Time, Spacetime, and the Logical Possibility of Time Travel

Common, phenomenological sense of time:
- Feels like an irreversible “river” constantly flowing forward.
- Marcus Aurelius quotation used to illustrate: “Time is a sort of river of passing events … another takes its place, and this too will be swept away.”
- Intuitive beliefs embedded in that image:
- Objective flow, same for everyone.
- Irreversibility (cannot “make the river flow backward”).
Immanuel Kant’s view (review from earlier lectures):
- Time (and space) are categories of mind—subjective frameworks we impose to organize sensations.
- Therefore, on Kant’s model, time is not an independently existing thing “out there.”

Einstein’s Special & General Relativity fuse space and time into a single, 4-D manifold $(x,\;y,\;z,\;t)$ called spacetime.
Key implication stressed in lecture:
- If time is simply a fourth dimension, then past and future are “out there” like north/south or left/right.
- Hence, in principle, one could travel to earlier or later temporal coordinates the way one moves through spatial coordinates.
Empirical status:
- Relativity’s predictions (time dilation, gravitational lensing, etc.) experimentally confirmed → gives physical plausibility to the notion that the future & past “exist.”

“Does it make logical sense for time travel to exist?”
- I.e., can we reconcile it with coherent metaphysics & logic, or do paradoxes render it impossible regardless of physical feasibility?

Characters: Jane (F), “unmarried mother” (M), bartender (M), infant.
1972: Unmarried mother (a man) meets bartender (secret time-traveler) in bar.
Bartender brings him back to 1963 → he meets & falls in love with Jane.
Jane becomes pregnant; before birth, bartender removes the man (unmarried mother) from 1963.
Hospital discovers Jane is a true hermaphrodite; surgeons remove female organs → Jane now lives as a male (becomes the “unmarried mother”).
Bartender later snatches Jane’s newborn in 1963, deposits baby in 1945.
Baby grows up to be Jane herself.
Unmarried mother ages → recruited by future self → becomes the bartender.
Net result: all four “people” are temporally displaced versions of one and the same individual; life history forms a perfect causal loop with no external progenitor.

Implies backward causation: later events (bartender’s actions) cause earlier events (Jane’s birth).
No-destination objection: asserts past/future do not exist as targets; only the present is real.
Two-places-at-once paradox: identical person located at multiple spacetime coordinates simultaneously.

Backward causation re-framed:
- For the traveler, the interfering action is done in their present → not “future causes past” but “present causes future-past.”
Einsteinian rebuttal to “no destination”:
- In relativistic spacetime, all points $t$ “exist”; hence genuine destinations are available.
Two-places paradox softened:
- Spatial co-presence analogy: one body spans multiple infinitesimal regions simultaneously; maybe temporal co-location can be similarly conceived.

Classic formulation: If I can travel to the past, I could kill my grandfather pre-conception → I would never exist → impossible to pull the trigger → logical contradiction.
Shows time travel seemingly entails mutually exclusive states of affairs.

Fixed-History / Consistency Principle
- You may travel to the past only in ways that already formed your present (Novikov self-consistency conjecture).
- Attempting to kill grandfather inevitably fails (misfires, slips, fate-like events).
Novikov-style “the universe won’t let you”
- Physical laws engineered so that probability of paradox-producing events is $0$ .
Branching Timelines / Many-Worlds
- Stepping back creates a divergent timeline B; timeline A (origin) continues without you.
- No contradiction because you never altered your past, only created an alternative history.
- Raises question: is hopping universes truly “time” travel or inter-dimensional travel?

Premise: If time travel eventually exists, time tourists should be ubiquitous now.
Observational fact: we see none (or so it seems).
Conclusion: time travel (to the past) probably impossible.

Civilizational rules forbid interference / visibility.
Only consciousness (not bodies) can travel, making tourists undetectable.
Technological extinction: humanity self-destructs before inventing such devices.
We have seen them (e.g., alleged cases like “John Titor”), but disbelieve.

Spacetime coordinates: $(x, y, z, t)$ with $c \Delta t$ treated analogously to spatial separation.
Loop logic: event $E1$ causes $E2$ , yet $E2$ temporally precedes $E1$ relative to external ordering, violating classical causality unless a block-universe view is adopted.
Grandfather paradox reduction:
$\text{Travel}(t0 \to t{-n}) \land \text{Eliminate(GF)} \Rightarrow \neg\text{Exist(Me)} \Rightarrow \neg \text{Travel}$ (contradiction).

Free Will: If spacetime has all events “laid out,” future is predetermined; raises compatibilist vs. incompatibilist debate.
Kantian vs. Einsteinian conceptions illustrate tension between phenomenal time (subjective) and noumenal/block time (objective).
Ethical/Psychological stakes: Responsibility, regret, fatalism, possibility of changing atrocities.

“All You Zombies” (Robert A. Heinlein, 1959) as philosophical thought experiment.
Grandfather paradox omnipresent in pop media (Back to the Future, Terminator, etc.).
John Titor internet phenomenon (circa 2000–2001) offered real-world example of “claimed tourists.”

Are self-consistent loops logically acceptable or do they merely relocate, not dissolve, paradox?
If fixed-history is true, how do we interpret moral responsibility (e.g., could you choose not to act)?
Does Many-Worlds “save” free will by multiplying outcomes, or trivialize it by ensuring every possibility happens somewhere?
Empirical detectability: What signatures would confirm or falsify branching-universe travel?

Everyday intuition ≠ philosophical or physical reality.
Einsteinian spacetime suggests objective existence of past & future, reviving serious discussion of time travel.
Three principal anti–time-travel arguments explored:
1. Self-contained loops (logical incoherence via backward causation & duplication).
2. Changing-the-past paradox (grandfather variant).
3. No-tourist observational argument.
Multiple counter-strategies offered: self-consistency, timeline branching, hidden observers, technological extinction, etc.
Implications spill into debates on determinism and free will (topic of upcoming lectures).