[continued from previous message]   
      
   FFLIRF is accelerating *down* at an acceleration of g with respect to the   
   surrounding-building IRF, the (stationary) surrounding building (and the   
   elevator, which is stationary with respect to the building) must be   
   accelerating *up* at an acceleration of g with respect to the FFLIRF.   
      
   	[Aside: It's instructive to compare the previous paragraph   
   	with what we'd think about a different physical system:   
   	suppose that the building and elevator were in space far from   
   	any other masses, and the building's foundation were replaced   
   	by a huge rocket that's accelerating the whole building (and   
   	the elevator suspended inside the building from cables which   
   	haven't yet broken) upwards at an acceleration of g relative   
   	to a Newtonian IRF.   
      
   	Given our assumption of "in space far from any other masses",   
   	a Newtonian IRF is a FFLIRF, and vice versa.  So, this   
   	"rocket-accelerated elevator in space" would have the same   
   	upward acceleration with respect to a FFLIRF as our ordinary   
   	elevator here on Earth (again, BEFORE the cable-break) in the   
   	GR perspective.   
      
   	This is an example of the "equivalence principle" (EP) which,   
   	in its simplest form, says (roughly) that a uniform gravitational   
   	field has the same local effects as a steady acceleration.   
   	In Newtonian mechanics it's not apparent why the EP should   
   	be true; GR sort of assumes the EP as a postulate.  In fact,   
   	assuming the EP can take you most of the way to deriving GR,   
   	and this was roughly the route that Einstein took in originally   
   	obtaining GR.  (I'm glossing over lots of technical details here.)]   
      
      
   To summarize, then, in GR *free-fall* plays a similar role to that which   
   *uniform motion* plays in Newtonian mechanics.  Newton's 2nd law   
     a = F_net/m   
   is formally the same in GR and in Newtonian mechanics, but a and F_net   
   are interpreted somewhat differently:   
   * In Newtonian mechanics, "a" is interpreted as acceleration with respect   
     to (relative to) an IRF, and gravity is viewed as a force contributing   
     to F_net.   
   * In GR, "a" is interpreted as acceleration with respect to (relative to)   
     a FFLIRF, and gravity is *not* viewed as a force and does *not* contribute   
     to F_net.  As I'll explain in a following article, gravity actually shows   
     up as spacetime curvature, evidenced by the relative acceleration of   
     FFLIRFs at different places (e.g., the relative acceleration of my   
     FFLIRF and the FFLIRF of someone 1000 km away).   
      
   --   
   -- "Jonathan Thornburg [remove -color to reply]"    
      on the west coast of Canada   
      "the stock market can remain irrational a lot longer than you can   
      remain solvent" or (probably the correct original wording) "markets   
      can remain irrational a lot longer than you and I can remain solvent"   
            -- A. Gary Shilling (often misattributed to John Maynard Keynes)   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)