrap/ Ontological Paradox

For example, 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 – a paradox. Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place.

3: Grandfather Paradox

The Butterfly Effect is a reference to Chaos Theory where seemingly 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.

Are Time Paradoxes Inevitable?

They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans, and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Story Of The Paleontologist Cont.

There is nothing in Einstein’s theories of <a href="http://www.astronomytrek.com/black-holes-relativity-and-time-travel/">relativity </a>to rule out <a href="http://www.astronomytrek.com/interesting-facts-about-time-the-fourth-dimension-and-time-travel/">time travel </a>, although the very notion of <a href="http://www.astronomytrek.com/is-traveling-back-through-time-possible/">traveling to the past </a>violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientist and <a href="http://www.astronomytrek.com/10-notable-time-travel-movies-of-the-21st-century/">time travel movie </a>buffs alike more than a few sleepless nights over the years. The <a href="http://www.astronomytrek.com/7-profound-questions-about-time/">time </a>travel paradoxes which follow fall into two broad categories:

Background 

I have always been interested in the concept of time and time travel, as it is always as mind boggling as it is simple, and this article I found useful to introduce paradoxes which arise as part of time travel.

Time paradoxes, whether to the past or future, are an inherent feature of time travel, exemplified by the grandfather paradox and the bootstrap paradox.

3: Grandfather Paradox

Similar ideas to 3: Grandfather Paradox

The Grandfather Paradox