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Time moves forward, what if you move UP or DOWN? Does time stop?

  1. Sep 9, 2013 #1
    Time moves forward, what if "you" move UP or DOWN? Does time stop?

    I know this is a really "dumb" question but I was thinking about the time continuem that moves from left to right according to the "big bang theory" that set the "arrow of time" into motion. Im sorry if this question seems silly but im really trying to understand. if you move "upwards" in space instead of left to right (like a car driving across the united states") does time stop or stay still? -----> the arrow of time does not make sense in my head. What if you were to move DOWN or move UP without moving side to side? Ahhhh. I know this question sounds crazy and please forgive me if you cant understand what im saying, its hard to describe.

    thankyou for anyone who cares to respond, i was just curious as i think about random things like this alot
  2. jcsd
  3. Sep 9, 2013 #2


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    "Moving with the arrow of time" is just fancy speak for "growing older". As far as we know, we can't grow younger, hence the "arrow". It points "where" we go.
  4. Sep 9, 2013 #3
    I'm no expert in the subject but, since time is considered as one dimension, wouldn't it mean that you can only move either way of the hypothetical "line of time" and you can't go in other directions (such as up and down) because that would require at least 2 dimensions (such as a plane surface)?
    I apologize if I said something incorrect
  5. Sep 9, 2013 #4
    (Note: This post was edited by DH to conform with the site's copyright rules.)

    From http://www.hawking.org.uk/the-beginning-of-time.html
    Stephen W. Hawking writes "It seems that Quantum theory, on the other hand, can predict how the universe will begin. Quantum theory introduces a new idea, that of imaginary time. ...".​
    Follow the link for more.

    I googled that and I think that was my question but I guess i should have posted the question in the quantum category because it seems a little wacky.

    Thanks for the response !
    Last edited by a moderator: Sep 9, 2013
  6. Sep 9, 2013 #5


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    The arrow of time applies only to the time dimension--it doesn't apply to any of the three spatial dimensions so it won't matter whether you are moving up or down or left or right or in or out--time still marches on in the same direction as always.

    By the way, the only reason why there is an expression called the "arrow of time" is because in equations that include time, if we didn't know better, solutions could have time going in either direction, but we are smart enough to know that the negative direction is meaningless. It's kind of like if we had an equation for the area, A, of a square determined from the length of a side, s, like this:

    A = s2

    and then we wanted to turn it around given the area and we want to determine the length of a side, we would have:

    s = √A

    But since we know that a square root has both a positive and a negative solution, we are smart enough to know that the negative solution is meaningless.
  7. Sep 9, 2013 #6
    I'm sorry I didn't get what you meant, lindsayb026. Anyway I think I heard of that hypothesis before, isn't it the one proposed by Hartle and S.Hawking? Or maybe I remember it wrong..
  8. Sep 9, 2013 #7


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    This thread requires a lot of valid references of the stuff being discussed. Unless you are able to provide ample support for some of the proposals being discussed, then it will be in violation of the Rules of this forum. If these ideas are backed by peer-reviewed journals, as required in our rules, then please make proper citations to them like you would in writing a paper.

    Otherwise, this thread will be closed.

  9. Sep 9, 2013 #8


    Staff: Mentor

    It could equally well go there or here, since it involves both quantum mechanics and relativity.

    However, I think Hawking was indulging in a little poetic license in the way he described imaginary time. It's not actually a second "direction" of time; it's just a different way of mathematically describing the single direction of time that we observe. Suppose, for example, that we're in flat spacetime, where we would write the metric, using our ordinary "real" time, like this (note that I'm using units where the speed of light is 1):

    ds^2 = - dt^2 + dx^2 + dy^2 + dz^2

    Now, we define "imaginary time" ##T## such that ##t = iT##. Then we have ##dt^2 = i^2 dT^2 = - dT^2## and the metric becomes

    ds^2 = dT^2 + dx^2 + dy^2 + dz^2

    In other words, the metric now looks like an ordinary Euclidean metric in a 4-dimensional space. But we haven't changed any physics; we've only changed the way we describe the physics in the math. (In quantum field theory, this trick is often called "Wick rotation" to further confuse the uninitiated.) Changing the mathematical description this way makes some scenarios easier to analyze; Hartle and Hawking's proposal was that the Big Bang was one such scenario. However, AFAIK their proposal is still speculative.
  10. Sep 9, 2013 #9
    Sorry Zapper, that was stephen hawking hypothesis that i googled on the internet. I dont know what exact website i found it on when it came up in the google search.

    he was basically saying that the state of entropy increases overtime due to the second law of thermodynamics or "arrow of time" something that gives direction to time and distinguishes past from the future...and there are three different "arrows" of describing time...thermodynamic (entropy) psychological (where you feel time passing in your life) and cosmological arrow of time which is the direction of time that the universe is expanding rather than contracting, hawking stated that the psychological arrow was determined by the increase of disorder ( a baby growing old....) and only can work along side with cosmological time (big bang universe expanding)

    what if the universe stopped expanding? would entropy stop? would time stop? is time just relative ? so confusing.

    thanks to all who responded

    Last edited: Sep 9, 2013
  11. Sep 9, 2013 #10


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    The Hawking lecture you linked to makes clear that the answer to these questions is "no":

    "[T]he arrow of time will not reverse. People will continue to get older, even after the universe has begun to contract."

    It's also worth noting that our best current model of the universe indicates that it will never recollapse; it will continue expanding indefinitely. However, AFAIK there is still enough uncertainty in the data vs. the model that we can't completely rule out the possibility of a recollapse.
  12. Sep 9, 2013 #11
    Right, i wanted to see if anyone had any other opinions on it though. It seems he contradicts himself with this topic through some youtube videos ive watched.

    Thanks for the reply!
  13. Sep 9, 2013 #12


    Staff: Mentor

    I think it's more accurate to say his opinion has changed as new theoretical developments occurred. He mentions that in the lecture you linked to.
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