hartlw said:
Are you saying that there are areas of physical space not accessible to matter?
No.
hartlw said:
If you include time as a "dimension," then I can't "access" a point that occurred three days ago.
Now you're talking about spacetime, not space, and what
you can access in
your future is irrelevant to what we're talking about.
hartlw said:
The problem is still that you are confusing mathematical space with physical space.
No, it isn't. (And I don't). The problem is that you believe that your intuitive ideas about space and time are somehow "better" than any theory. You don't seem to reallize that your intuitive ideas (which are the same as everyone else's intuitive ideas) is just another theory about the real world, which by the way has been thoroughly disproved by experiments.
hartlw said:
x,y,z,t are independent variables. A sequence of events may occurr for which the position of an object is given by x,y,z as a function of time . You can arbitrarily specify the functions (mathematics) or invoke some physical law (phyusics). To do anythiing other than pure mathematics, x,y,z,t must have meaningful definitions. The first step is defining x,y,z and t. Without that, you can still do all the mathematics you want, and talk about MATHEMATICAL space, but it still doesn't mean anything.
Assuming the variables in Einsteins Equatiion are x,y,z,t, what is the definition of x,y,z,t and what is the physical basis for the formulation of the equation?
I can't teach you general relativity here, but I can give you a brief outline of some of the basic ideas. Spacetime is a 4-dimensional manifold M. A coordinate system is a function x:U\rightarrow\mathbb R^4, i.e. the coordinate system is the function that assigns coordinates to events:
x(p)=(x^0(p),x^1(p),x^2(p),x^3(p))
If you'd like, you can use the notation (t,x,y,z) for the thing on the right. These are however
not the variables in Einstein's equation. The variables are the components of the metric tensor, which contains all the information about the geometric properties of spacetime. Einstein's equation describes the relationship between the metric tensor and the stress-energy tensor, which represents the properties of matter. You asked specifically about mass. Mass enters the equation through the equivalence between mass and energy (E=mc
2) because one of the ten independent components of the stress-energy tensor is energy density.
The relevant solutions of this equation are found by first assuming that spacetime can be "sliced" into a one-parameter family of spacelike hypersurfaces \Sigma_t[/tex] (we can think of each \Sigma_t[/tex] as "space, at time t"), such that each \Sigma_t[/tex] is homogeneous and isotropic (according to a precise mathematical definition of those terms). There are only three solutions of Einstein's equation that are consistent with that assumption. (<a href="http://en.wikipedia.org/wiki/Friedmann-Lema%C3%AEtre-Robertson-Walker_metric" target="_blank" class="link link--external" rel="nofollow ugc noopener">Wikipedia link</a>). These three solutions describe space as a 3-dimensional version of a sphere, a plane and a hyperboloid respectively. Spheres are finite in size. Planes and hyperboloids are not.<br />
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It's convenient, but not necessary, to define a coordinate system x that assigns time t to all the points in \Sigma_t[/tex]. (I.e. x^0(p)=t when p\in\Sigma_t). If we do, we find that t can't be defined for all real t. There exists a t<sub>0</sub> such that t is only defined for t>t<sub>0</sub>. It's convenient to choose t<sub>0</sub>=0.<br />
<br />
The fact that each \Sigma_t looks like a sphere, a plane or a hyperboloid means that there's also a very natural way to assign the spatial coordinates to points on \Sigma_t. This gives us a way to identify a point on \Sigma_t with a point on \Sigma_s when t\neq s, and this allows us to define the distance between any two points in space as a function of time. It can be shown that this distance goes to zero as t goes to zero. That's why the limit t→0 is called "the big bang".