From: julio@diegidio.name   
      
   On Sunday, 8 May 2022 at 09:41:41 UTC+2, Luigi Fortunati wrote:   
   > Julio Di Egidio sabato 07/05/2022 alle ore 15:00:52 ha scritto:   
   > >>> A free-falling brick is an inertial frame?   
   > >   
   > > A free-falling brick is *in* "an" inertial frame. Which more precisely   
   > > means that we can find an inertial frame in which the brick is in   
   > > uniform motion, then just a special case is an inertial frame in   
   > > which the brick is at rest, and and even more special case is the   
   > > frame in which the brick is at rest at the origin of space. And we   
   > > might call that last one "the brick's own frame", because there is   
   > > indeed something "privileged" about it as far as that brick is   
   > > concerned: OTOH, though, notice that the fact that the brick is in   
   > > *free-fall* requires no frame to state or verify at all, it's altogether   
   > > a *true* physical property that can be verified *locally*.   
   >   
   > How small must this be "locally"?   
   > As small as a brick? A half brick? A tenth of a brick?   
      
   [Apologies for my first reply, not very constructive: please disregard,   
   I am trying an actual answer.]   
      
   It's literally the *point*, which is an ideal condition of course, but one   
   of those that can be approximated arbitrarily well: keeping in mind   
   that this is classical physics, not quantum, so we don't need further   
   cautions.   
      
   In practice, it's just a little bit more articulated than that, it goes this   
   way: *you* start moving until your motion is in sync with that of the   
   brick, which you can check by having a rigid rod guarantee that your   
   distance and orientation relative to the brick does not change (and   
   then would you ask why and how that works?), then, by *transitivity*   
   of the condition of being inertial, you just check that *you* are   
   inertial: and that is as local as it needs be, modulo approximations   
   that are simply structural to doing experiments.   
      
   And I am sure most here could rephrase that in better terms, but I'd   
   rather invite you to take a step back and this point and reconsider   
   how the very progression goes: what it even means for a property   
   to be a *true physical property* vs e.g. an artefact of the coordinate   
   system.   
      
   Julio   
      
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