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#61




Looking into the past with a telescope
"Ioannis" wrote in message
news:[email protected] 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. 
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#63




Looking into the past with a telescope
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 
#64




Looking into the past with a telescope
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 spacetime." Therefore, it need not be "everything there is." If that piece of spacetime is embedded in some higher space (such as fourdimensional 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) fourdimensional 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 'fourdimensional', 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 fourdimensional, 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 3sphere, not a 3torus. I'm a little too tired to think about those at the moment.... Stupot  higher dimensional layperson. 
#65




Looking into the past with a telescope
Stuart Chapman wrote:
OK. My understanding of (one variety of) fourdimensional 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 'fourdimensional', 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 rightthat is the same as saying that the universe need not be actually embedded in a fourdimensional space. Only the topology and the metric are "real." My question is: If the universe is described as fourdimensional, 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 4space, 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 
#66




Looking into the past with a telescope
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 pseudodynamic 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. 
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