- No science-fiction staple poses more philosophical difficulties than time-travel, but there is still no consensus as to whether time-travel fictions exhibit logical, metaphysical, or physical impossibility. Time travel1's defenders often assume that granting its logical possibility makes all other oddities about it disappear, but even noncontradictory time-travel poses problems in causation2, identity, epistemology, and probability.
- This paper offers a taxonomy of fictional time-travel, including Wellsian, Nietzschean, Freudian, Eschatological, Self-fulfilling, and Many-Worlds varieties. I argue that while the last is the least objectionable, it has its own insurmountable difficulties. None of these scenarios can occur in our world, or what we think is our world. All are wanting, but in such different ways (physical, biological, epistemic, logical) that their diversity all but precludes a general objection to time-travel.
- Herein, proper time is a traveller's elapsed journey-time as measured by a clock or other physical process on the same path (i.e., time proper to the traveller). All causal processes for the traveller (e.g., ageing and memory) increase with proper time. Global time is time measured in a suitable reference frame (e.g., by a stationary observer). A world-line is a path through space-time.
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