Temporal Paradox

Temporal Paradox is an apparent contradiction or logical contradiction associated with the idea of time travel. Temporal Paradox has two main part i.e. Casual Loops and Grandfather Paradox. Other paradoxes are such as Newcomb's paradox. 

Casual Loops

In this, a sequence of events is among the causes of another event, which is in turn among the causes of the first mentioned event. Such loop exist in spacetime but their origin can't be explained. Backwards time travel would allow for causal loops involving events, information, people or objects whose histories form a closed loop, and thus seem to "come from nowhere." The notion of objects or information that are "self-existing" in this way is often viewed as paradoxical, with several authors referring to a causal loop involving information or objects without origin as a bootstrap paradox, an information paradox, or an ontological paradox. The use of "bootstrap" in this context refers to the expression "pulling yourself up by your bootstraps" and to Robert A. Heinlein's time travel story "By His Bootstraps". The term "time loop" is sometimes used to refer to a causal loop, but although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting.

An example of a causal loop paradox involving information is given by Everett: suppose a time traveler copies a mathematical proof from a textbook, then travels back in time to meet the mathematician who first published the proof, at a date prior to publication, and allows the mathematician to simply copy the proof. In this case, the information in the proof has no origin.

Grandfather Paradox

The grandfather paradox is a paradox of time travel in which inconsistencies emerge through changing the past.[1] The name comes from the paradox's common description: a person travels to the past and kills his or her own grandfather before the conception of his or her father or mother, which prevents the time traveler's existence.[2] Despite its title, the grandfather paradox does not exclusively regard the contradiction of killing one's own grandfather to prevent one's birth. Rather, the paradox regards any action that alters the past,[3] since there is a contradiction whenever the past becomes different from the way it was.

Some advocate a parallel universe approach to the grandfather paradox. When the time traveler kills his or her grandfather, the traveler is actually killing a parallel universe version of the grandfather, and the time traveler's original universe is unaltered; it has been argued that since the traveler arrives in a different universe's history and not his or her own history, this is not "genuine" time travel. In other variants, the actions of time travelers have no effects outside of their own personal experience, as depicted in Alfred Bester's Men Who Murdered Mohammed.

Fermi Paradox

The Fermi paradox, named after Italian-American physicist Enrico Fermi, is the apparent contradiction between the lack of evidence for extraterrestrial civilizations and various high estimates for their probability (such as some optimistic estimates for the Drake equation).There have been many attempts to explain the Fermi paradox,[10][11] primarily suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) we see no evidence.

Although he was not the first to consider this question, Fermi's name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi allegedly said suddenly, "But where is everybody?" 

Newcomb's Paradox

In philosophy and mathematics, Newcomb's paradox, also referred to as Newcomb's problem, is a thought experiment involving a game between two players, one of whom is able to predict the future. Newcomb's paradox was created by William Newcomb of the University of California's. However, it was first analyzed in a philosophy paper by Robert Nozick and appeared in the March 1973 issue of Scientific American, in Martin Gardner's "Mathematical Games." Today it is a much debated problem in the philosophical branch of decision theory. Newcomb's paradox can also be related to the question of machine consciousness, specifically if a perfect simulation of a person's brain will generate the consciousness of that person.^[14] Suppose we take the predictor to be a machine that arrives at its prediction by simulating the brain of the chooser when confronted with the problem of which box to choose. If that simulation generates the consciousness of the chooser, then the chooser cannot tell whether they are standing in front of the boxes in the real world or in the virtual world generated by the simulation in the past. The "virtual" chooser would thus tell the predictor which choice the "real" chooser is going to make.