jeffinchiangmai":27c95h3i said:
I suppose this must be one of the most frequently asked questions since 1905 but I am still trying to get my head around space-time. I think I understand the analogy of the ball of current dough which started expanding 13.7 billion years ago until now. I'm trying to see how any point in that ball of dough can look in any direction for 13.7 BY and see the same thing. Suppose our galaxy is a current right near (or on) the surface of the ball of dough, if we look inward, we look back 13.7 BY but if we look outward we can see nothing (i.e no other galaxy - nothingness). I can understand, if I am right that in looking out, that small distance (if any) is still 13.7 BY where time has gone very slowly (in that distance), (or even stopped if we are right on the surface of the ball of dough).
If we are right on the surface of the expanding ball of dough and look inwardly in all directions so that we can see the inner shell of the ball of dough (or maybe balloon in this case), I can see how that inner surface, at distances from zero to maximum would represent the CMBR and that these distances would still represent 13.7 BY due to those distances materializing in the same time span.
If our galaxy was right on the surface of the balloon and that surface represents the CMBR, which occurred 13.7 BY ago, would we would also be the same age as the CMBR, zero years old?
Another analogy could be a great circle map (used in ham radio) where the circumference of the great circle map represents a single point on the opposite side of the earth. May be I'm being too complex here.
I just want the penny to finally drop. So glad I found this sight.
The ball of dough or the balloon analogy are intended to help you think about space-time as a manifold without using the word "manifold" or invoking the associated mathematics. It can be a bit confusing.
A manifold is mathematical construct, a topological space, that "locally looks like" ordinary Euclidean space of the appropriate dimension. Space-time is a 4-dimensional Lorentzian manifold. The term "Lorentzian" tells you something about the geometry of the manifold and what is called the "differentiable structure", but we won't worry about that here.
The surface of a balloon, or the surface of a ball of dough, is a 2-dimensional manifold. Locally it looks like a plane, just as the surface of the earth looks like a plane at small scales, but at larger scales you see the effect of curvature.
While the examples that you are being given are of manifolds that are contained, or embedded in some higher-dimensional space (the surface of the ball of dough for instance is a 2-manifold that is embedded in the 3-dinmensinal space of the kitchen) is it not at all necessary for a manifold to embedded in anything at all. Such manifolds are called "intrinsic manifolds". Space-time is an intrinsic manifold. It is not necessarily embedded in anything else. That point is critical -- an intrinsic manifold is complete unto itself and there need not be any larger space anywhere.
So, now think of the surface of the balloon or the surface of the ball of dough as intrinsic manifolds -- just forget about the larger space in which they are realized. The surface is all that there is. If you not imagine yourself as an ant on that surface, then things look the same in all directions -- remembering that by a "direction" we mean along any line that lies totally on the surface. Further, by a line we mean a geodesic on the surface -- a curve that is locally the smallest distance between points. On a sphere the geodesics are great circles. (This gets a little more complicated with Lorentzian manifolds but that complication is not necessary to point here).
So the reason that things look the same in all directions on your ball of dough is because you are only allowed to look along curges that lie on the surface. You are not alllowed to look "inward" or "outward" because those directions lie in dimensions that do not exist with respect to the intrinsic manifold.