Intro to Philosophy – Week 7 – Time Travel

  • this module focused on the paradoxes of time travel and some ways to defend the logical possibility of backwards time travel (mostly from a David Lewis paper)
  • time travel involves:
    • external time = “time as it is registered by the world at large” – e.g., movement of times, rotation of the Earth; “time as it is registered by the majority of the non-time-travelling universe”
    • personal time = ” time as it is registered by a particular person or a particular travelling object” – e.g., your hair greying, the accumulation of your digestive products
    • normally, external time = personal time
    • but for time travel, the two diverge
  • forward time travel – “external time and personal time share the same direction, but have different measures of duration
  • backward time travel – “external time and personal time diverge in direction” and duration (in that you are travelling, for example, -50 years of external time while personal time goes forward)
  • Einstein’s Special Theory of Relativity says that if you travel fast enough, forward time travel does occur (because of time dilation)
  • backward time travel is more speculative – it’s debated whether physics supports the notion of backward time travel, though the General Theory of Relative “seems to predict that under certain circumstances” (e.g., enormous mass, enormous speed of mass) “it is possible to create circumstances where personal time and external time direct in duration and direction)
  • Lewis provides an argument that backward time travel is “logically possible” – not that it is physically possible
  • the grandfather paradox is basically that backward time travel is not possible because:
    • “if it was possible to travel in time it would be possible to create contradictions.
    • it is not possible to create contradictions.
    • Therefore, it is not possible to travel backwards in time”
  • e.g., if you  could travel backwards in time, you could kill your grandfather before they father your parent, which would prevent you from ever being born, but if you didn’t exist, how could you go back in time to kill your grandfather?
  • another example, you can’t go back into time to kill Hitler in 1908 because you already know that Hitler lived until 1945, so if you did travel into the past, you are guaranteed not to succeed in killing Hitler. So your actions in the past are restricted, but that’s not the same as saying it’s impossible you traveled back in time
  • Lewis agrees that contradictions can’t occur, but argues that time travel need not necessarily create contradictions
  • compossibility: possible relative to one set of facts may not be possible relative to another set of facts.
    • e.g., it’s compossible that I speak Gaelic, in the sense that I have a functioning voice box, but I can’t actually speak it because I’ve never learned it
  • so it’s “compossible” to kill Hitler in the past (he was mortal, I am physically capable of shooting a gun), but relative to the fact that Hitler was alive in 1945, it’s not “possible” for him to be killed in 1908
  • two senses of change:
    • replacement change: e.g., if I knock a glass off a table, I’d replace whole glass with a pile of glass fragments
    • counterfactual change: “the impact that you have assessed in terms of what would have happened (counterfactually) if you hadn’t been present”
      • e.g., my alarm clock going off this morning changed the course of my day (relative to if it hadn’t gone off)
  • Lewis thinks replacement changes can happen to concrete objects, but not to time
  • he also says that time travellers could cause a counterfactual change – i.e., the time traveller can affect things in the past (compared to if they hadn’t been there) – they don’t cause a replacement chance (i.e., it’s not like the past happened one way and then it changed to another way – it always happened only one way
  • causal loops are “a chain of events such that an event is among its own causes”; they aren’t paradoxes, but they do “pose a problem for the intelligibility of backward time travel”
  • e.g., imagine you travel back in time with a 2012 copy of Shakespeare’s complete works and give them to the young Shakespeare, who then claims them as his own – well, the only reason the 2012 copy exists is because you gave it to Shakespeare – but who wrote it? where did the information in it come from?
  • [or could become your grandfather by sleeping with your grandmother in the past, but you could only do that if you existed and you couldn’t exist unless you’d fathered your own parent, which could couldn’t do if you didn’t exist first.]
  • Lewis agrees that causal loops are strange, but they aren’t impossible
  • there are 3 possible chains of events:
    • infinite linear chain: ever event has a prior cause, so you can never get an answer of what the first cause was because you can always ask “but what caused that?”
    • finite linear chain: the first event int he chain has no cause – e.g., the Big Bang wasn’t just the first event in time, it was the beginning of time – no time existed before that (As Hawking says, asking “hwaht happened before the Big Bang is like asking “what’s north of the north pole?”) – so you still have the problem of “where does the information come from?”
    • finite non-linear chain: (causal loops) – again, we still have no explanation of where the information originally came from, but it’s no more problematic than the other two
  • there are other questions that philosophers think about with respect to time travel:
    • how can you bilocate? i.e., how can you from the future be standing next to you from the present
    • what physical laws govern time travel?
  • there’s also the idea of branching histories – you could go back to the past and kill Hitler, but you’d have killed Hitler in one version of history but in the version of history where you came from still had a Hitler who lived until 1945 (which raises the question: is this really time travel if you traveled to what is really a different history?)
  • another “interesting question is whether the mechanisms from time travel that general relativity may permit, and the time travel mechanisms that quantum mechanics may permit, will survive the fusion of general relativity and quantum mechanics into quantum gravity”
  • Hawking has posed another challenge to the “realistic possibility of time travel” – if time travel is possible, where are all the time travellers? Why haven’t we seen them?
  • “closed time-like curve is a path through space and time that returns to the very point whence it departed, but that nowhere exceeds the local speed of light. It’s a pathway that a physically possible object could take, that leads backward in time.” – it’s debated if this is realistic
  • but if it’s true, you could only access history once a closed time-like curve has been generated (e.g., if it is generated in 2017, then people in the future can travel back only as far as 2017)- so perhaps we haven’t seen time travellers yet because no one has yet generated a closed time-like curve
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