Has the Dimension a mathematical defination?
HallsofIvy said:More generally, "dimension" is used to tell how many numbers are needed to identify something of interest. If we working with problems on a single line or curve, each point can be identified by a single number (simplest way: choose 1 point on the curve to be 0, identify every other point by its distance along the curve from that point, one way being +, the other -)- dimension 1. If we are working with problems on a plane or other surface, set up a coordinate system (i.e. parametric equations for the surface which will involve two parameters) and we can identify each point by two numbers- dimension 2. The extension to 3 dimensions is obvious. Physicists work with "events"-things that happen at a particular point in space and a particular time. That requires 3 numbers to identify the point and one to identify the time: physicists work in 4 dimensions.
If I were doing a study of all possible spheres in space, I might record each sphere by the position of its center (3 numbers) and its radius (1 number). That is, again, 4 dimensional.
In Thermodynamics, we might have to imagine a cloud of N particles, each with a given position (3 numbers) and momentum (3 numbers) as well as time (1 number). That would be 6N+ 1 dimensional! (Which is why thermodynamics uses statistical methods.)
I don't think I've ever seen a definition of time that's not to some extent circular. My personal favorite is :Borogoves said:As a non physicist, I ask you, how do you physicists define time ?
mathwonk said:dimension is often defined inductively, based on the idea that the boundary of something has dimension one less than the something, boundary in the sense of boundary of a manifold, not topological limit points.
this is explicitly used as a definition by poincare in his essays. he defines a continuum as having dimension one if it can be separated into more than one connected component by removing a finite set of points, say as a circle can. so a circle is one dimensional.
then a continuum is 2 dimensional if it can be separated by the removal of a one dimensional continuum.
etc, etc.. this actually occurs in riemann, as a "fragment on analysis situs", some 50 or 60 years earlier, as apparently almost everything does.
these ideas are justified as theorems by the results in another thread on the jordan brouwer separation theorem. i.e. there the dimension of a manifold is defined in terms of saying th dimension of R^n is n, and that any manifold, locally homeo to R^n also has dimension n. then it is a therem that a compact n-1 dimensional manifold separates R^n.
the lovely little book by hurewicz and wallman, dimension theory, says something like this for more general topological spaces:
a space has dimension zero at a point, if there is a basis of neighborhoods at that point consisting of sets with empty topological boundary, i beieve.
so the rationals are zero dimensional for example.
then one goes on up inductively. i.e. finite sets are zero dimesnional too.
so then the reals are one dimensional everywhere since each point has a basis of nbhds with finite boundary.
in algebra and algebraic geometry, dimension of a domain is defined in terms of "krull dimension", i.e. the maximal length of a chain of prime ideals, or transcendence degree of the field of fractions. the transcendence degree allows the definiton to be relative. i.e.th field of fractions of a complex plane curve has dimension one over the complex field C, but dimension zero over the field C(z) of rational functions in one variable z.
this allows the concept of families of curves, i.e. a familiy of curves parametrized by a variety V is a variety mapping to V and whose function field has tr. deg. one over that of V. i.e. a surface S mapping to a curve V, can be viewed simply as a curve over V.
it is natural to restrict dimension to domains since domains correspond to spaces having only one irreducible component, and dimension should be defiend separately on each component.
or again, actually dimension should be defined locally at each point, so one uses the krull dimension of the local ring at a point.
one can also proceed as in manifold theory, saying the standard object, i.e. affine space k^n has dimension n, and then defining an appropriate family of maps that preserve dimension, in this case finite maps, amd defining the dimension of an affine variety as the minimum dimensionl affine space to which one can map your variety by a finite map. finite maps are defined either as proper maps with finite fibers, or maps inducing on pullback of rings of functions, a finite module map.
there are also homological notions of dimension. i.e. the homology of a compact n manifold vanishes above dimension n, but also in lower dimensions for non orientable manifolds. so one ahs again a local homological definition interms of the homology of a deleted neighborhood of a point.
also there is a homological definiion in ring theory, pioneered by hilbert, and also developed by auslander and buchsbaum, serre and others.
i.e. the length of a projective resolution. e.g. the ring of polynomials in n variables, has transcendence degree n, and also admits a projective resolution of length n and no shorter. however auslander buchsbaum showed that a local ring which is not regular has infinite homological dimension. so homological dimension only works for regular local rings.
the number of generatros of a maximal ideal is another approach but must be defined carefully.
so this is such an important idea it has appropriate definitions in every setting.
for exmple a pid is nothing but a one dimensional ufd in ring theory.
a dedekind domain is a one dimensional "regular" local ring, i.e. the local ring of a smooth curve.
Wow, that's about as presumptuous a statement as I've seen in a bit. Why do you think mathwonk should be responding to your question? I think it's pretty obvious that he is responding to the OP's question.Borogoves said:I can see that you've deliberately deviated from the question which I posed, namely, how physicists define time.
Too advanced for whom ? Please let the OP (whom you call "this gentleman") determine what is too advanced for him/her. Personally, I found mathwonk's post extremely enlightening.Furthermore, the response you gave to this gentleman is way too advanced.
Why should he respond ? for the same reason that he chooses to respond to other members, whatever that reason may be. It's called equality.Gokul43201 said:Wow, that's about as presumptuous a statement as I've seen in a bit. Why do you think mathwonk should be responding to your question? I think it's pretty obvious that he is responding to the OP's question.
Too advanced for whom ? Please let the OP (whom you call "this gentleman") determine what is too advanced for him/her. Personally, I found mathwonk's post extremely enlightening.
If anything, you caused this thread to deviate from its original purpose. If you have a fresh question to ask, please start a new thread.
Dimensionality of an object in the Cartesian space is the number of different (nonidentical) points you need to identify that object uniquely, minus one. Thus:shekoofeh said:Has the Dimension a mathematical defination?