Where Next?

Although not the main focus of this section, the speaker reminds us that David Lewis has already:
- Defined backward time-travel in terms of divergence between personal time and external time.
- Offered solutions to the classic “grandfather paradox.”
- Explained how genuine change in the past is compatible with determinism once we distinguish internal vs. external perspectives on history.
- Analyzed causal loops (self-existing information or objects) and shown why they are logically coherent even if counter-intuitive.

Thought experiment: You travel back and meet your younger self, have a face-to-face conversation (akin to Lewis’s telephone call example).
- Puzzle: How does a single person manage to be in two spatial locations at once?
- Raises questions about persistence (continuity of personal identity) and how ordinary metaphysical categories handle time-travel cases.

Key physicists/philosophers: David Deutsch and Michael Lockwood.
Problem in a one-world (Lewis-style) model:
- You go back to 2700 and attempt to assassinate Hitler.
- Physics seems to allow pulling a trigger, but you cannot succeed—something always prevents the fatal shot.
- Such “mysterious blocks” violate what Deutsch & Lockwood call local autonomy: what is physically possible in a region should be fixed by local conditions, not by constraints from the whole of spacetime.
Solution: Many-worlds / branching histories.
- Time travel still occurs, but your arrival point lies on a new branch.
- You may kill a Hitler whose timeline now diverges; meanwhile, the original Hitler in your native branch still dies in 1945.
- Freedom of action is preserved without paradoxes, at the “cost” of accepting ontologically real multiple histories.
Philosophical follow-up: If your destination is not on the same historical chain, is that really “travel in time,” or merely travel to an alternate universe?

Theoretical background:
- General Relativity (GR) suggests mechanisms (e.g., rotating universes, wormholes) that allow closed timelike curves.
- Quantum Mechanics (QM) has its own exotic proposals (e.g., quantum-tunneling wormholes).
- We still lack a final quantum-gravity theory unifying GR and QM; any verdict on time machines must wait on that synthesis.
Philosopher John Earman (transcript says “Erwin”) proposes:
- A real time machine might be physically constructible, yet uncontrollable.
- You could set up the right boundary conditions, but predicting the actual temporal displacements produced would be impossible.

Stephen Hawking’s question: “If time travel is possible, where are the time travelers?”
Lewis’s stance: He defends only logical possibility; physical reality could still forbid it.
Paul Horwich’s Coincidence Argument:
- Imagine a busload of would-be assassins arriving in Vienna in 1908 with machine guns, bazookas, poisoned hats, exploding cakes, etc.
- Historical record shows Hitler survived; therefore every attempt must somehow fail, producing implausibly long chains of coincidences.
- Horwich claims that such statistically extreme coincidences would be detectable; their absence counts against real backward travel.

Gödel Universe (rotating infinite distribution of matter):
- Any spacetime point reachable from any other ⇒ entire history accessible.
- If we lived in a Gödel world, the absence of visitors would be fatal for time-travel optimism.
Localized Closed Timelike Curves (CTCs) in mainstream physics:
- Definition: A closed timelike curve is a path that remains timelike (never exceeds light speed) yet loops back to its own origin in spacetime.
- Consensus: Whether CTCs are permitted is still an open research question.
Time machines as regions, not vehicles:
- Fiction shows brass-and-quartz sleds roaming history; physics envisions a region of curved spacetime that functions as the machine.
- Example: You build a CTC generator in 2015.
- From that moment onward, future travelers can return to 2015.
- No one can travel to times before the generator’s first activation (e.g., pre-2015 events remain unreachable).
- Hence Hawking’s challenge might be answered: Time travel could be possible but not yet inaugurated.

Logical vs. Physical Possibility: Even if coherent, backward time travel might never be physically realized given our world’s actual laws.
Autonomy Principle: Any acceptable physical theory must allow local agents genuine options, without mysterious external constraints.
Many-Worlds Trade-off: Frees agents but multiplies realities; raises ontological and semantic questions about what counts as “the” past.
Engineering Hurdles: Constructing controllable CTC generators requires breakthroughs in quantum gravity—still on the horizon.
Empirical Signature: If time travelers ever begin arriving, we should expect either (a) spectacular anomalies and coincidences, or (b) unmistakeable appearances post-creation of the first time-machine region.

Kurt Gödel – rotating cosmological solutions to Einstein’s equations.
David Lewis – “The Paradoxes of Time Travel” (1976).
David Deutsch & Michael Lockwood – many-worlds approach to time travel.
Paul Horwich – improbability argument against real time travelers.
Handout (not included here) reportedly contains diagrams, expanded bibliographies, and extended discussions.

End of summarized lecture section on contemporary issues in the philosophy of time travel.

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