Looking into the past with a telescope

"Ioannis" wrote in message
news:1170106772.429559@athnrd02...

To me, it looks as though "the edge" of the universe around any individual
observer is simply a theoretical sphere of radius c*t where t is the age
of
the observer and c is the speed of light.

By that definition, if the speed of light were infinitely fast, the edge
would be infinitely distant.

Just a thought.

On Mon, 29 Jan 2007 16:24:14 -0800 (PST), (Brian Tung)
wrote:

Chris L Peterson wrote:
The Universe is almost certainly much bigger than 13 billion ly in
"diameter" (I use that term loosely, because I don't think the Universe
is a 3D construct).

You don't need the universe to be three-dimensional to use "diameter"
rigorously. It just has to have a consistent metric; the diameter is
then simply the greatest distance between any two points.

I know. I just wanted it to be clear to some that by using "diameter" I
wasn't presuming the Universe to be a sphere, and I was too lazy to
attempt a more rigorous explanation.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:
I know.

I knew you knew.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Brian Tung wrote:
Paul Schlyter wrote:
...and where is that "somewhere else" supposed to be?

I mean, isn't the universe supposed to be "everything that is, everything that
was, and everything that will be", instead of merely everything we could
in principle observe? If the universe really is *everything*, there cannot
be any "somewhere else".

From the context, I hope it is clear that we have been talking about the
universe as "a connected piece of space-time." Therefore, it need not
be "everything there is." If that piece of space-time is embedded in
some higher space (such as four-dimensional Euclidean space), which is
not actually necessary, then there can be a center to that universe that
does not lie within it.


OK. My understanding of (one variety of) four-dimensional Euclidean
space is, that if we head off any direction, after an arbitrarily long
time we will arrive at our starting position. Reduce all dimensions by
one, and you have the 'ant on a balloon'.

However, I always understood this to be an analogy just used to aid in
visualisation, so that the centre of the balloon was simply an artifact
of the geometry of the analogy, and not a 'place' in any meaningful
sense of the term. Therefore, when we describe our universe as
'four-dimensional', all we are really saying is, that if we travel in a
straight line for long enough, we eventually end up at the starting
position. We are not making any claims on whatever higher geometry there
may be.

My question is: If the universe is described as four-dimensional, does
that mean that it necessarily has a geometrical centre. That is to
ask, is there a place that is an equal distance from every place that we
can observe, (and travel to).

Of course I'm thinking of a 3-sphere, not a 3-torus. I'm a little too
tired to think about those at the moment....

Stupot - higher dimensional layperson.

Stuart Chapman wrote:
OK. My understanding of (one variety of) four-dimensional Euclidean
space is, that if we head off any direction, after an arbitrarily long
time we will arrive at our starting position. Reduce all dimensions by
one, and you have the 'ant on a balloon'.

However, I always understood this to be an analogy just used to aid in
visualisation, so that the centre of the balloon was simply an artifact
of the geometry of the analogy, and not a 'place' in any meaningful
sense of the term. Therefore, when we describe our universe as
'four-dimensional', all we are really saying is, that if we travel in a
straight line for long enough, we eventually end up at the starting
position. We are not making any claims on whatever higher geometry there
may be.

That's right--that is the same as saying that the universe need not be
actually embedded in a four-dimensional space. Only the topology and
the metric are "real."

My question is: If the universe is described as four-dimensional, does
that mean that it necessarily has a geometrical centre. That is to
ask, is there a place that is an equal distance from every place that we
can observe, (and travel to).

If it is an ordinary hypersphere embedded in 4-space, then yes, it has a
geometrical center.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On Jan 29, 11:18 pm, Chris L Peterson wrote:
On Mon, 29 Jan 2007 23:39:30 +0200, "Ioannis"
wrote:

To me, it looks as though "the edge" of the universe around any individual
observer is simply a theoretical sphere of radius c*t where t is the age of
the observer and c is the speed of light.

You need to distinguish between the edge of the Universe, and the edge
of the observable Universe. They aren't the same thing. You are
describing the latter.

_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

Easy knowing that none of you are astronomers.

http://veimages.gsfc.nasa.gov/607/PIA00232_md.jpg

With incredible images of the Earth from space you would think that
people would appreciate the planet more.The angle which your location
entered and exited the orbital shadow would be different from a month
ago because the orbital path of the Earth changes the orientation of
the orbital shadow/solar radiation boundary against fixed axial
orientation.

The same people who talk of the celestial sphere/balloon universe
still attribute a pseudo-dynamic of variable axial tilt to explain why
a location experiences variation in daylight/darkness when it is
easier and more enjoyable to appreciate how the orbital path of the
Earth changes the angle by which a location enters and exits the
orbital shadow.

http://upload.wikimedia.org/wikipedi...easonearth.png


Amateur astronomers would love to work on how to mesh the astronomical
motions of the Earth with climatological studies and this is the
direction most of humanity has headed in.You could not get people to
consider a ridiculous 'expanding balloon' universe if you paid them
insofar as they already know it is rubbish and have heard it too many
times.

I use actual images whenever possible and this is the job of a real
astronomer whereas ,the ridiculous 'expanding balloon ' universes
which nobody cares for now only highlights the excesses of
meaningless non geometric equations and the people who promote them.