Is Time Travel Impossible?
Summary
PBS Space Time explores whether time travel is physically possible, examining special relativity’s tachyons and general relativity’s wormholes, Tipler cylinders, and rotating black holes. The episode highlights that while equations permit these mechanisms, they require impossible conditions like infinite length or negative mass. It concludes by discussing causality paradoxes, the Novikov self-consistency principle, and Hawking’s chronology protection conjecture, suggesting a full theory of quantum gravity is needed to resolve the issue.
Topics discussed
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Introduction: The appeal of time travel
Special Relativity: Time dilation
Faster-than-light travel and backward time
Tachyons and imaginary mass
General Relativity and wormholes
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Creating a time machine with wormholes
Stability issues and negative mass
Casimir effect and exotic matter
Energy conditions in General Relativity
The Tipler Cylinder
Limitations of the Tipler Cylinder
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Gödel universe and rotating black holes
Causality and the Grandfather Paradox
Hawking's Chronology Protection Conjecture
Novikov Self-Consistency and Many Worlds
Quantum gravity and Thorne's hypothesis
The argument from lack of time travelers
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Hawking's party and conclusion
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