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What actually is a dimension?

  1. Apr 22, 2012 #1
    I wanted to know what is the definition and what actually is a dimension. I mean I get that length, width and height story, but how does time fit in as a dimension. Then what about higher dimensions. Like if a certain system was defined in 7 dimensions what exactly would those dimensions be? 4 could be time, length width and height but what about the other 3?
  2. jcsd
  3. Apr 22, 2012 #2
    Actually, I have a similar question (that I hope won't interfere with the original OPs question) so I thought I'd add it here.

    I can see us dividing our space up into dimensions of x,y,z and for myself - though probably not for others - I can see myself adding time as a dimension of movement and also acceleration as a dimension of force or reaction.

    My question is about the extra dimensions too. Obviously we can refer to our x,y,z dimensions without absolutely needing a preference ie. x can describe any direction in space and the others are then just 90deg to this and each other. You can rotate your graph any revolution in space and it can still be used to plot points in space.

    So what then prevents the simple movement through those other dimensions; the movement that is so easily observed to remain in the 3 dimensions that we do see? What gives them a preference of exclusion?

    Obviously, if you referred to mathematical dimensions; as I have to space, movement and acceleration, then you get obvious mechanisms of exclusion ie x,y,z can not represent moving without time as a dimension nor direction changing without reaction as a dimension.

    Are there any similar established mechanisms of exclusion for those other dimensions that keep them separate from x,y,z? If there are what are they?
  4. Apr 22, 2012 #3


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    The number of variables required to completely specify the state of your system is the number of dimensions of the system.

    It takes 3 numbers to specify the location of a object in space, therefore we speak of 3 spatial dimensions. If you throw in time you get a 4th dimension.

    If you are tracking the fluid level of 100 different tanks you have a system with 100 dimensions.
  5. Apr 22, 2012 #4
    Thanks Integral. By this it then perhaps becomes more logical to refer to extra dimensions. Maybe it is okay then for theories like string theory to have extra dimensions without needing to resort to physical 'braines' and such.

    I would wonder if the real ultimate trick is in trying to reduce a model down to the fewest needed dimensions for its representation then?

    I guess this leaves the question of what can propagate through those dimensions as the limiting factor. If for example it were that there were four spatial dimensions (one hidden from us) then I would have to wonder what prevents us smoothly rotating through the fourth direction as easily as any of the other three directions. What provides the barrier to this movement?
  6. Apr 22, 2012 #5


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    I don't think this is sound logic. In string theory, the extra dimensions look, feel, and smell just like the other ones, which is why we assume they're on exactly the same footing (well, of course subject to compactification. But they're still genuine spatial dimensions).
  7. Apr 22, 2012 #6
    Thanks Nabeshin. This then raises the question, which I mentioned above, as to what ties us inside our three dimensions? What excludes us from moving through those other dimensions as easily.

    The three space dimensions we attribute ourselves x,y,z don't have any preferred direction, as I said, so you can claim any direction as being the x dimension and the other dimensions then only need to be 90deg to this and each other. We can freely rotate within this without restriction.

    What stops us from as easily rotating into those other 'hidden' dimensions if they are all equal with ours?
  8. Apr 23, 2012 #7


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    First, it is not proven that they even EXIST.

    As posited by string theory they are TINY. For all we know, we DO move through them.
  9. Apr 23, 2012 #8
    You can think of it in different ways...and it does require some different thinking....

    for a start think about plotting d = vt...what do you do?? typically you'll plot time as one of your 'variables', right??....just think of that as a 'dimension'. It's a simple 'spacetime' diagram with ONE dimension of space. In these 'low speed' physics, space and time remain separate entities....they do not mix.

    YOU can also plot two or three dimensions....no need to stop with three.....but visualizing beyond that is tough for our senses....

    I've found this visual helpful: consider an x,y plane with time t perpendicular to the plane[two space dimensions for simplicity] : then the general plot of a particle moving at constant velocity is a straight line away from the x,y plane at some angle [it'smoving through space and time in a straight line]; circular motion [a merry go round] becomes a corkscrew plot since motion is both rotational in x,y and also moves thru time...say roughly parallel to the constant velocity plot; and a linearly accelerating particle moves away from the x,y plane thru time, but now is a curved plot.

    Have you ever plotted, say, energy versus say height...as in potential energy??? There typically as height increases relative to a reference, so does PE. So here you have PE as a 'dimension'...but we don't think of that one as a physical 'direction' merely an increasing 'value' of some measurement...analogous to conventional time.

    Another perspective is this :

    A reference frame is composed of origin, a set of axes along with some convenient coordinates [reference points]. Reference frames are not a property of our universe; they are mathematical devices helpful to us for analyzing physics, not physical features themselves. The word "Euclidean" applies to 3 spatial dimensions; The 3+1 geometry (3 space, one time) is Minkowski spacetime used in special relativity.

    Things get more involved in special relativity...where neither distance nor time remain the same to different observers...there we talk about 'length contraction' and 'time dilation' due to relative speeds....what different observers do all see the same is, oddly, the speed of light. In essence, space and time mix together .....and became more involved than even Einstein first realized.... so as to keep observations of light speed constant...THAT is not part of classical [Newtonian] physics....

    so my view is "If Einstein's math professor changed Einstein's view from space and time to 'spacetime' then it's not so dumb that I have some questions,too."
  10. Apr 23, 2012 #9


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    There are totally different usages of the term dimension. Which one needs to be figured from the context. Here are a few examples:
    1) any degree of freedom. This usage is more common in classical mechanics with generalized coordinates, thermodynamics, robotics
    2) number of independent basis vectors in a vector space. related to 1). Usual definition in quantum mechanics.
    3) fractal dimension, especially Hausdorff dimension. If you are messing with fractals.
    4) space-time-like dimensions in the sense that all of our vectors and tensors need to be extended for all of these extra dimensions. It makes sense to rotate from one dimension into another. Fields exist over all these dimensions, and certain force laws follow a 1/r^(#spacedimensions-1) relationship. This is used by some grand unified theories. Since there is no evidence for extra dimensions, they are presumed to be small.
  11. Apr 24, 2012 #10
    I imagine that we would have to move in such a way as to maintain our overall rigid or elastic shape shape in our space with maybe some fluctuation at the miniature level or something in and out of these 'tiny' dimensions?

    I wonder if the string theory dimensions are supposed to be 90deg to our space and to each other?

    As an example, a 2D person looking at a 2D-circle would only see a shaded line. If the circle were actually a cylinder with a hidden 3rd dimension then somehow the cylinder must maintain orientation to that 2D world through exactly the same slice. So some mechanism must be preventing the cylinder from rotating in the 3 dimensions and only being able to move in relation to the 2 dimensions although when it moves it moves in all dimensions.

    The other alternative is where the 2D-circle bubbles in and out of the 2D plane into the extra hidden dimension while maintaining it's 2D-circle appearance. The extra dimensions then somehow become like overflow or spillage buffers.

    The later seems unlikely and the former begs a mechanism to prevent free rotation in all the dimensions...

    If the extra dimensions are tiny are they tiny in respect of having limited depth or are they tiny in respect of severely compacting anything that moves into them. Another sort of question: like our space dimensions; do the extra dimensions of string theory have 'long' distances in them?
  12. Apr 26, 2012 #11
    Thinking about it mathematically I wonder if it is possible to consider that Einstein added a second time dimension when he developed his relativity?
    Things no longer move with straight forward relative movement but now can move through 'time' at different speeds due to this added mathematical 'dimension'.
    Is it possible to think of it this way?
  13. Apr 26, 2012 #12


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    A SECOND time dimension? I don't see how you get that, and it certainly is not how anyone else that I'm aware of views it.
  14. Apr 26, 2012 #13


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    One can move at different speeds along any single dimension in space too, but that does not require us adding an extra dimension for each existing one.
  15. Apr 26, 2012 #14
    Well, GR and SR actually could be considered to add two extra dimensions to equations couldn't they? I'm not talking about physical dimensions; though science does treat physical aspects mathematically. I'm talking about adding an extra dimension to the classical models which treated things in a direct fashion. They certainly add some extra dimensions to how we think about simple movement.
    I'm not saying that anyone has thought of it that way. I'm saying it could be thought of in that way.
    I can't see that having two time dimensions means that you are in one or the other.
    To me a second (or more dimension) simply affects how you would be thought to move through the spatial dimensions. And GR and SR certainly affect how we animate through the spatial dimensions.
    We don't have a fixed x dimension nor y dimension nor z dimension. They are all mutable.
    But mathematically we can describe them that way.
    In the same mathematical sense I can't see why we can't say that GR & SR add an extra dimension each to our equations?

    To move at different speeds along a direction in space you need to be acted upon which I'm wondering if this reaction also adds a dimension itself to our equations as well?
    New ways of thinking are not a new thing in science as long as they fit within their logical context...
    Take our current debate over big bang, crunch, bounce, expansion, string theory, etc.

    All I'm saying is that they can be thought to add extra dimensions to our formula that we didn't think were there before Einstein.
    Last edited: Apr 26, 2012
  16. Apr 26, 2012 #15


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    Gonegahgah, your extra dimensions don't fit the same definition as the classic 4 we talk about. Inventing new definitions won't get us anywhere.
  17. Apr 26, 2012 #16
    Which is exactly what string theory is attempting to do?
    And again as I say our cosmologists are coming up with new definitions of our universe.
    Blocking all new definitions isn't generally the way forward in physics. Resistance is certainly fine.

    The question then comes back to whether what I'm saying can have it's own context.
    And then the next question is can that context be of any use...

    Even if it ends up not being useful to anyone it still may have validity within its own framework.
    Again, look at string theory.
    Last edited: Apr 26, 2012
  18. Apr 26, 2012 #17


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    The dimensions in string theory are all defined within that theory to work together and make sense. They aren't exactly the same, just as our space and time dimensions aren't, but they all fit together.

    I have no problems with new definitions. If you develop a full fledged theory of spacetime and gravity and all that, feel free to create as many dimensions as you want to make it work. Until then, please stick to current mainstream theories and their definitions, otherwise the conversations on the forum break down as people won't be able to follow them and they stop making sense.
  19. Apr 26, 2012 #18


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    Clarification for gonegahgah:
    In GR, we are certainly not modifying anything about the dimensionality of spacetime. In normal special relativity, we have the invariant interval given by
    [tex]ds^2=\eta_{\mu \nu} dx^\mu dx^\nu = -dt^2+dx^2+dy^2+dz^2[/tex]
    That's really all there is to the dimensionality story here, Einstein didn't do anything special on that front.

    All that happens when we move to GR is that in general we promote [itex]\eta[/itex] to a metric tensor which is in general a function of the spacetime coordinates
    [tex]ds^2=g_{\mu \nu} (x ^\alpha) dx^\mu dx^\nu[/tex]
    But the tensor is still of dimension four, we really haven't altered the dimensionality at all.

    In short, no it is not possible that we've somehow added an extra dimension. Keep in mind that the coordinate vectors we choose to represent our dimensions have to be linearly independent. So it's not as through defining a new vector in normal 4-space promotes us into the fifth dimension, since such a vector is spanned by the existing space.

    With re: to the rest of what you've written trying to explain why this makes sense to you, I cannot make heads or tails of it, but I hope my explanation has clarified a bit.

    Also, as a general note, it doesn't make any sense to lean on string theory when trying to state that currently established theories somehow add extra dimensions nobody else seems to be aware of. String Theory is an untested proposal, while something like relativity is (as close as can be) scientific fact.
  20. Apr 27, 2012 #19
    and for the less educated people like myself, this makes sense using string theory lol http://www.tenthdimension.com/medialinks.php

    i understand that string theory is, as stated above, an "untested proposal" but when people start talking about "dimensions" i find that this video is an easy way out
  21. Apr 27, 2012 #20


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    Just to clarify, I've seen this video before and while it's an interesting exercise, everything he talks about dimensions above the usual four dimensions is unrelated to string theory. Specifically, you can define these extra 'dimensions' in any way you like, but they're unrelated to any 'dimensions' we are familiar with in the natural sense. So really it's a purely MATHEMATICAL exercise, and his attempt to link it to string theory at the end is very flawed, for it gives the impression to the viewer that this is a PHYSICAL rather than mathematical conclusion.
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