What is time? According to Albert Einstein, time is an illusion; it is relative, it is the fourth dimension that can vary for different observers depending on your speed through space. Conventionally it moves only forward. Also, Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein’s theory of general relativity, gravity can bend time.
Time travel is the concept of movement between certain points in time, like movement between different points in space by an object or a person; that is travel between time and time, typically with the use of a hypothetical device known as a time machine.
Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motion were possible. Many physicists believe that backward time travel is highly unlikely. Any theory that would support time travel would produce potential problems of causality. The classic example of a problem involving causality is the “grandfather paradox”.
A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time and time travel. The time travel paradoxes fall into two broad categories:
1) Closed Causal Loops- The paradoxes consist of 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. The Predestination Paradox and the Bootstrap Paradox are examples.
2) Consistency Paradoxes- Paradoxes like 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 logical possibility of altering the past.
1. Predestination Paradox
A Predestination Paradox occurs when the Cause leads to an Effect which then leads back to the initial Cause. That is, the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. This results in a circular loop of events ensuring that history is not changed by the time traveler and that any attempts to hinder something from happening in the past will simply lead to the cause itself, instead of stopping it. This paradox suggests that things are always destined and cannot be altered. That is, whatever has happened must happen.
Sounds complicated? Imagine that your friend dies in a hit-and-run car accident and you travel back in time to save him from his fate, only to find that on your way to the accident you are the one who accidentally runs him over. Your attempt to change the past has therefore resulted in a predestination paradox.
2. Bootstrap Paradox
A Bootstrap Paradox is a type of paradox in which an object, person, or a piece of information sent back in time results in an infinite loop where the object has no distinguishable origin and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being, or existence.
For example, Suppose someone goes to a bookstore and gets a copy of Hamlet, then using a time machine he travels back to 1599 and gives the copy to Shakespeare. He’s impressed and produces the play as if he wrote it. People like the play and it is published for generations afterwards.
3. Grandfather Paradox
The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. The consistency paradox or grandfather paradox occurs when the past is changed in any way, thus creating a contradiction. That is, if a time traveler were ever to go back in time and kill his grandfather in his childhood, it would result in one of the time traveler’s parents, and consequently the time traveler, not being born. If the time traveler weren’t born, then it wouldn’t be possible for him or her to kill the grandfather in the first place. Therefore, the grandfather lives to offspring the time traveler’s parent, and thereby the time traveler. There is, thus no predicted outcome to this.
4. Let’s Kill Hitler Paradox
Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect”, killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.
The butterfly effect is an idea that is more commonly used in chaos theory. A small change can make much bigger changes to happen; one small incident can have a big impact on the future. So, 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.
5. Polchinski’s Paradox
American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which if you sent a billiard ball through the wormhole into the past such that it collides with its previous self, deflecting it so that it never enters the wormhole in the first place.
6: Fermi’s Paradox
The Fermi paradox can be adapted for time travel, and phrased “if time travel were possible, where are all the visitors from the future?” Answers vary, from time travel not being possible, to the possibility that visitors from the future cannot reach any arbitrary point in the past, or that they disguise themselves to avoid detection.
However, Scientists are eager to avoid the paradoxes presented by time travel with a number of ingenious ways in which to present a more consistent version of reality. Some of them are:
Solutions?
It is predicted that time travel is impossible because of the very paradox it creates.
The Multiverse or “many-worlds” hypothesis: According to this theory, an alternate parallel timeline or universe is created each time an event is altered in the past by a time traveler.
Self-healing hypothesis: This hypothesis depicts that successfully altering events in the past will cause another set of events which will cause the present to remain unchanged.
Erased timeline hypothesis: In this, a person traveling to the past would exist in the new timeline, but have their own timeline erased.
Source: Astronomy Trek
Also Have a Look at: Calendars: Where Ancient Astronomy Meets The Modern World
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