ZEITREISEN - IAAC

A wormhole
Time Travel Paradoxes
Even the slightest possibility of time travel raises the question on how to resolve possible paradoxes.
These can be divided into at least two broad categories:
The Grandfather Paradox (The time traveler travels into past and kills some of his predecessors or
even his earlier self),
a) Causality Violation Paradoxes (The Time traveler travels into past and hides the plans how to
build a time machine, which he finds later in the future. Consequently the time machine has
never been invented; it just is.)
Perhaps one of the most thought-provoking SF novels, which also includes sexual transformations,
that deals with the time travel paradoxes, is “All You Zombies”, written by Robert Heinlein. The main
character in this novel is his/hers own child, mother, father and friend.
There are at least three possibilities to resolve time travel paradoxes:
1. The impossibility of time travel
This approach can be comprised in the statement of SF writer Isaac Assimov:
“To my thinking it is precisely because time travel involves such fascinating paradoxes that we can
conclude, even in the absence of evidence, that time travel is impossible”.
2. The assumption of global causality
In this approach that has been proposed by Stephen Hawking, the past is totally defined, i.e.,
everything that has happened or must happen, including the time traveler’s attempt to kill his
grandmother, cannot be altered and nothing will change the course of history. In other words, the time
traveler will experience endless “mishaps” in trying to kill his grandfather and will never achieve the
murder, thus keeping time (or at least events) intact.
In other words, Hawking invokes a sort of “cosmic censor”, an omnipotent entity that intervenes to
prevent the occurrence of a paradox. It operates something like this. If a man goes back in time and
attempts to alter his personal history, for example by trying to kill his father, then the cosmic censor
would have it that the murdered man was not really his father. What if he attempts to commit a form of
retrograde suicide by killing his earlier self? Presumably in this kind of situation the cosmic censor
takes a more active role, depending on the method employed. If, for example, the time traveler elects
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ZEITREISEN 16. INTERNATIONALE PROJEKTWOCHE DES IAAC IN TANZENBERG