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When it comes to time travel And paradoxesthe first image that comes to mind – it could not be otherwise – is that of the family photo of Marty McFly (per century Michael J. Fox), in which the silhouettes of the brother and sister gradually fade, almost to disappear: all because Marty, moving into the past with the DeLorean built by the friend Doche inadvertently seduced his mother Before that he knew his father.
And here’s the paradox: if Marty’s mom doesn’t know her father, Marty can’t be born; and if Marty can’t be born, he can’t go back to the past. The name of this problem – to be precise it is a ‘antinomythat is, the type of paradox that indicates the coexistence of two contradictory statements both demonstrable or justifiable – is grandfather paradox: his first description, by the science fiction writer René Barjavelrefers to the hypothetical situation (analogous to that of Back to the Future, as well as many other films and books dealing with the theme of time travel) in which a grandson goes back in time and kills his grandfather before he meets his grandmother, so before he can get married and have offspring; in this way, once again, the action of the time traveler makes both existence and time travel impossible.
Things just got (even more) complicated. Recently, two physicists – Venkatesh Vilasiniof the Eth ZurichAnd Roger Colbeckof the University of York – have in fact proposed a new bizarre approach to the paradox, the details of which are described in two articles (this And this) uploaded to the server of pre-print ArXiv. Vilasini and Colbeck’s analysis focused on a particular case of possible solutions to the grandfather paradoxthe so-called causal loopor causal cyclewhich describes a (hypothetical) situation according to which a certain event (an action, information, the existence of an object or a person) is among the causes of another event, which in turn is itself among the causes of the first.
In this situation it no longer makes sense to talk about the it causes of an event: to understand why we can refer to the enlightening example offered by the physicist Allen Everett. Suppose a time traveler delivers the equations of relativity theory to Albert Einstein Before that the latter has formulated them, authorizing him to disclose them: in this case the equations that will end up in the textbooks will not have a real origin, due (!) to this overlapping of causes.
How tells the New Scientist in an in-depth study dedicated to the topic, causality can be defined in two ways: the first implies the description of the relationships between two entities in space-time (i.e. how far they are and whether one comes before the other or vice versa), while the other involves the analysis of flow of information from one entity to another. “We are used to saying that correlation does not imply causation – explains Vilasini – and instead we focused on the opposite, ie on the fact that causality does not imply correlation, or more precisely the ability of the two entities to exchange signals “. That is, to return to the previous example, the possibility that Einstein can discover his equations based on information coming from his future but without ever communicating directly with the time traveler. All this can happen in a universe similar to ours but not too much: “The three spatial dimensions of our universe – explain the two authors of the study – could change the equations of causal cycles making them impossible, but we are not sure yet “. Let alone us.
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