Dimensions Definition and 1000 Threads

In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one (1D) because only one coordinate is needed to specify a point on it – for example, the point at 5 on a number line. A surface such as a plane or the surface of a cylinder or sphere has a dimension of two (2D) because two coordinates are needed to specify a point on it – for example, both a latitude and longitude are required to locate a point on the surface of a sphere. The inside of a cube, a cylinder or a sphere is three-dimensional (3D) because three coordinates are needed to locate a point within these spaces.
In classical mechanics, space and time are different categories and refer to absolute space and time. That conception of the world is a four-dimensional space but not the one that was found necessary to describe electromagnetism. The four dimensions (4D) of spacetime consist of events that are not absolutely defined spatially and temporally, but rather are known relative to the motion of an observer. Minkowski space first approximates the universe without gravity; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity. 10 dimensions are used to describe superstring theory (6D hyperspace + 4D), 11 dimensions can describe supergravity and M-theory (7D hyperspace + 4D), and the state-space of quantum mechanics is an infinite-dimensional function space.
The concept of dimension is not restricted to physical objects. High-dimensional spaces frequently occur in mathematics and the sciences. They may be parameter spaces or configuration spaces such as in Lagrangian or Hamiltonian mechanics; these are abstract spaces, independent of the physical space we live in.

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  1. K

    A Alternative topology of our 3D space

    Mathematically, a flat 2D surface folded down over the edge of a desk has no curvature, so a Flatlander could not detect the fold. Same for us in space if our 3D space was folded down in a 4 dimensional sense. Suppose 3D space was folded down in two different perpendicular directions, both...
  2. billtodd

    I Experiments to check for extra dimensions

    Suppose we want to verify that there are curled-up dimensions. What sort of experiments can you devise to check for them? I mean what sort of experimental data are we looking for? (I guess it would be in high-speed particles' accelerators, but sure there are also cosmological observations that...
  3. F

    Insights Aspects Behind the Concept of Dimension in Various Fields

    [url="https://www.physicsforums.com/insights/aspects-behind-the-concept-of-dimension-in-various-fields/"]Continue reading...
  4. cianfa72

    I About the definition of vector space of infinite dimension

    Hi, a doubt about the definition of vector space. Take for instance the set of polynomals defined on a field ##\mathbb R ## or ##\mathbb C##. One can define the sum of them and the product for a scalar, and check the axioms of vector space are actually fullfilled. Now the point is: if one...
  5. H

    I Gravitation In Higher Dimensions

    It is assumed that gravitation in n dimensions would follow an approximate 1/d^(n-1) law. In our 3D world the attraction of a uniformly dense sphere is the same as if all the mass were concentrated at its center. I have read for n>3 this is not so. I want to find out what the result would...
  6. E

    A Dimensions: Emergent from the Big Bang?

    If we accept the concept that the universe began as a zero-dimensional point, wouldn't that imply that our 3+1 dimensions, or 9+1 / 10+1 dimensions of string theory, emerged at the moment of the Big Bang or some presumably Planck-order time after? Are there any theories on this?
  7. Kyuubi

    How Does Dimensionality Affect Phonon Polarizations?

    When going from 3 to 2 dimensions, I am unsure about how the number of polarizations will be affected. I know the following though: The 1/8 factor becomes a 1/4 since we are now integrating over the positive quadrant in 2d rather than the positive octant in 3d. The ##4\pi n^2## becomes a ##2\pi...
  8. H

    How would your body change if you were in a 4D space?

    I don't believe this question will interest anyone else, but remains worthy of notice because the solution is a formula that depends on the 36th root of a number. You unexpectedly find yourself in a space with four Euclidean dimensions. Fortunately your molecules have been rearrange into a four...
  9. E

    I What is the velocity of a photon through the four dimensions?

    What is the velocity of a photon through the four dimensions? What is the velocity of a photon through the three spatial dimensions x1, x2, x3? What is the velocity of a photon through the fourth dimension x4?
  10. H

    Equator of a Planet with Four Spatial Dimensions is a 4D Torus

    A 4D planet has no axis of rotation. Nothing special about planets, all freely rotating 4D bodies have two perpendicular planes of rigid rotation. (Clifford proved this in the 19th century.) Now there is nothing stopping us from thinking of planes of rotation here in 3D Universe. It's the...
  11. James1238765

    I Geodesic path in 2 dimensions?

    The object takes a step [x, y] in 2 dimensional space. This is represented the change in coordinate ##x \vec e_x + y \vec e_y## where ##e_x## and ##e_y## are basis vectors in this space. Suppose we define a non-linear / parametric transformation of this ##\vec e_x## and ##\vec e_y## basis...
  12. A

    B Need some clarification to get dimensions for a volume

    I have a volume of 0.04 cubic centimeters. I need to convert into something I can work with in my shop. Tolerance is not important. To get dimensions of a cube with this volume, I calculated the cube-root which came out to 0.341. So, my cube would be (rounded) 3mm on a side, correct? Next...
  13. sol47739

    B Shape & Dimensions of Containers: Impact on the Maxwell Boltzmann Distribution

    1.Does the Maxwell Boltzmann distribution change depending on the shape of the container? Pressure and the volume is constant. How is the Distribution affected whether the gas is in: a,sphere b,cube c,cuboid? Why does/doesn’t the distribution change depending on the shape of the container...
  14. N

    B How to describe complicated dimensions?

    Hi, just wondering if the dimension of velocity is m/s that can be described as what distance is passed in a specific time, then how can I describe volt which is kg•m²÷s³÷A? Mass in the area that moved... I can't even imagine. Thanks.
  15. Feynstein100

    I Does topology distinguish between real and imaginary dimensions?

    Let's say we have four 3D spaces: (x, y, z) , (x, y, iz) , (x, iy, iz) and (ix, iy, iz), with i being the imaginary unit. Now, let's say we have a donut in each of these spaces. Geometrically, the donuts are different objects, have different equations and different properties (I think) but would...
  16. T

    I Why do dimensions curl up or expand?

    In string theory, the universe can have 9-10 spatial dimensions and the reason why we experience 3 is because those higher dimensions compactify. Under right conditions the extra dimensions can decompactify into the macroscopic dimensions we see. Why do some dimensions curl up or expand? Is...
  17. Euge

    POTW Gaussian Integrals in Two Dimensions

    Let ##a, b##, and ##c## be real numbers such that ##a## and ##c## are positive and ##ac > b^2##. Evaluate the double integral $$\int_{-\infty}^\infty \int_{-\infty}^\infty e^{-ax^2 - 2bxy - cy^2}\, dx\, dy$$
  18. G

    Kinematics in Two Dimensions (Monkey and Hunter Thought Experiment)

  19. Al-Layth

    How do Engineers Calculate the Dimensions of their Design Ideas?

    You have a design problem statement, which gives you numerical specifications for performance, spatial constraints etc, and you have finally come up with some mechanism ideas you synthesised into a machine concept. You’ve sketched your design’s parts and the final assembly. So: How do you now...
  20. DaveC426913

    B Time Dimensions: Astronomy & Beyond

    This is really a conceptual question about time dimensions bit it came up in the context of astronomy so I'll put it here. I was gazing up at the sunset sky the other night and saw a very bright (twinkling) star. I was about to make a note about its location so I could look it up later. I...
  21. ohwilleke

    I Trying To Conceptualize Two Dimensions Of Time

    Some BSM theories, often but not exclusively string theory inspired, have an extra time dimension, as well as extra spatial dimensions. I'm trying to make sense of what it even means to have more than one time dimension, ideally, with a concrete example that illustrates how the second time...
  22. H

    I Dimensions vs. degrees of freedom

    Are dimensions the same thing as degrees of freedom? Would you say that a circle is a one dimensional object embedded in a two dimensional space?
  23. J

    I Dimensions of quantum cell automata's state space

    In the paper C. S. Lent and P. D. Tougaw, "A device architecture for computing with quantum dots," in Proceedings of the IEEE, vol. 85, no. 4, pp. 541-557, April 1997, doi: 10.1109/5.573 about quantum dots, it is stated that the basis vectors in the space of quantum states for a single cell...
  24. JandeWandelaar

    A What does the metric of a 6D space with 3 compactified dimensions look like?

    I'm interested in describing a 6-dimensional space of which three are compactified to small circles. Globally this space looks 3-dimensional, like a 2-dimensional cylinder looks 1-dimensional globally. Kaluza and Klein did a similar thing in the context of 4-dimensional spacetime. They extended...
  25. S

    Typical PWR fuel assembly dimensions

    What are the typical dimensions of the coolant channels and the upper and lower core support plates?
  26. K

    I How Does the Flow Equation Apply in Higher Dimensional Phase Spaces?

    We are in phase space of ##6N## dimensions. Each point ##\mathbf r## in this space has ##6N## coordinates. Pathria writes "Consider an arbitrary "volume" ##\omega## in the relevant region of the phase space and let the "surface" enclosing this volume be denoted by ##\sigma## then the net rate...
  27. S

    Motion in two dimensions: Launching up an incline

    But it seems to get too complex and am not sure whether it is correct.. my solution got a bit messy + incorrect Thanks in advance!
  28. I

    B Calculate needed dimensions of table - Center of mass, levers etc

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  29. T

    I Do extra dimensions lead to expansion of our universe?

    https://arxiv.org/abs/hep-th/0303031#:~:text=In%20gravitational%20theories%20with%20extra,of%20our%20four%2Ddimensional%20world. This paper seems to suggest that if there are extra dimensions , then they are unstable and can expand turning our 4d universe into a higher dimensional space.
  30. T

    I What Shapes Can Extra Dimensions Take in String Theory?

    I read Brian Greenes book "The Elegant Universe". In it there is one chapter talking about extra dimensions. The first idea was proposed by Kaluza and Klein. They suggested that the extra fifth dimension exists as a circle, is it right. Is this called a KK circle? Now Greene also mentioned that...
  31. L

    I Dimensions of our physical world

    Is there anything we know of in this physical world that is less than 3 spatial and 1 time dimension? Or is that pretty much everything, no more, no less?
  32. H

    A quite verbal proof that if V is finite dimensional then S is also....

    If a linear space ##V## is finite dimensional then ##S##, a subspace of ##V##, is also finite-dimensional and ##dim ~S \leq dim~V##. Proof: Let's assume that ##A = \{u_1, u_2, \cdots u_n\}## be a basis for ##V##. Well, then any element ##x## of ##V## can be represented as $$ x =...
  33. S

    I If one of the dimensions of our solar system contracted ....

    ... by a factor of 10, then would it operate exactly the same way as if it did not contract? I ask this because there could be an observer at a sufficient speed for which our solar system contracts by a factor of 10. Wouldn't our solar system and its contents have to behave differently for the...
  34. P

    MHB New Billboard Dimensions: 4m x (4m + 96cm^2)

    An amusement park wants to place a new rectangular billboard to inform visitors of their new attractions. Suppose the length of the billboard to be placed is 4m longer than its width and the area is 96cm^2. What will be the length and the width of the billboard?
  35. S

    A Branes with any number "n" of dimensions and laws of physics?

    I had a few questions about this paper by Nima Arkani-Hamed, Georgi Dvali and Savas Dimopoulos (https://arxiv.org/abs/hep-th/9907209) which is closely related to the concept of branes (https://en.wikipedia.org/wiki/Brane) 1. The authors of the paper mention that the branes in the model could...
  36. Lynch101

    B Proof Pertaining to Possible Paths Through 3 Dimensions

    I'll try to phrase this as clearly as possible but my use of terminology might need to be refined. That may be what ultimately comes of this thread, but hopefully the question as I phrase it will make enough sense. I'm not necessarily asking that a proof be provided, rather, I am interested to...
  37. TheHeraclitus

    I String theory in 4 dimensions?

    Non-critical string theory, on Wikipedia it says: Has it been experimentally disproven or are other reasons for its apparent obscurity?
  38. C

    Calculate dimensions of floating laptop stand

    I am planning to build my own laptop stand with an acrylic sheet. The final product should look like the one in the attached image (bottom right) Material: most laptop stands are made with 6mm acrylic thickness, hence this is what I will use. Also I already have this material at home. I am...
  39. Paige_Turner

    I Negative curvature in low dimensions

    It's probably the hyperbola, but I don't see how it's curvature is negative. It looks like 2 parabolas. thanx, paige turner
  40. D

    Motion in 2 dimensions -- Total displacememt of an ant walking

    I got the answer right and here's my work, but I want to know how to get the final direction correct [E 76 S]: Dtx = 2.78 [W] + (6.25)(sin40) Dtx= 1.237 Dty= (6.25)(cos40) Dty= 4.95 Dt= 4.95 (pythagorean thereom involving 1.237 and 4.95) Dangle = tan^ (4.95/1.237) = 76 Therefore, it is...
  41. AHSAN MUJTABA

    I Dimensions of Cosmic Scale Factor ##a(t)##

    I know the formula for Hubble's parameter, ##\frac{\dot{a}}{a}##, but I cannot infer any dimension of ##a(t)## from it. Please guide me. Thanks.
  42. AHSAN MUJTABA

    I Dimensions of P and ##\omega##

    I am studying polymer quantum mechanics. In it, they say that the momentum, ##p## eigenvalue, has the dimensions of ##(mass)^{-1}## and similarly ##\omega## has the dimensions of ##mass##. How it is possible, please someone explain that to me. Even a little hint would work. I don't get it...
  43. Diego Josephia

    B Can We Discover Multiple Time Dimensions Like Space?

    Heloo guys, diego here, I was wondering if there exist multiple space dimensions, and we can only see in the 3th dimension, it is entirely possible to have multiple time dimensions. And another thing, we all learn(simplified version coming) 1 dimension is basically a point, 2nd dimension is a...
  44. patric44

    How does Generalized Work have dimensions of Work?

    hi guys my analytical mechanics professor asked a question the other day about, how come the generalized forces##Q_{\alpha}## doesn't need to have a dimension of force, and the generalized coordinated ##q_{\alpha}##as well doesn't need to have a dimension of length, but the generalized work...
  45. MathematicalPhysicist

    A How to Find Authors' Emails for SU(N) Gauge Theory Paper?

    I want to find the email of the authors. The problem is I cannot find hers. How to find it? Any help? Thanks in advance!
  46. L

    Elastic collision in 2 dimensions

    https://en.wikipedia.org/wiki/Elastic_collision By the angle θ they mean some angle before or during the collision, or after the collision?
  47. JD_PM

    A Renormalizability conditions for a real scalar field in d dimensions

    I am studying the real scalar field theory in ##d## spacetime dimensions as beautifully presented by M. Srednicki QFT's draft book, chapter 18 (actually, for the sake of simplicity, let us include polynomial interactions of degree less than or equal to 6 only) \begin{equation*} \mathcal{L}...
  48. S

    I Brane cosmology without string theory?

    There are several models of brane cosmology (https://en.wikipedia.org/wiki/Brane_cosmology) and several physicists working in this field (e.g Lisa Randall and Raman Sundrum), but as you will notice, apparently they are all directly related to string theory. This has several consequences, for...
  49. mustafamistik

    Throwing Objects - Motion in Two Dimensions

    I stuck on part c and d. My attemption is attached.
  50. person123

    I Computing the Speed of Traveling Waves in 2 Dimensions

    I have a 2-dimensionsal smooth function ##f(x,y,t)##. There may be multiple traveling waves across the domain. None of them are precisely traveling waves (the shape of the wave changes as it travels). Here is how one of these waves would look in 1-dimension: I want to find the speed of these...
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