How Far Beyond Earth Could Humanity Travel?
Summary
PBS Space Time explores the maximum extent of future human expansion, defining the "affectable universe" as a 16.5 billion light-year sphere containing roughly 20 billion galaxies. Drawing on research by Toby Ord, the episode uses spacetime diagrams to illustrate how dark energy creates a shrinking horizon that limits intergalactic travel and communication. It concludes that while humanity could potentially populate a sextillion stars, eventual isolation will cause distant colonies to diverge into causally disconnected civilizations.
Topics discussed
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Introduction: Limits of human expansion
Context from previous episode on cosmic horizons
Assumptions for intergalactic travel thought experiment
Spacetime diagrams and light cones explained
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Hubble horizon and dark energy effects
Conformal coordinates and cosmological event horizon
Calculating the size of the affectable universe
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Impact of travel speed and delay on reach
Ultimately observable universe for future explorers
Communication limits and era of isolation
Strategic considerations for launching colony ships
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Conclusion: Responsibility and Carl Sagan quote
Promotion: PBS Earth Month and Untold Earth
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