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The presesnt

  1. Aug 28, 2004 #1
    I've been reading all about time cones, and space-time, and those general ideas. i forget if i already posted about it a whil ago, but if i did, obivously i didn't quite get the right answer because i still don't understand it... but i was looking at a picture too in an astronomy book a while ago. and it was a drawing of a circle with the big bang as the perimiter. the very center was the present. and space-time went outward from t here towards the big bang. at the time i thought it was a great representation, but the more i thought about it the more confused i started to get. then i was thinking about light cones. and that picture sort of makes sense again... i think. in those light cones, the present is only one point in space time, just like the center in that picture. and i guess we're always traveling towards the center of the picture, which is the future, but we're actually always exactly at the center. and we can only observe the space that expands outward from us, which is the past. i dunno if i'm missing anything else......

    umm, i had a question when i started... but its kind of dissapeared when i was trying to figure out how they were related. either way, does that sound about right? or make sense? and is that a good way to think about it at all?
  2. jcsd
  3. Aug 29, 2004 #2
    As long as you are completely aware that any drawing or even 3-d representation is only an incomplete rendering to help visualize something that can not be visualized, the one with the origin at the edge is as good as the one with the origin at the center.

    It maybe renders better the fact that any direction we look at around us, is the origin.

    But it's less effective in for example, showing that our visible universe is only a part from the total, in representing the paths of light etc.

    I personally feel more comfortable with the one in which the origin is the center. Then, our visible universe can be represented by the surface of a sort of melon (or american football ball) from the center to our current location.
    Once again, all ok as long as you don't take any of them too seriously.
  4. Aug 29, 2004 #3


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    A time cone is a 4 dimensional geometric. Very hard to visualize by even the most gifted physicists [Hawking said he has difficulty picturing the universe in 3 dimensions!]. Try to visualize a pair of two dimensional universes. The first is a flat piece of paper plotting x,y coordinates. The second is a flat piece of paper plotting z,t coordinates. After that, align and rotate both flat pieces of paper around any axis, x,y,z or t. This gives you a four dimensional reference frame. Time [how long between revolutions] is the the only scalar quantity in this reference frame. This is the most awesome demonstration of space time dimensions I ever witnessed. It is also, for me, a beautiful way to demonstrate how time and space are coupled.
    Last edited: Aug 29, 2004
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