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If Time Is The Fourth Dimension

  1. Apr 17, 2006 #1
    does that mean that the past and the future exist somewhere? It makes it seem like everything we're experiencing is one big solid object in some higher dimension we can't visualize, naturally, just as an ever widening and then shrinking disk seen by a two dimensional person is really a 3 dimensional sphere passing through his space, and the other parts of the sphere that aren't seen exist somewhere, don't they? I ask partly because I've heard some say that the past and the future don't exist, so there can be no time travel. Many thanks.
     
  2. jcsd
  3. Apr 18, 2006 #2
    Counting ten characters...

    So, anyone?
     
  4. Apr 18, 2006 #3

    JesseM

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    Have a look at the illusionary time thread--it sounds like you're talking about the concept of "block time" which is discussed there. And yes, relativity makes the idea that the past and future "don't exist" problematic, because relativity allows for different reference frames which have different definitions of "simultaneity" (meaning they disagree about whether two events at different locations happened 'at the same time' or not), and there's no physical reason to prefer one frame to another.

    Also, you may want to check out this article by physicist Paul Davies which I posted on the earlier thread:

    http://urgrue.org/lib/mysterious-flow.html
     
  5. Apr 28, 2006 #4
    Hello!

    I will agree here. Traveling thru time will not be an issue really which was being discussed, seriously.

    I but think we do also have as far, with regards to the informational content, as scientific approaches
    have come today to face the dimensions of space itself. Here it is a good option that at further places
    in different galaxies same evolution at the end have taken place. If so we, as we could get there, had
    to face the same 'time' running. Everything would be same. This could change slightly on again other
    planets as far as not just the event would have changed or human mankind self but the species living
    on these planet. This, as I think, was a valid sight towards space seen from a human anatomy from
    this planet earth.

    Steve
     
    Last edited: Apr 29, 2006
  6. Apr 28, 2006 #5

    HallsofIvy

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    No, it doesn't. The word "dimension" simply refers to a number used to label something of interest. Ordinary space is "three dimemsional" because in any coordinates system (Cartesian, Cylindrical, Spherical, etc.) three numbers are required to specify a point in space. Physics is concerned with "events" that happen at a specific point in space at a specific time. Saying that "time is the fourth dimension" just means that we need 3 space dimensions and time to specify an event.
     
  7. Apr 28, 2006 #6

    robphy

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    Maybe there's some ambiguity in your first sentence [of my quote of your post]...

    In relativity, it certainly does make sense to describe the "past" of an event... it is the set of all events on and inside its past light cone, which are those events which could have an effect on what happens at that event. In addition, this notion of "past" is unambiguous and independent of frame. An analogous notion of the "future" of an event also can be defined.

    It is true, however, that there is no frame-independent notion of a set of past events which are simultaneous.... i.e., no frame-independent notion of "all space at an instant in the past".

    In Galilean relativity, where there are no bounds on signal-speeds, there is a frame-independent notion of which events which are simultaneous. In that case, the past of an event is the whole of spacetime before "now", the set of events simultaneous with our event. In some sense, we get this picture by taking the light cone in SR and flattening it out.
     
    Last edited: Apr 28, 2006
  8. May 20, 2006 #7
    let's try this.if one full orbit of earth around sun is one year, how about if we are living in mars?we will also make one full orbit of mars around sun as one years.so which one will we take?if there is an alien in mars claim like this.
     
  9. May 20, 2006 #8

    JesseM

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    This has nothing to do with relativity or time as the fourth dimension though, it's just a matter of how you define the word "year". The earthling and the martian will both agree on the ratio between the time of the earth's orbit and the time of mars' orbit.
     
  10. May 20, 2006 #9

    pervect

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    My very quick comment is that this is basically a philosophical question. It might be possible to answer it scientifically, if you give us the necessary and sufficient properties for something to "exist", or an experiment we can do to determine if something "exists" or not.
     
  11. May 21, 2006 #10
    If time is to be called a dimension,
    then I would rather label space as just one dimension instead of three.

    Actually, I am really wondering whether time is really something similar to space.
    For if I hold a variable constant,
    when that variable is space, I can still make some kinds of measurements, such as the temperature at one point in space over time.

    However, when that variable is time, I cannot see what I can measure at all. Since all means of measurements cannot take two events spontaniously (the fastest it can go is the speed of light), eventually I can just measure one point in space given one point in time, and with only one data available it can hardly be called a measurement afterall.

    A straight horizantal line on the Minkosky diagram is just some bird eye in our imagination...
     
  12. May 22, 2006 #11
    Yeah, time itself cannot really be measured directly, we can only measure the motion of some physical thing and compare it to the motion of another physical thing.

    To reply to the original question, basically what Jesse said; the framework of relativity does imply a static block of 4D-spacetime to be a valid model of reality.

    Anyhow, a while back there was a thread that I started about this same issue, and after some confusing exchange, I think the consensus was that there is no real consensus, and that the ontological interpretation of relativity is very much open in this respect.

    But I think this is one of the more interesting facets of relativity, so let's talk about it :)

    Consider yourself in a lab frame looking at a clock one light-second away. Ask yourself, "what is the actual state of the clock at the moment?" Ordinarily you might be inclined to say the actual state of the clock is 1 second ahead of the state you see (in your "subjective" reality), but you must also assume the clock exists in different state in the "subjective reality" of any observer that is moving in the lab-frame, even if this observer is right next to you.

    Basically if you jump onboard of the moving observer now, it could be said that the subjective reality around you should also change its state accordingly. For example, we could just say that the clock went from +1 seconds to +0,5 seconds (half a second backwards) during the change of direction. Of course you wouldn't see any backwards moving clock, but this is the conclusion you'd come to if you assumed there really is a subjective "now"-moment around you the way it is implied in the timespace diagrams, and that framework of relativity is true.

    But the math of SR doesn't necessarily say this, or much of anything about the reality of the "present" moment around an observer. It's how Lazycai said; the horizontal line (the subjective "now"-moment) may well be just a product of our imagination, just something that exists in our model of reality.

    But I would add that I think Einstein believed there is a subjective "now"-moment. This is implied in the thought experiment about the relativity of simultaneity (with the moving train), and in all the talk about length contraction. (If there is no subjective "now"-moment, then there would be no subjective length-contraction either)

    And if you assume there is a subjective "now"-moment (= a 3D-slice of the block-time), then what this would mean to the nature of past and future is that they must be fixed, and anything that happens in your future has already happened in the subjective reality of some observers in relative motion, and also things you've already done may not have happened yet in the subjective reality of another observer.

    Elaboration; Let's say you are in a lab frame with a pole at some distance from you. You send another observer towards the pole (at great speed) and clap your hands when you *know* (by figuring out the speeds and distances) the observer is just about to pass the pole.

    But when this other observer later figures out when did you clap your hands (from the isotropic speed of light), he will conclude you had not yet clapped your hands even well after he had already passed the pole.

    So it can be said that when you had clapped your hands and you knew the observer was at the pole, the reality around that observer was such that you had not clapped your hands yet.

    Or other way around, if there is an observer approaching you, and you decide to clap your hand at the moment you figure the observer must have just passed the pole (in lab-frame), then it will turn out that in the subjective reality around the observer you clapped your hand well *before* it had reached the pole. (assuming he won't stop at the pole but continues onwards until he observes your clapping)

    So we can say anything you will do in the future, you have already done in the subjective reality around other observers.

    Whether the above assertions have anything to do with reality or if they only exist in our model of reality, is up to debate.

    What I would like to say about the model of 4D block time is that while it is understandable how at any arbitrary moment of our worldline the state of our brains could only have memories of the past and semantical predictions of the future - and in this sense it could be said that time is only an illusion - I personally find it philosophically very very difficult view as far as considerations of reality goes, because then there is absolutely no meaning or reality to our conscious experience where we experience as if time exists in one state "at a time" and flows forwards.

    I find it practically impossible to wrap your mind around the sort of reality where "this" moment is not truly occuring, but rather all the so-called "moments" exists at once and everything is still. I find it far more likely that block time model of reality is simply too much of an approximation to be able to address these issues properly, and bitw, this is also where the philosophy of the mind needs to be taken into consideration; another not-so-light subject of metaphysics :)

    I wish I had more time to communicate these things more clearly, but I hope you can pick up just what a huge issue this is for anyone considering the metaphysical nature of reality.

    Let it be still said that if we accept the framework of SR, I personally find it most likely that there is a subjective reality around us where there is a subjective "now"-moment in which things may move forwards and backwards when we change direction, as described by the "horizontal line" of Minkowsky diagram. You can still "visualize" this as if there is a block-time or whatever, as long as you also imagine there really are 3D slices flowing through the block (in different orientations) in a fundamental sense. This is because nothing can deny the fact that our subjective reality *does* consist of time flowing forwards, not a static block of events.

    And as always, reality is painfully, painfully elusive...
     
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