Time travel : Theories&Paradoxes

Coherence Paradox - Grandfather Paradox
Suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's grandmother. As a result, one of the traveler's parents (and by extension the traveler himself) would never have been conceived. This would imply that he could not have traveled back in time after all
-first described (in this exact form) by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (Future Times Three).

Fitch's paradox of knowability
Fitch's paradox asserts that the existence of an unknown truth is unknowable. So if all truths were knowable, it would follow that all truths are in fact known
-The paradox appeared as a minor theorem in a 1963 paper by Frederic Fitch

Physical paradox
Un altro paradosso è questo: supponiamo, di nuovo, che il viaggio nel tempo sia possibile e che un oggetto qualsiasi torni indietro nel tempo. Limitiamo l'infinita gamma di momenti passati in cui potrebbe tornare, a quelli in cui l'oggetto già esisteva. Dal punto di vista dell'universo al momento di arrivo nel passato, la massa costituente l'oggetto comparirebbe praticamente dal nulla; la "copia ridondante" sarebbe dunque priva di passato. Ciò sembra inconcepibile in quanto violerebbe molte delle leggi fisiche (oltre che logiche) esistenti.

Novikov self-consistency principle:
Asserts that if an event exists that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero. In short, it says that it's impossible to create time paradoxes.
-developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel

Chronological censorship - Chronology protection conjecture - Destiny
-The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales.
-Destiny or fate refers to a predetermined course of events, is a concept based on the belief that there is a fixed natural order to the cosmos, maybe due to the presence of a god.

law of conservation of energy
afferma che, sebbene possa essere trasformata e convertita da una forma all'altra, la quantità totale di energia di un sistema isolato è una costante, ovvero il suo valore si mantiene immutato al passare del tempo.

Parallel Dimensions - Many-worlds interpretation
Implies that all possible alternative histories and futures are real—each representing an actual "world" (or "universe").
-original relative state formulation is due to Hugh Everett in 1957