From: ram@zedat.fu-berlin.de   
      
   To better understand symmetries, I have a few questions, but for   
     simplicity's sake I'll start with one:   
      
     Let a hypothetical one-dimensional world consist of a ray with   
     values x>=0. This world is completely empty except for a mass   
     point with unit mass 1 at x=1. This is described by a "mass   
     density" R(x), which is zero everywhere except for R(1)=1.   
      
     Now a scientist comes along and says: I formally extend this world   
     to a two-dimensional world with the coordinates (x,y). The mass   
     density R(x,y):=R(x) is everywhere equal to 0, except for a mass in   
     the form of a straight line {(1,y)|yeR}. This world is invariant with   
     respect to y. A translation y'=y+y0 results in the same world again.   
     So, there is a preserved quantity, which I call the "y-momentum".   
      
     Now, there are two reactions: One praises the "deep result". Others   
     say that y is just a "redundant, unphysical coordinate" that has   
     no meaning at all, and that the result is completely irrelevant.   
      
     So, is the y-invariance of the two-dimensional world irrelevant   
     or meaningful? Why?   
      
   [[Mod. note -- In order to have unit mass, doesn't your mass density   
   need to be a Dirac delta-function?  -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)