Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does Time Really Move Forwards?

  1. Dec 11, 2008 #1
    When I make plots and graphs, time is just another dimension. When I write an equation, it is just another variable. The present does not erase the past, and it does not preclude the future. Clearly our consciousness moves in the positive direction along the time axis, but time itself, does not appear to be moving anywhere. If you could travel back in time, my body would still be there. If you could travel forwards in time, my body would be there as well(or at least the mass and energy that used to be me). If you made a plot of me vs. time, I would exist everywhere along the time axis. Do you think that time is static in the same way that height, width, and depth are?
    Last edited: Dec 12, 2008
  2. jcsd
  3. Dec 12, 2008 #2
    The universe appears to be evolving, changing from instant to instant. The past is our record of physical configurations that no longer exist. The future is our prediction, based on records of the past, of physical configurations that might evolve.

    Time is motion, the recording of different physical configurations. The fact that each recorded configuration is unique, and that each unique configuration in our index is more like its close neighbors than its distant ones, suggests a preferred evolutionary direction -- ie., an arrow of time.

    You can't travel to something that doesn't physically exist any more -- except in your imagination of course.

    That's the point. The matter and energy of the universe are continually changing. 1000 years from now there won't be any you.

    Yes, for as long as you actually exist. But even then, each instantaneous configuration of you would be unique. You, as all of us, are evolving to nonexistence.

  4. Dec 12, 2008 #3
    I could also define past and future as volumes in space-time that merely have a value of "t" greater than or less than the present value.

    This continuity exists in the other three dimensions as well, and I think we are in agreement that these are relatively static.

    If space-time were defined as a function, say,


    it would exist. It would just be at a different location along the t-axis.

    I once had a physics teacher that said "We don't see people getting younger, we see them getting older, therefore time goes forwards." I believe this logic is flawed, because even if time did go backwards we would be unable think. Our thoughts would be undone. Our acquired knowledge would be leak from our consciousness like video tape being rewound. My understanding of time is that it is stagnant and moves neither forwards nor backwards. In mathematical terms, I see now reason why


    should exclude the existence of



  5. Dec 13, 2008 #4
    Yes, I see what you mean.

    I was using the word time as a synonym for change or motion, and I see now that that's not correct.

    Time is an index of recorded spatial configurations. Psychological time refers to our personal, subjective experience of the world wherein the index is made via our human biological data processing faculties following the direct detection of physical variables via our sensing faculties. Objectively, time refers to the generation of indexes via standardized protocols and time-keeping devices, and the physical data might be generated, accumulated, and processed via instruments used to amplify physical variables and augment our human biological capabilities.

    Time is just an index, no more no less -- nothing mysterious there.

    Spacetime refers to a coordinate backdrop onto which spatial configurations and time indexes are jointly mapped. Nothing mysterious there either.

    The phrase 'time moves forward' refers to the observation that each spatial configuration in an index of real world spatial configurations is unique -- that is, no configuration is repeated.

    There's an archetypal pattern regarding physical processes that's most clearly seen in the radiative 'arrow of time' (eg., the evolution of a wave produced by dropping a small round object into calm water).

    So, while it's not quite correct to say that 'time moves forward', nevertheless all real world time indexes have a common feature. They're all asymmetric in the same way, in that as you look backward through the indexed configurations they appear increasingly different from the most recent.

    Yes, but the convention is, t_past < t_present < t_future.

    Records of past spatial configurations exist, but not the spatial configurations themselves.

    Insofar as the word, time, is used in ordinary language as a synonym for change or motion, then saying that it's static or stagnant without any elaboration can result in communication problems. But I now see what you're saying. Time is an index of motion (not motion itself) -- and the concept of an index is static in the same sense that the concepts of height, width and depth are static.

    That is what you're saying, isn't it?
  6. Dec 13, 2008 #5


    User Avatar
    Gold Member

    This is a standard debate in philosophy of time. Cfr Eternalism and Presentism and there is no final word on it.
  7. Dec 13, 2008 #6
    I think that is more or less what i was trying to get at. There maybe other communication problems as well. I used words like "is," "would," and "are" and they can imply a specific point in time.
  8. Dec 14, 2008 #7
    Thanks xantox. I don't think that the eternalist or presentist philosophies of time have it right. And, the conventional model of time vis Wikipedia is a sort of layman's philosophical extension of the way that the word, time, is used in everyday discourse, but not equivalent to it.

    Time is just an index of a set of events or spatial configurations. Time indexes are generated by associating (correlating) configurations of some set or sets of spatial configurations with some other set or sets of spatial configurations. This interpretation is a generalization of SR's definition (or convention, regarding the procedure for generating time indexes) of the time of an event as the reading on a clock that is in the frame of reference of the object event or object configuration.

    Time is a dimension in the sense that it is a set of measurements (correlations) that, collectively, organize (index) data accumulated via measurements of the spatial dimensions.

    I think that that's all there is to it. Note that I'm not saying that time is like an index, but that it is an index (wrt both standard ordinary and technical scientific usage), no more, no less, no mystery, no problem. The word, time, as with any word, can be defined as something other than what it refers to in ordinary and technical scientific discourse. But what would be the point of that?

    Can we revisit the spatial configurations catalogued in our recorded history, our time index, of the universe? This is the same as asking if we can rewind the evolution of the universe or any significant part of it? The evidence (salient features and emergent patterns revealed in the time indexes) suggests that we can't. The question of whether past universal configurations exist now is a nonsensical one.

    Asking if we can take a shortcut through the natural evolution of the universe and sort of previsit temporally distant spatial configurations of some possible evolutionary path (wrt some possible future time index) that haven't happened yet, but might, is also a nonsensical question.
  9. Dec 14, 2008 #8
    I'm not sure what you mean. Can you provide some examples where using those words would present problems?

    Maybe we should talk about this in the philosophy forum?
  10. Dec 14, 2008 #9
    For example, I used the sentence
    This contained the phrase "is stagnant." Literally, this would mean that time is stagnant at the present time. It 'would' not, however, specify whether time 'would be' stagnant in the past or future.

    It may have been better to put this in the philosophy forum, although some (more) mathematical arguments would be nice. Maybe the entire thread could be moved.
  11. Dec 14, 2008 #10


    User Avatar
    Gold Member

    Eternalists do not suggest that we can revisit the past, but rather that there is a time-less ontology which is defined globally, and that we just happen within a photogram of a global movie. It also implies that the present is observer-dependent. For presentists, only the present has a defined ontology. It also implies that the present is absolute.
  12. Dec 14, 2008 #11


    Staff: Mentor

    Actually, if you are going to talk about consciousness and time then I would say that we travel backwards in time. If I walk "forwards" towards the north then I percieve objects to the north of me which I have not yet reached. On the other hand, if I walk "backwards" towards the north then I see objects to the south of me which I have already passed and have to guess about the position of objects I am approaching. Similarly with time, as I walk towards the future I remember only things in the past and I have to guess about things that I am approaching in my future. I think it is therefore pretty clear that as far as consciousness goes we travel backwards through time.
  13. Dec 14, 2008 #12
    If we define time as an index (a certain sort of index, but an index nonetheless) of a set of physical states -- whether it's observed states in the near or distant past or extrapolated states which might include the future as well as the past -- then insofar as the index is a set of corrolations of 'stagnant' physical states (ie., each state is a 'snapshot' of the universe or some part of it that is associated with a 'snapshot' of a clock of some sort -- note that wrt time indexes the states aren't simply assigned a value according to some more or less arbitrary indexing scheme like, say, the Dewey Decimal System or whatever), then the states, or any subset thereof, specified in the index are all 'stagnant' whether they are past or present or future states.

    Time indexes themselves, as records, aren't motion, and don't necessarily imply it. The word, motion, refers to the observation that at least some states in a time index are unique. An interesting feature of the time index of the universe is that all the states in that index are different, and they are different in a way that suggests that the universe is evolving away from previous states.

    This feature of time indexes wrt any physical scale is what is commonly termed the 'arrow of time'.

    I'm not sure if this adequately addressed your consideration, but it's the way I'm currently thinking about time. I used to equate time with motion or change, and I now think (as explained in my recent posts) that that's not quite correct.

    I think we might be on the same page wrt this issue, but I'm not sure. The index that we call time is stagnant in the same sense that any index is stagnant. The general concept of an index is static in the same sense that the concepts of height, width, and depth are static.

    The concept of time, just as the concept of motion, doesn't tell us anything about the physical world. These terms just refer to ways of organizing recorded data about the world and talking about it. And, as I mentioned, the evidence is that the world is indeed changing and will continue to change in ways revealed by our time indexes of it.

    Quantifiable mathematical shorthand representations are necessary to unambiguously communicate detailed statements about the world. But it's a good idea, I think, to have these sorts of semantic discussions in order to refine our ideas of what we're talking about and how it might be mathematically modelled.

    For example, during the course of this discussion I discovered at least one error in the way I was thinking about the word, time.

    If you think we're not on the same page wrt time, or if I've got it wrong in any of my ramblings, then your comments and criticisms are welcomed.

    And, thanks for starting the thread.
  14. Dec 14, 2008 #13
    Commonsensically we recognize that the word, time, refers to an index of sorts. But, without a detailed semantic analysis to more precisely ascertain what the word is referring to, we tend to reify it, and phrases like the flow of time or that we are moving in time emerge to confuse the issue. I view eternalism and presentism as byproducts of this confusion.

    The ontological status of time is that it's a certain sort of index.
  15. Dec 14, 2008 #14


    User Avatar
    Gold Member

    It's not about the reification of time. Eternalists and presentists talk about events. Eternalists consider that all events share the same ontology. Presentists consider that only present events have an ontology.
  16. Dec 14, 2008 #15
    When Koko the signing gorilla was asked to point to the future she would point behind her. She could see the past, so it was in front of her. Or something like that :smile:

    Anyway, what you wrote is compatible with the definition of time as an index of our experience.
  17. Dec 14, 2008 #16
    Define an event as an indexed set of snapshots (subjective, ie., biologically sensed and recorded; or objective, ie., instrumentally recorded or via agreement of two or more subjective reports) of the world.

    Subjective events and objective events share the same ontological status.

    What we call the present, either wrt a subjective or an objective time index of the world, is the most recent past.

    What we call the future is a projection or prediction regarding possible, but not yet recorded, events. A past might also be a backward extrapolated time index of possible, but not recorded, events.

    Events (subjective and objective) and possible events don't share the same ontological status.

    The word, time, doesn't refer to the object events of a time index or to any motion or change that might be revealed by the index. It refers to the index itself. An operational definition specifies a procedure for generating such an index. That's the ontological status of time.
  18. Dec 14, 2008 #17


    User Avatar
    Gold Member

    Recorded by who? Eternalists consider that all events no matter their time coordinate are recorded by some observer, so they all exist in some local present. They are just like events happening in a different space, say a different city : they all exist for someone who lives there. This means that the ontology of an event is independent from its space and time location, and that all events are recorded in Nature, which is like a giant Babel's library of all events.

    For a presentist this is not true: only present events are real, and an event which happened in year 1020 only exists in present memories of it (which are in fact, other events), but the "original" event itself does not exist in Nature, which has a single memoryless state.
    Last edited: Dec 14, 2008
  19. Dec 14, 2008 #18
    Originally Posted by ThomasT
    What we call the future is a projection or prediction regarding possible, but not yet recorded, events. A past might also be a backward extrapolated time index of possible, but not recorded, events. All events don't share the same ontological status. Some are recorded and some aren't.

    By us or instruments that we construct.

    Our time indexed records, our objective histories, seem to indicate that this isn't the case. The conjectured occurance of certain events doesn't establish their existence in any sense other than as constituents of the projected or extrapolated time index in which they occur.

    Of course, not having an objective record of some possible event or other doesn't necessarily mean that it can't or didn't happen. And the universe of our records is itself a record of sorts that preserves at least some aspects of its more distant history.

    However, the idea that recorded past events exist in some physical sense other than as records would involve a reification of the time index, and there's no reason to do that. The time index, like any index, is just an index, a data organizing tool.

    The ontological status of an event is a function of our apprehension of it in time indexes of spatial configurations.

    As I mentioned, remnants of more distant past events might be found in more recent events, but there's no reason to suppose that (say) the Chicago of 1920 exists in any physical way other than the remaining remnants of it and as part of our historical records of it.

    I think it's reasonable to reject eternalism.

    This makes more sense. However, as I've mentioned, what we call the present is actually a subset of the past.

    Nevertheless, we can reasonably assume that there is single, memoryless spatial configuration that is the object of the most recent snapshot in our time index of an event.
  20. Dec 15, 2008 #19


    User Avatar
    Gold Member

    However, only a physical theory, not reason alone, can be invoked to falsify whatever interpretation of reality (provided it is logically consistent). Special and general relativity falsify vanilla presentism, and are compatible with eternalism, so eternalism is the mainstream view amongst physicists. Since there is no full theory of quantum gravity, there is still a possibility that eternalism, or both eternalism and presentism, are wrong. According to some current incomplete theories of quantum gravity, some also falsify presentism and a very few others are compatible with both.
    Last edited: Dec 15, 2008
  21. Dec 15, 2008 #20
    Physical theories are the result of reason applied to observations of the world.

    I don't think so. It's just that since we get all of our info about the world via em, and since the rate of em propagation is finite, then we need a standard set of definitions, conventions, and procedures for generating time indexes in order to unambiguously communicate the information that those time indexes contain.

    The idea that there is a single spatial configuration of the universe corresponding to each spatial configuration in a time index of an event isn't at odds with either SR or GR.

    Yes, SR and GR can't be used to decide between presentism and eternalism.

    I don't think that's true. I don't think you'll find many physicists who think that the phrase "Plato exists" has any physical meaning other than wrt our historical records of him.

    The grounds for rejecting eternalism and Plato's continued existence is in the information revealed in our time indexes of the world.

    Our experience of the world is the final arbiter of statements about it, and our experience seems to indicate that time is an index -- no more, no less. Presentism is a reasonable extension of that. Eternalism isn't.

    Both presentism and eternalism are nonsensical in that our experience can never (and hence no physical theory can ever unequivicably) establish the truth or falsity of either.

    But, eternalism is the more nonsensical. :smile:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook