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A problem with time travel.

  1. Feb 5, 2009 #1
    Okay, I read this on a blog so I'd post it the way it ias with the link:

    "There is one aspect of time travel that nobody considers, whether in science or science-fiction (at least, i am not aware of it), something that has the potential of rendering time travel, even if it is possible, totally futile. So, what is it? In time-travel, we are only moving through time, and not through space. So, if we are at a given set of co-ordinates X, Y, Z at the present time A, and we go back in the past to some moment B, we would still, essentially, be at the same co-ordinates XYZ. Now, here's the point. Earth is moving through space; moving with velocities we might not even have a clear idea of. For definite, we know the earth is moving around the sun, the sun is moving around the centre of the galaxy, and the galaxy itself is moving. That makes up a lot of motion. So that at any given two instants earth would be at two different places. So, if i travel in time, whether in past or future, while retaining my location in space, i would end up in empty space, cause earth would be somewhere far far away..."


    What's the guy missing here? I've been thinking for some time and I'd really like to know.

  2. jcsd
  3. Feb 5, 2009 #2

    Vanadium 50

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    Why is he necessarily missing anything?
  4. Feb 5, 2009 #3
    So is that a valid problem?
  5. Feb 5, 2009 #4


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    None of the time travel schemes proposed in general relativity actually involve the sci-fi idea of dematerializing in one time and rematerializing in another, instead they involve travel through a warped region of spacetime like a wormhole--and with a wormhole, where you end up in the past would just depend on where the "mouth" of the wormhole happened to be located at the time you exit. For example, if the mouth was in orbit around the Earth then you'd end up near Earth no matter what time you travelled to.

    That said, the guy's idea that there is some objective truth about which point in space in the past is the "same position" that the Earth is in today is in total contradiction with relativity. See this section of the wikipedia time travel article:
  6. Feb 5, 2009 #5
    Hmmm, this is an interesting problem. Take into account a Minkowski diagram (the three spacial dimmensions on the X axis plotted as a funtion of time on the Y axis). If you move back into time, which would be moving down in a verticle line, you yourself stay in the same position it seems at a different location. However since the Earth is included in your frame of reference, it too would travel with you it seems, and stay in the same location instead of travelling back to where it started. I mean i could be completely wrong but thats what it seems like with a Minkowski diagram

  7. Feb 7, 2009 #6
    Any argument about the mere futility of time travel is obviated if you give TIME an objective, operational (physical) definition -- and take into account some observation-based inferences about the Universe.

    In keeping with the SR definition of the time of an event, TIME, in general, refers to indexes of observations of physical systems -- with or without the use of regular oscillators (clocks) as indexing tools.

    Lets say the system is an expanding circular wavefront created by dropping a pebble into a calm pool of water. We can talk about the time of this system in terms of the radius of the wavefront circle, or in terms of a number associated with each picture in a series of pictures or each frame of a movie of the evolving wavefront, or in terms of the configurations of some clock associated with configurations of the system, or just in terms of configurations of the system itself.

    A time of a physical system, some set of physical objects, whether it's a regular oscillator like a conventional clock or an expanding wavefront or a basketball game, etc., can be said to be some instantaneous configuration of the physical system itself.

    Observations (our time indexes) indicate that physical configurations are transitory -- continually changing -- and that physical systems at all scales tend to evolve according to certain physical laws.

    So, physically travelling back in time, ie., not just looking through some pictorial record but actually revisiting the history of some physical system, would require the reproduction of the physical configurations that correspond to the time of that system that we would like to revisit. If we want to go back to, say, 1967 New York City, then what would that require?

    In a sense, we are travelling forward in time, but travelling to, say, 2050 New York City in less that 41 years would require the production, in less than 41 years, of the physical configurations that correspond to 2050 New York City -- or you could move around real fast for a while.:smile: In the latter case you wouldn't really have travelled in time, because 41 years would have passed for you as well as the rest of us in terms of, say, the evolution of the Earth-Sun system or the Solar System or the Universe, but you would have aged less due to your history of acceleration.

    We live in an expanding, evolving universe and we can't travel to something that no longer exists -- or to something that won't exist for another 41 Earth-Sun revolutions, without those revolutions having happened.

    All physical systems are part of some larger, encompassing system -- up to universal scale. So time travel would involve either reversing the evolution of the Universe, or speeding it up.
  8. Feb 7, 2009 #7


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    It wouldn't require "reproducing" it, it would just require traveling to the region of spacetime where it existed. Something I posted on this a long time ago on this thread:
  9. Feb 7, 2009 #8
    Thanks Jesse. Yes, I'm aware of this interpretation of GR. I just don't think it has the physical significance that some do. Imho, it's sort of akin to the point of departure of the MWI wrt QM. That is, physical significance is being attributed to something which doesn't warrant it.

    Considering only a generalization of the operational (physical) definition of TIME, per SR, and observations that support an expanding, evolving universe -- then time travel is impossible.
  10. Feb 7, 2009 #9


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    Well, if it's just a matter of different interpretations then you shouldn't disagree about the physical possibilities implied by the theory--so since closed timelike curves are possible in certain spacetimes allowed by GR, how would you interpret them? A spacetime where a configuration of matter from an earlier time is recreated at a later time is clearly different from a spacetime involving a closed timelike curve that loops around to revisit a region of spacetime it's already crossed through.
  11. Feb 7, 2009 #10
    As nonphysical artifacts of the theory.

    The former is clearly physically impossible in an expanding, evolving universe. The latter is physically meaningless, at least afaik.
  12. Feb 7, 2009 #11


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    But then it's not just an "interpretation", your ideas are based on the assumptions that certain predictions of GR are incorrect, which would require a new theory (such as quantum gravity, which most likely will in fact rule out backwards time travel) to tell you the correct predictions in these scenarios.
  13. Feb 7, 2009 #12
    If spatial configurations are transitory, and the observational evidence seems to support this view, then in order to experience, say, your yesterday again, then it would be necessary to reproduce the set of universal spatial configurations corresponding to your yesterday.

    I don't know what it means, physically, to travel "to a region of spacetime where it existed" -- outside of some theoretical manipulations. I don't think that spacetime curvature, CTC's, etc. should be taken as literal descriptions of the physical world.
  14. Feb 7, 2009 #13
    It's not that CTC's are wrong. It's that they're not even wrong. I'm interpreting them as physically meaningless artifacts of the mathematical model.

    I don't think you need a new theory to rule out time travel. Just stick with a physical definition of time and assume that the universe is expanding and evolving, which fits the data.
  15. Feb 7, 2009 #14


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    You seem to be arguing for the philosophy of presentism over eternalism--but what "observational evidence" could support presentism over eternalism? I suppose a physically preferred definition of simultaneity would do so, but that doesn't seem to be what you're proposing. What observations do you think would differ in a universe where different physical configurations just exist in different regions of a single 4D spacetime rather than "ceasing to exist" in any objective sense, but where the laws of physics were still the same as the ones we observe and therefore there was no easy way (or no way at all) to actually revisit past regions?
    Can you conceive of the possibility that time is just another dimension in a 4D manifold, and we are all just frozen worldlines in this manifold, with the "flow" of time being subjective rather than objective, caused by the fact that at any point on our worldlines our brains only contain memories of events in the past light cone of that point? If so, then if you can imagine pieces of string in a 3D block of ice looping around to pass through a region they've already passed through, it shouldn't be hard to conceive of something analogous in 4 dimensions.
    That doesn't really make sense to me--all predictions in GR are derived from the mathematical model, you can't just arbitrarily say that some predictions are "artifacts" and yet still claim that you're not disagreeing with the theory itself. If someone believed that black holes are not actually possible, yet insisted that they were just "interpreting" GR differently rather than saying the theory was wrong, would this strike you as a coherent position? Surely they'd have to make predictions about what would happen to a large collapsing star that were different from the predictions of GR, for example. Of course, unlike with black holes there's no easy way to get CTCs in a spacetime that doesn't already have certain features which might not be present in our universe, like a region of spacetime with the topology of a wormhole, or a universe where the matter and energy has some overall nonzero rotation. So I suppose you could "interpret" GR in such a way that certain classes of spacetimes were just declared impossible, and as long as there's no evidence that our own spacetime falls into any of these classes, and there'd be no way for even the most advanced civilization to influence it so it did (unlike with black holes where you'd just have to gather enough matter in one place), then this could be a way of avoiding CTCs.
    Last edited: Feb 7, 2009
  16. Feb 7, 2009 #15
    Pretty much all of it, I think.

    None, but arguing for the possibility of time travel because it's allowed by GR is like arguing for the possibility of spontaneous self-reassembly of all broken eggs in New Mexico at 4pm today (Eastern Standard Time) because it's allowed by statistical mechanics.

    I'm saying that in a universe that is expanding and evolving, neither of the above is possible.

    I don't take this as a literal, physical description. GR is a useful calculational tool. Like QM. That's all. We can all agree on that. Those who would ascribe more physical significance to these theories than that, those who would consider them descriptions of Nature bear the burden of proof and the baggage that those views carry.

    I don't want to think of the physical world in these terms. I don't think that these terms describe the physical reality.

    I don't think GR is a description of Nature. I think it's a proven and useful calculational tool. In a model as rich as GR it's not so astonishing to generate some physically meaningless stuff. Is it?

    Black holes are a different consideration. Especially since they (or their predicted effects) have actually been observed.

    Ok, so are CTC's physically meaningful in an expanding, evolving universe (whose spatial configuration is transitory -- continually changing)?
  17. Feb 7, 2009 #16


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    That doesn't make sense--do you agree that by definition, for some observation to qualify as "evidence" of theory A over theory B, the observed event must be more likely in a hypothetical universe where theory A is true than in a universe where theory B is true? But when I asked:
    You replied "none", implying that you don't think any of our observations would be different in a reality where eternalism was true.
    Of course the second is physically "possible" according to all our current theories of physics, but the probability might be something ridiculous like 10^-100, so it isn't a possibility we have to consider in any practical sense. Still, it would be silly to say there could be an "interpretation" of existing theories where it was not even physically possible, since quite clearly it is according to these theories.

    And I'm not arguing for the real-world "possibility of time travel", by the way. I think it's very likely a theory of quantum gravity will say it's impossible. I'm just saying that your claim that even if we assume GR is the correct theory of gravity in our universe, one can "interpret" GR such a way as to rule out CTCs, doesn't make a lot of sense to me.
    You think that somehow the expansion of the universe makes it physically impossible for "thermodynamic miracles" to occur, as opposed to just exceedingly improbable? Why?
    I know you don't, I'm asking if you can conceive that the philosophy of "eternalism" might be true...this is a question independent of GR (GR could be wrong but eternalism true, and in principle you can interpret relativity in a presentist way by imagining a definition of simultaneity that is 'metaphysically preferred' even if it's not preferred in any measurable physical way).
    But different "interpretations" of QM don't involve throwing out any of its physical predictions as "artifacts". Again, if you propose that some of the predictions made by a mathematical model are not correct in the real world, then by definition you are saying the model is physically inaccurate in some way, and assuming you believe the universe always behaves in a lawlike way, this means there must be some better mathematical model which can make physically accurate predictions about the same situations (like quantum electrodynamics making more accurate predictions than classical electromagnetism).
    I'm just asking if you can conceive of a universe that works this way as a logical possibility.
    Again, it just doesn't make sense to call this a different "interpretation". If GR predicts that certain configurations of matter/energy lead to CTCs or black holes or whatever, and in the real world these configurations are possible but they don't actually lead to these things, then these predictions are wrong, not "meaningless", and presumably GR would have to be replaced by some more accurate theory whose predictions about these same configurations match the physical reality.
    Dense dark objects have been observed, but we haven't observed the distinctive properties predicted by GR like event horizons or singularities. And black holes were first brought up as a theoretical possibility before any astrophysical candidates had been spotted. So suppose someone took the position I described above back in those early days--again, would this statement even seem coherent to you? It wouldn't to me--if they are saying they don't think black holes are possible, fine, but that wouldn't be an "interpretation" of GR, it would be a situation where GR gives inaccurate physical predictions and needs to be replaced by a theory which reproduces all of GR's observationally-verified predictions and also makes correct predictions in the situations where GR doesn't.
    In GR they would be--you could have wormholes in an expanding universe for example. GR's predictions may be (and probably are) wrong in some cases, but they aren't meaningless, there is nothing logically impossible or undefined about these predictions.

    Also, when you say the "spatial configuration is transitory", are you referring specifically to the presentist idea that only present configurations are real, or do you just mean "transitory" in the physical sense that the configurations are different at one time than they are at another, regardless of whether we believe only one time is actually real (presentism) or whether we believe all locations in time have equal reality just like different locations in space (eternalism)?
  18. Feb 8, 2009 #17
    I wasn't thinking of that in my reply, but now that you mention it I recently learned about the presentism-eternalism thing vis discussions with xantox in this thread:

    If there's no observation that I can make that would either support or falsify eternalism, then it might be true, albeit physically meaningless (As a consolation, this contingency would seem to hold for all eternity. :smile:).

    On the other hand, the 'present' is physically meaningful. Just look around you. You call this the present, don't you? So do I. So does everybody else, afaik.

    What might it mean to say that the physical configurations of my past experience exist for eternity? I don't know. My experience is of a world where spatial configurations are transitory. Of course, they might be eternal (whatever that might mean) and just appear to be transitory. :smile:

    That's true, but I think (I hope anyway) that physics will evolve to contain physical laws or fundamental dynamics which prohibit these sorts of 'processes'.

    I look at it this way. Modern physics is very young. There's no reason to think that any physical theory is THE CORRECT (in the sense of describing the deepest dynamics) THEORY of anything. Thinking of CTC's as physically real is one way of interpreting that particular artifact of GR. Thinking of them as not physically real is another. Attributing physical reality to 'spacetime curvature', or thinking of it as the deep cause of gravitational behavior, is one way to interpret the physical significance of GR. Thinking of it as a simplification of a deeper reality, and of GR as a step toward a deeper understanding of the nature of gravitational behavior is another.

    Yes. The isotropic expansion of the universe is the fundamental wave dynamic that governs all wave motion in all media at all scales. "Thermodynamic miracles" (including 'backward time travel') involve behavior that's contrary to this fundamental dynamic.

    I don't think it has any physical meaning. There's nothing else I can say about it.

    Insofar as CTC's allow for backward time travel, they aren't a physical prediction, imo. Neither is the metaphysics of MWI or dBB, or the 'quantum nonlocality' of orthodox QM, or 'advanced potentials'. None of these are 'descriptions' of the physical world afaik. But there's no reason to label the models that produce these sorts of things as 'wrong' because of that. There's no reason to think that everything that comes out of a generally very useful calculational tool is a statement about the real world. Maybe CTC's are useful in other ways. I don't know. Are some GR experts working on ways to sort of disallow CTC's, or at least backward time travel, using GR itself?

    Yes. In "some" way(s).

    Yes, physics is evolving. Don't you think so?

    No, I don't think that's on the right track to describing the deep nature of reality. I don't think it's a logical possibility.

    There's a physical definition of TIME as an index of spatial configurations. (So, time is more properly thought of as an ordering parameter, than as a physical dimension.) There's observations that suggest that spatial configurations are transitory, and that the Universe is evolving. There's a fundamental wave dynamic (ie., the radiative arrow of time) at the deepest physical scale, the isotropic expansion of the Universe. All of this taken together logically precludes backward time travel.

    Can you conceive of the nature of reality in terms of wave mechanics in a hierarchy of media?

    If a solution is a nonphysical one, then it isn't wrong per se. It's physically meaningless. GR also produces accurate physical solutions. Anyway, it seems that it WILL be superceded some day by a more 'unifying' theory (which, as you noted, might disallow, or at least not produce, CTC's), and possibly eventually by a more physically 'descriptive' theory whose dynamics actually prohibit time travel.

    As I mentioned previously, the time of a particular spatial configuration (or set thereof -- since no 'snapshot' of the world actually captures an 'instantaneous picture' of it) can be expressed in terms of some other spatial configuration that's associated with the particular spatial configuration, or it can be expressed in terms of the particular spatial configuration itself. The PRESENT refers to the highest ordered (or most recent) additions to our time indexes of the world.

    So, my answer would be, yes, I mean transitory in the presentist and in the physical sense.

    The latter belief has no physical foundation, no objective physical referents other than symbols. I don't see any reason to believe that the history of the universe exists in any way other than in the form of historical records.

    What do you believe?
  19. Feb 9, 2009 #18
    You and I can perform experiments which show that we are not in the same Present, that we do not share a Now.

    How does Presentism hold against that simple truth?

    I was having a conversation with my girlfriend about time earlier.

    We were driving home from somewhere, and I told her "We are already at home, upstairs, changing into our leisure clothes. We are still at the book store, beginning this conversation. We are still in the moment where I began telling you this."

    Then, when we got home, I continued "We are still back there at the corner, when I said we are already upstairs changing, and we are upstairs changing. We are still at the book store, we are still in the moment when we first met. We can't see these periods of time because they are... around the corner from us. That does not invalidate them, nor does our inability to directly perceive events we clearly remember in our past make them stop existing. You and I are lines scribbled through time, imagining each snap shot we perceive to be the truth of reality."
  20. Feb 10, 2009 #19
    Max, I'm going to start a new thread, replying to your statements here, in the Philosophy forum.
  21. Feb 10, 2009 #20
    I largely agree with Thomas' statement that GR is a quantitative correlation of observations. The reason everyone seems to disagree and/or be confused about the physicality of "time travel" is because nobody has bothered to define time or physical scientifically.

    The scientific definition of "physical" is: shape. Now, logically, something is either physical or not by definition. That which is physical (is an object, a thing, i.e. something not nothing) may serve as the subject of a sentence. It may be qualified by adjectives, it may act or be acted upon by another object (verb), and its actions may be qualified/described (adverbs). That which is nonphysical cannot logically serve as the subject of a sentence.

    Does time have shape? Is it physical? If someone says it is the only way to test this claim is for them to present the object itself, or at least a model of it. If they can't they were just bluffing, trying to get you to believe their religion. Nobody has ever been able to show me 'a' time. Therefore the question of "time travel" itself is nonsensical.

    A scientific definition of time: comparative motion

    Motion invokes at least two locations of an object and "compare" invokes an observer to record/remember the locations. Time and velocity are mathematical inverses and the quantity an observer calls the velocity of A relative to B as it traversed the distance D s/he may also call the amount of time for B to traverse a distance D relative to A. Both time and velocity are entirely conceptual, i.e. we *understand* time and velocity rather than *visualize* them, which is what we do with objects.

    "Time dilation" is also a nonsense term. The logically correct description is "clock slowing". One observer's clock runs slower than the other's, objectively that's all we have before us. In order for the two observers to exchange data and observations they will need to account for this. Why does one run slower? Nobody knows, we are still seeking the physical explanation for this phenomenon. What is it about the internal machinery of an entity such as a clock or an atom that makes it release light (or move its internal parts such as a clock hand) more slowly when the overall entity moves? If we understood this internal machinery physically we could explain this phenomenon.
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