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The grandfather paradox is a proposed paradox of time travel which results in an inconsistency through changing the past. The paradox was described as early as 1931, and even then it was described as "the age-old argument of preventing your birth by killing your grandparents". Early science fiction stories dealing with the paradox are the short story ''Ancestral Voices'' by Nathaniel Schachner, published in 1933,〔; the story refers to ''an ancestor'' of the time traveller not his grandfather.〕 and the 1943 book by René Barjavel ''Future Times Three''.〔; actually, the book refers to ''an ancestor'' of the time traveller not his grandfather.〕 The paradox is described as follows: the time traveller goes back in time and kills his grandfather before his grandfather meets his grandmother. As a result, the time traveller is never born. But, if he was never born, then he is unable to travel through time and kill his grandfather, which means the traveller would then be born after all, and so on. Despite the name, the grandfather paradox does not exclusively regard the impossibility of one's own birth. Rather, it regards any action that eliminates the cause or means of traveling back in time. The paradox's namesake example is merely the most commonly thought of when one considers the whole range of possible actions. Another example would be using scientific knowledge to invent a time machine, then going back in time and (whether through murder or otherwise) impeding a scientist's work that would eventually lead to the invention of the time machine. An equivalent paradox is known (in philosophy) as autoinfanticide, going back in time and killing oneself as a baby.〔 When the term was coined by Paul Horwich, he used the term autofanticide.〕 Assuming the causal link between the time traveller's present and future, the grandfather paradox that disrupts that link may be regarded as impossible (thus precluding the arbitrary alteration of one's fate). However, a number of hypotheses have been postulated to avoid the paradox, such as the idea that the past is unchangeable, so the grandfather must have already survived the attempted killing (as stated earlier); or the time traveller creates—or joins—an alternate timeline or parallel universe〔See also Alfred Bester, ''The Men Who Murdered Mohammed'', published in 1958, just the year following Everett's Ph.D thesis〕 in which the traveller was never born. A variant of the grandfather paradox is the Hitler paradox or Hitler's murder paradox, a fairly frequent trope in science fiction, in which the protagonist travels back in time to murder Adolf Hitler before he can instigate World War II. Rather than necessarily physically preventing time travel, the action removes any ''reason'' for the travel, along with any knowledge that the reason ever existed, thus removing any point in travelling in time in the first place. Additionally, the consequences of Hitler's existence are so monumental and all-encompassing that for anyone born after the war, it is likely that their birth was influenced in some way by its effects, and thus the grandfather paradox would directly apply in some way. ==Scientific theories== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「grandfather paradox」の詳細全文を読む スポンサード リンク
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