> > >1. What is at the edge of the universe?
> > What is at the edge of the Earth?
>
> I don't know about the edge of the universe, but the only thing
> currently at the edge of the earth is atmosphere. Where the
> atmosphere ends is where the earth's edge is. If you ask what the
> edge of a box is, you find that it is the outer extremity. This is
> also true of a circle, or a shpere. The earth is a sphere. Thus the
> earth DOES have an edge. The universe, if infinite, has no edge,
> because it would have no shape. If the universe does have an end,
> then what is at that edge is unknown.
In classical general relativity it is generally assumed that spacetime is
a 4-dimensional manifold without boundary. The boundary points,
if included as part of the manifold, would require some different
kind of physics. If you trace back in the past timelike direction in
spacetime you will come to a singularity, according to Hawking's
singularity theorem. This singularity is called the Big Bang, and it
could be thought of as a boundary point of spacetime, although it
is not part of spacetime itself. There is currently much debate as to
whether there is another singularity, called the Big Crunch (if it exists),
in the future timelike direction, or whether spacetime (the universe)
will continue expanding forever.
Most physicists will tell you that it is a mistake to imagine spacetime
as a 4-D manifold embedded in some higher dimensional space.
Spacetime is all that exists. So it's not like a 2-D surface embedded in
3-D, which you could think of as being its own edge, or the edge of the
solid region it encloses. For simplicity you can imagine it as a curved 2-D
surface, but it is not "in" any 3-D space.
--Bill Haloupek haloupekb@uwstout.edu http://www.mscs.uwstout.edu/~billh/home.html My employer doesn't share my opinions, my webpage is under construction, and my life is based on a true story. Will solve math problems for food.