From: ram@zedat.fu-berlin.de   
      
   In "Gravitation" by Misner et. al., in chapter 1, it says in box 1.6,   
      
   |[O]ne can "look at the separations between these nearby [two]   
   |test particles and from the second time-rate of change of   
   |these separations and the 'equation of geodesic deviation'   
   |(equation 1.8) read out the curvature of spacetime."   
      
     . To me, a "force" is something that is causing an acceleration   
     (thinking of "F = ma"). So, I'd be inclined to say that the   
     curvature of spacetime is a /force/ that is accelerating the   
     two test particles relatively to each other.   
      
     If someone thinks that this curvature is /not/ a force, maybe   
     he could explain why the curvature of spacetime should not   
     be called a "force" although it causes an acceleration?   
      
     And while I'm at it: Wikipedia says: "Most fermions decay by   
     a weak interaction over time.". This weak interaction also   
     is called "weak force"; so, this weak force does not seems   
     to cause accelerations, but decays. Why is it still called a   
     "force"?   
      
     What is a force?   
      
   [[Mod. note -- Applying the same force to different-mass test bodies   
   results in different accelerations, as per Newton's 2nd law $a = F/m$.   
   But spacetime curvature induces the *same* relative accelerations   
   between different-mass test bodies.  So to call spacetime curvature   
   a "force" you have to posit that's really a   
   "force-proportional-to-inertial-mass", which is a funny sort of beast.   
   It seems cleaner to just call it "spacetime curvature".   
      
   As to the weak interaction, I think particle physicists usually call   
   it the "weak interaction".  Calling it a "force" is colloquial usage.   
      
   As to your general question... there's a rather extensive discussion   
   of "what is a force" and the operational definition of same, in the   
   context of teaching introductory physics courses, in the excellent   
   book   
      Arnold B Arons   
      "A Guide to Introductory Physics Teaching"   
      Wiley, 1990, ISBN-10 0-471-51341-5   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
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