Subject: Re: Reasons for Christianity (was Re: Gay Mexicans Endure Hate)
Date: 27 Aug 1999 04:03:50 GMT
From: Chris Ho-Stuart <>
Organization: Queensland University of Technology, Australia
Newsgroups: aus.religion.christian,aus.religion,nz.soc.religion
In aus.religion.christian Brendan <> wrote:
> On Thu, 26 Aug 1999 05:46:24 GMT,  (Allistar Melville)
> wrote in message [< 37c4d407.27110]:
> >From what I've heard (correct me if I'm wrong) scientists say that the
> >closer you get to the "edge" of the universe (hypothetically) the
> >harder it would be to continue. Kind of like approaching the speed of
> >light. Therefor you couldn't ever reac the each, making it infinite.
>
> That's right - like I said :)

Actually, I think this is wrong. There is no real objective edge you can
approach. There is an event horizon, which is a different kind of thing.

> The edge IS expanding at the speed of light or greater, therefore, you
> would have to travel faster. But you are still damned, as no matter your
> speed, the universe expands with you as you are part of the universe.

The edge which expands at the speed of light is actually just an
event horizon. It is not a real edge -- just a bound on how far we
can see. As time goes by, light originating from the very earliest
times of the universe has had longer to reach us, and thus can have
travelled a greater distance, and thus we can see further.

> >It's like having a large round room and you were tied to a bungy fixed
> >in the middle. The bungy is just the right length so that you couldn't
> >quite stretch your finger tips to touch the wall. That room is
> >definitely bounded, but would you consider it to be infinite?
>
> No, that's wrong. The universe is not actually an expanding balloon. The
> edge of the universe is defined as being where light has 'shone' out to so
> far. If the universe is 15 billion years old, then it has a radius of 15
> billion years.

Subtle confusions abound.

In fact, the expanding balloon analogy is a very good one -- once you
get your head around the notion that the skin of the balloon is all
that exists. The universe (3 spatial dimensions) is like the skin of
the balloon, but with an extra dimension. (The skin of the balloon
is only two dimensional.) 

Another way to think of the universe is as an infinitely large loaf of
rising bread. As the bread rises, the raisins in the dough keep moving
further and further apart, but there is no actual boundary to the loaf.

Just like dots on a balloon move further apart as it expands, without
there being any edge to the skin of the balloon.

The really really tricky thing is think imagine that the unbounded
loaf of bread may actually wrap around on itself (like the skin of a
balloon, but in three dimensions rather than in two dimensions).

There is no edge to the universe in the sense of being "where the
light has shone so far".

There is an edge to the "visible" universe, which is the point
from which light travelling since the origin of the universe has
had time to reach us. This edge depends on the observer. It is
analogous to the horizon: there is no real "edge", just a limit
to how far we can see. In fact, the word "horizon" is often used;
which is much better than edge. In the balloon analogy, it is
an indication of how much of the balloon can actually be seen
from a point on the balloon. You (probably!) can't see the whole
thing.

As time passes, you can see more and more of the balloon. However,
this has nothing at all to do with the expansion of the balloon or
its size.

> But not actually, as apparently there was FTL expansion of the universe in
> it's first microseconds. So it's a bit more. Light was dragged with it of
> course, as it is part of the universe.

Yes.

This plays subtle tricks with the notion of a horizon. The very
very early universe was opaque to light. The horizon is often
defined as the points from which light can reach us since the
universe became transparent.

> The diameter is of course about 30 billion light years, but because of the
> finite speed of light, we only get to see half way. Not even that actually.
> It's called the light horizon or something.

Bingo. This is spot on.

But note this diamater is NOT a measure of the size of the universe.

It is a measure of how far we can see.

The actual size of the universe itself is an open question. The
smart money at present says it is finite (but still without an
edge, due to curvature, or wrapping in on itself) and yet that we
can only see a tiny fraction of it.

Cheers -- Chris