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Trouble with the concept of dimensions and time

  1. Apr 2, 2005 #1

    Its been a number of years since ive actively studied physics, and I find my knowledge extremely lacking, however there are some fundimental concepts I seem to get lost in and would ask for some (polite preferably) assistance.

    My problem is with regards to dimensions and our (or my particular) understanding of them/use of them.

    For instance why do we only use the term dimension? When we talk of somethings position we give a 3 dimensional coordinates - arent we actually describing something which has 3 coordinate dimensional characteristics?

    Another problem I have is with its use with regards to String theory.
    My understanding is we use time as a dimension, giving us 4d which we all understand. String theory talks of 11+ dimensions, but i dont appreciate what it means by dimensions. Do we use dimensions to mean characteristics?

    If yes, could it be argued that an entity has as many dimensions as we can think of properties for that entity?

    Also if we know an entity has at least this many dimensions, could it be argued time itself may consist of more than 1 dimension?

    Im totally confused :yuck:
    Last edited: Apr 2, 2005
  2. jcsd
  3. Apr 2, 2005 #2


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    To 'conceive' of 11 dimensions, start by thinking what the world would be like if you were a two dimensional drawing on a page. There's no way to get around anyone except by climbing other them, you have to regurgitate your food because if you had a 'real' digestive system you would be cut in half. A 3d person could easily touch your insides and see your insides, but you would have no idea how.
  4. Apr 3, 2005 #3

    Thanks for your help, however it doesnt help me get past the 4th dimension (lol). I can easily grasp the concept of an entity in 2 dimensions, 3 dimensions, and posibly 4 dimensions with the 4th being an overlay on the other dimensions. Its the idea of dimensions curled up at a point - and the diagrams used to represent them.

    To me - any dimension for a given entity is related to the the dimensions of all entities around it - however this "wrapped" up dimension information makes me think the extra ones arent related between entities - i guess a good comparrison is that everything in the universe is a universe - or more precisely everything in the universe contains a universe?
  5. Apr 3, 2005 #4
    i think of dimensions fitting in groups:
    1: X
    2: Y
    3: Z
    4: chronological time
    5: choice/chance (i've never heard much about this, but i beleive in it anyway, otherwise we're probablly sticking to religion)

    theres also the 4th dimension of space, which is sorta confusing...

    these are the only ones i can theorise, but i could definitly say there can be infinate amounts of dimensions, there might be people out there who live in a 502,632 dimension existance
  6. Apr 3, 2005 #5
    Well thanks again,

    I still cant help but find all the terms misleading. To me (well yes im a nobody) there can be no more than 3 dimensions because thats what I percieve everything in. Sure i can map loads of variables which rely on those 3 base dimensions to that 3 dimensional view, but why do we say there are more than those 3 easy to understand ones?

    Even time doesnt itself seem to fit into the same category - you can map it to those 3 dimensions, but does that make it a dimension in itself - other than it HAS dimension, and we notice the passage of it on things that we can map into those dimensions?

    *brain starts to hurt*
  7. Apr 3, 2005 #6
    Each of the three dimensions need each other in order to make sense too. It isn't only time that needs the three to notice it as a dimension. How would you know if an one dimensional plane is going back/forth, up/down or left right without using the other dimensions to notice the direction your traveling.
  8. Apr 3, 2005 #7


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    "Space" is really a mathematical construct. The fact that we visualize the world in a three-dimensional position space is more a consequence of our biology than anything else. Anytime we have a set of linearly independent variables, we can make a space out of them, but the objects we're describing in that space may not always have the same properties in every dimension. For example, I could describe a color printout in three dimensions: two of position and one of color (or, equivalently, frequency of light). This 3-D space is just as mathematically valid as the three-dimensional one we see every day, but the contents of the paper behave differently in the "color" dimension than they do in the two spatial dimensions. Newton's Laws can't be used to describe the change of "color" as a function of time, but they can describe the behavior of the paper in the two spatial dimensions.

    Thus, we group the properties of the physical world by the laws that they follow and look for similarities. Usually, if two linearly independent variables behave in the same way, we'll group them together into a "space". What was discovered with relativity was that time and position act in very similar ways, so much so that it became useful to group them together into a single space and describe objects according to their position on "spacetime" rather than just in position space. Because of a minor difference between position and time, however, our bodies decided not to perceive them in the same way. As a result of this, our brains have a lot of trouble coming to grips with the concept of spaces larger than three dimensions.

    I find that the best way to think about these things is to imagine the question at hand in fewer than three dimensions and then extrapolate up to three. Once I've done this, it's usually pretty clear how to continue on to four, five, or more; at least from the mathematical point of view. In the context of relativity, this idea was popularized as "flatland", an imaginary two-dimensional world that was being effected by the forces of an unseen third dimension.
  9. Apr 3, 2005 #8
    Thank you very much for your excellent explanation - however it still doesnt help me get past a maximum of 3 real dimensions..

    I can imagine 3 dimensions no problem because the dimensions themselves dont overlap. I can even bring my somewhat feeble mind to imagine a 4th dimension (though to me it isnt a dimension as such which is what causes me such problems) by overlapping the existing dimensions. and thats where my problem lies.

    Because the first 3 dont overlap - they are what I cant help but understand as dimensions. I cant map our known universe to less. I cant picture more without some overlap .. please excuse my ignorance.

    Now i percieve time overlapping on the same space that was here before which is maybe where i get some of my problems. This overlapping is what breaks my whole mental model, and the basis of why I cant percieve it in the same group of things we call dimensions, other than a variable. Its also why I cant seem to get beyond 4 dimensions at most.

    But when i try and think about it - if i didnt view it that way - it would mean time itself might be mapped to a seperate 3 dimensions the same as everything else we view ?
  10. Apr 3, 2005 #9
    Try to think of a dimension as a direction. In what we perceive as the world, we have three directions and time. Time is also a dimension because it has the fundamental concept of having a forward and a backward.

    The fact that we seem to be moving forward has to do with our brain containing info on the past and not the future, moving through time forward as we seem to do may well be merely an illusion.

    It is not such a strange idea to think of more directions that can be, but we cannot perceive them, because we're build the way we are. Very much like seeming to go forward in time.

    Consider the following. What if someone (call him Paul) knew the world as it is this instant. Everything stands still, but Paul can move around, move things around too, but nothing moves out of itself, only if Paul sets it in motion.

    Now consider us living in our world. We go past the point in time where Paul lives and we see in a split second a lot of things moved, but we didn't see them move.

    What I just described is a 5-dimensional world with two time dimensions, ours and Pauls. In this way it is definitely possible to imagine a more than 4-dimensional world. You can do the same with dimensions of space, have Paul see three spacedimensions, two we share and a third one.

    What Paul is a disection of our universe at a very specific point in his other dimension. For example, from where paul is standing say at the height of one meter is the universe in length and in width precisely the same as in our universe in some point in our hight. If he moves upward from that plane he sees the universe changing in a similar way we do, but the universe will not be exactly the same in any plane other than exactly horizontal at height one meter.

    Now we described a universe with 5 dimensions, our three spacedimensions + Pauls spacedimensions + time. It's easy to expand the idea with more dimensions.

    Normally you need to do so in mathematical way, because it helps you with the calculations, but sadly nobody does it in this way usually because it really helps your understanding.

    Something to note with all this is that it's clear that there is no movement towards the other dimensions. If Paul would move anything in his extra time-dimension, we would see it move instantly for us. And if Paul would move something from the plane intersecting our dimensions in his world with the extra space dimension to a direction not in our dimensions, than that bit of matter would be lost to our world. We would see it disappear.
  11. Apr 3, 2005 #10
    Please bear with me since im getting a little confused as to how this could/would be possible.

    Well i have no problem with that.

    This concept confuses me a little. If it was only us and how we stored the things we percieved, why wouldnt we observe other things not obeying this (such as trees appear out of thin air and revert to a seed over time)?

    That all really does confuse me somewhat. Would Pauls other dimension exist in ours always? Would ours in his? I still seem to try and picture your description in a 2/3dimensional way which causes me more questions than it answers.

    Would Paul always exist to us but at that one instance in time appear to change and change the things we percieve - if so how would we even know other than noticing something changed (and since it only existed once - we wouldnt notice the change again)? or would he just suddenly "exist" and disappear to us - in which case again how would we know other than things have changed?
  12. Apr 4, 2005 #11
    You have to picture it in a 2/3dimensional way to understand it.What I'm trying to show you is that you can never understand the thing it completely is. Consider The following threedimensional world.

    The world is made out of two pices of paper crossed, so two planes. sideview: +. In both dimensions the world is smooth, so what you draw on either one of the planes is smooth. Now draw on one of the planes a cirkel and on the other an elliptical figure. To remain smooth, the elliptical figure must have at the line of intersection it's points in common with the cirkel. We can tell our universe is smooth in a similar way, so this must be true if the universe has more than 4 dimensions. Now say that the cirkel is a two-dimensional planet and so is the elliptical figure on the other plane. Both planets are obviously connected, but for no-one living on either planet is a threedimensional world conceivable. You can even say that they actually live on the same planet.

    So ours and Pauls dimension form one spacetime, but a spacetime both Paul and we can see only part of.

    If Paul lives in a different timedimension, then his dimension intersects only at one point with ours. Take a spacetime diagram with the movement of something, say a particleand we see a line. If we have a universe with two timedimensions, call them time1 and time2, we would have a curved plane. take a point in time1 and you have a line. take a point in time2 and you have a line. Say that we are standing still in time2, that means you can write our motion in a line. Paul would then be standing still in time1 and moving in time2. That would mean that his direction comes into contact with our world, as his time reaches the moment we are standing still at in time2!

    Also I notice now that I made an error in my first post, I stated that paul with a different time dimension could change something for us instantly, that is invalid because he ony touches our world in an instant and he can move nothing in a single instant. When he has one spacedimension different then he can change things.

    there is a difference in going from past to future and distinguishing a past and a future. Take for example a piece of would, colored so that from one end to the other, it's color slowly changes from green to orange. If we walk from green to orange over that stick, or from orange to green gives us only a changing to us in general. But still always right will be orange and left green.

    We cannot see if we move through time, we live only in the now and the now is at all times an instant. If we take for example an old man and rewind his life backwards. the old man has in the beginning of this 80years of long term memory and a couple of minutes short term. Moving him backwards in time erases part of his memory. So if we move him backwards in time, he will not remember moving backwards through time, but he will remember living untilkl the point we moved him back to. Say if he dropped a cup. When the cup is shattered, he remembers dropping it and seeing it break on the floor. Now we set time in motion backwards and we see the cup's pieces coming back together and flying up. now if we take the moment it is in th air, our man will rememberdropping the cup, but not it smacking into the ground. Our sense of how time moves has everything to do with our memory.

    There is absolutely no guarantee that we aren't standing still or moving backward. It's a very interesting concept.
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