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   From: wugi    
   Newsgroups: sci.physics.research   
   Subject: Re: Apparent rotation   
   Date: 12 Jan 2023 08:23:17 GMT   
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   From: brol@dinges.be   
      
   Op 11/01/2023 om 9:32 schreef Phillip Helbig (undress to reply):   
      
   >   
   > Yes.  No-one debates the fact that accelerations are absolute.  The   
   > question is WHY that is the case.  Imagine an empty universe with one   
   > object in it, say a merry-go-round.  Should it be possible to tell if it   
   > is rotating, as it would be under normal conditions?  If so, with   
   > respect to what is it rotating?  There is nothing else in the Universe.   
      
   Then even light would not "know" where to go. And there would be nothing   
   to tell matter how to stick together. At least there ought to be an EM   
   field and the spacetime metric that goes with it. Once we've got that,   
   we can talk about "absolute" acceleration/rotation, to wit, in relation   
   to inertial motion states.   
      
   > Some would claim that there would be no way to tell in such a case, i.e.   
   > no inertia.  Add a small amount of matter to the universe and there   
   > would be a small amount of inertia.  Add more and there would be more.   
      
   Do you mean that an object "gains" more and more inertia by filling   
   space with more and more matter? I don't believe that: inertia is an   
   inherent property of a given body, not of far away galaxies.   
      
   > And so on.  That would make sense if inertia is somehow caused by the   
   > presence of other matter, which is the essence of Mach's Principle.   
   > Certainly the Lense-Thirring effect indicates that the idea that   
   > relative rotation has physical effects is not absurd.   
      
   My personal view is that Mach's principle is not required to explain a   
   body's inertia and its absolute or not state of acceleration. But it may   
   (or may not:) have its role in the existence of the EM field and the   
   spacetime metric, which in turn are required to consider inertial or not   
   motion, at all.   
   Remember the layman's description of GR: "matter tells spacetime how to   
   curve, and curved spacetime tells matter how to move".   
      
   --   
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