Sunday, December 7, 2014

Physical time travel










Brad Dowden
Department of Philosophy
Sacramento State


  1. Brad, thanks for posting more on this great topic. Two questions.

    Is it possible to say anything more, and comprehensible to a general audience, about why General Relativity permits time travel into the past in our world?

    Also, is there currently any preferred notion of personal identity that is regarded as being most compatible with theoretical physics? From what I gather, General Relativity seems most compatible with the Many Worlds interpretation, which also requires us to be multiply instantiated.

  2. Randy, you have asked why General Relativity permits time travel into the past in our world. In a way this question is like asking why relativity theory puts a top limit on speed. The answer is that the existence of speed limits and possible time travel are theorems. But we can say a bit more. If you want to travel in time relative to your twin sister, you cannot sit still. You must move in space rapidly until your light cones start tilting far enough. But now we are getting too technical, I think.

    It is much easier to carry out this travel with an A.I. device in the spaceship than with an ordinary human being because of all the dust. If your high speed travel avoids collision with all the big objects such as planets, stars, comets and asteroids, it would not be practical to avoid each bit of dust. Yet your ship’s high speed collision with dust particles will produce a shower of gamma rays and x-rays that will annihilate ordinary human bodies.

    You also asked whether there currently is any preferred notion of personal identity that is regarded as being most compatible with theoretical physics. There is none that I know of, but if you travel back to meet your earlier self, then you absolutely must exist in two bodies at once, so that upsets any classical philosophical theory of personal identity.

    This personal identity needs only to be within just one world, the possible world we call the actual world. However, as you point out, General Relativity is compatible with the Many Worlds interpretation in which at every instant each world splits into multiple possible worlds. Metaphysicians dispute whether at a branch point your identity would split into many yous, or whether there would be just one you, the one that survives into the “actual” branch, while all the other supposed yous would merely be counterparts of you, but not actually you. Physicists avoid these discussions, and metaphysicians have no consensus on the topic.

    1. I love the idea of time travel. If I can come in on this discussion here... First, I think we should view meeting ourselves during backwards time travel as meeting an earlier time slice of ourselves. It's a bit like a worm stretching out until it went right around the world and met it's own tail. Second, I think the theory of personal identity that makes the most sense of this is the "no self" theory: we are an ever-changing bundle of experiences with no unique or unchanging core. On this view, you don't meet your self when you go back in time because there are no selves. But you do meet a being that is very much like you.

    2. Just to clarify... on my first point, I mean you meet what seems like an earlier time slice of your self, but it is a bundle of experiences that is very similar to the bundle that you are currently. Yikes!

    3. Dan, you said, in past time travel you meet an earlier version of “a bundle of experiences that is very similar to the bundle that you are currently.” What do you mean by similar? Can the experiences be similar and still be in different bodies having no atoms in common?

      So, is the following a good deal for you? Suppose I were to say I’ll give you thirty thousand dollars if you’ll step into this transport box here in Sacramento. The transport box has transported past clients to the places of their dreams; my evidence for this is that the beings who step out of the box have had no objections after they have arrived. Suppose your dream vacation is to go to Paris for a while. I claim that the human being who steps out of my box in Paris in 15 seconds will look just like you and will remember having all your experiences—teaching in New Zealand, and so forth. Inside the Sacramento box the info about your experiences will be collected, then your body will be disassembled and converted to energy. This energy and information will be sent to Paris where new hydrocarbon molecules are found to construct a human being very similar to the one that stepped into the Sacramento box, remembering very similar experiences. It’s the chance of a lifetime for you. Sign up now.

    4. The teletransporter would be fine. Death without the experience of dying or being dead after doesn't sound so bad... and that is just "adding another experience" on the no self view.

      Your first challenge I find more troubling intuitively. But an older version of the same atom being close by is no stranger than an older version of yourself being nearby.

    5. It is interesting that you speak of "close by" and "nearby." When "you" step out of the teleporter machine in Paris, your Paris atoms are not nearby your Sacramento atoms, are they?

  3. Dan, I don't understand why an experiential notion of the self should makes the most sense of what you say. Why can't the worm and the time slices you speak of be understood physically? I like your fundamental idea, and if you build it into the Many Worlds interpretation it becomes more like a hydra.

    1. Perhaps my two ideas don't go together that well. I like how the no self view attacks the intuition that we can't be in two places at once by undercutting the idea that there is a unique thing that constitutes us as individuals. Perhaps my worm analogy accounts for your atoms being near your old atoms. And perhaps the no self view best explains why there is no unique self that meets herself during backwards time travel (once the uniqueness idea of personal identity is dropped, a bundle of experiences meeting another similar bundle seems easy to accept). But you are probably right that we can do away with the no self view if we have easily ignorable intuitions.

    2. If a future "Dance of Reason" post were devoted to the problem of personal identity, we'd have a lively discussion.

  4. Kyle,
    That graphic is hilarious! Made my day.