What Might Time Travel Be Anyway?

Context & Key Text

• Speaker: Dr. Alistair Richmond, University of Edinburgh, specialist in metaphysics of time (esp. time travel).
• Central reading: David Lewis, “The Paradoxes of Time Travel,” 1976 (Lewis = 1941-2001).
  • Lewis’ goal: defend the logical possibility of back-wards time travel.

Lewis’ Two‐Time Framework

• Time travel requires distinguishing two independent temporal orderings:
  • External (public, world) time
  • Registered by tidal cycles, Earth’s rotation/revolution, sunrise/sunset—"time for everybody else."
  • Personal (proper) time
  • Measured by the traveller’s wrist-watch, heartbeats, digestion, greying hair, memory accumulation, cell death, etc.
• Ordinary life (non-time travellers): \Delta t{external}=\Delta t{personal} (same magnitude, same direction).
• Time-travel situations: the two magnitudes diverge; the divergence constitutes time travel.

Forward Time Travel (FTT)

• Definition: direction of both times coincides, but durations differ.
  • Formal: \Delta t{external}>\Delta t{personal}>0.
• Example (Richmond’s illustration)
  • Depart January 2013; arrive “February” 50 years later in external time.
  • Traveller experiences only 5 min of personal time.
  • Thus: 5\,\text{min}{personal}\;\mapsto\;50\,\text{yrs}{external}.
• Conceptual upshot: subjective ageing ≪ objective cosmological ageing.

Backward Time Travel (BTT)

• Definition: personal time increases while external time decreases.
  • Formal: \Delta t{personal}>0\quad\text{but}\quad\Delta t{external}<0.
• Example
  • Same 5 min personal journey from Jan 2013 ends in Jan 1863 (≈ 150 yrs earlier).
  • Travel’s temporal ordering in external time is inverted: journey begins after it ends.
• Logical point: possibility secured once we allow distinct personal/external axes.

Physics & Forward Travel: Special Relativity (SR)

• Einstein’s Special Theory of Relativity (1905) underwrites FTT:
  • Time dilation: rate of temporal passage is velocity-dependent.
  • (Canonical relation) \gamma=\frac{1}{\sqrt{1-v^2/c^2}}; higher v ⇒ larger \gamma ⇒ slower proper time.
• Practical implication (thought-experiment)
  • Suppose traveller has 40\text{–}50 yrs of life (personal).
  • By approaching c (speed of light), those 40 yrs can correspond to tens, hundreds, millions, even billions of external years—enough to see the Sun’s full remaining lifetime.
• Empirical status: decades of experiments (muon decay, GPS satellite clocks, particle accelerators) confirm SR’s unequal time passage.

Physics & Backward Travel: General Relativity (GR)

• Einstein’s General Theory of Relativity (1915): spacetime curvature due to mass/energy.
• Certain GR solutions predict possible BTT:
  • Require enormous mass/energy density or extreme rotation (“frame-dragging”).
  • Kurt Gödel’s 1949 rotating universe model: contains Closed Timelike Curves (CTCs)—paths through spacetime returning to one’s own past.
• Status: still speculative and debated; no empirical confirmation yet.

Constitutive Summary

• Time travel = any situation where
  \exists\;\text{traveller}:\;\Delta t{personal}\neq\Delta t{external}.
• Two species:
  1. Forward: same sign, different magnitude.
  2. Backward: opposite signs.
• Lewis’ philosophical contribution: by clarifying this two-time framework, logical paradoxes (e.g., “journey starts after it ends”) become intelligible rather than contradictory.

Illustrative Phenomenological Markers

• Personal markers used in talk: watch ticks, digestive product accumulation, memory chain, hair greying, cellular decay.
• External markers: tides, solar movement, calendar dates.

Broader Significance / Connections

• Lewis’ account addresses traditional paradoxes (grandfather paradox, information loops) by separating causal order (personal) from chronological record (external).
• Forward time travel is already part of accepted physics; backward travel remains an open frontier bridging metaphysics and relativity.