From: tjroberts137@sbcglobal.net   
      
   On 5/20/22 8:44 AM, Lou wrote:   
   > On Sunday, 15 May 2022 at 11:10:45 UTC+1, Tom Roberts wrote:   
   >> the key concept [is that] locally inertial frames [...] are   
   >> only APPROXIMATELY inertial [...]   
      
   > An interesting point you make.   
      
   It's not my point, this has been known for over a century, since   
   Einstein began thinking about gravity.   
      
   > It follows that there is no such   
   > thing as an inertial frame   
      
   Hmmm. Physics is not math. Yes, in the world we inhabit, and in any   
   manifold of GR with any nonzero energy density, there are no EXACTLY   
   inertial frames. But in physics, where measurements are always inexact,   
   approximately inertial frames can be very useful, because in a locally   
   inertial frame one can use SR rather than GR, which GREATLY simplifies   
   the analysis. The point is to make sure the approximation involved   
   affects answers less than measurement resolutions.   
      
   	[For example, at the LHC no individual event lasts more   
   	 than 1 microsecond. A truly inertial frame that is at   
   	 rest in the lab at the start of the event will fall at   
   	 most a few picometers during the event. As their   
   	 detectors have resolutions at least a million times   
   	 larger than that, the difference between the lab frame   
   	 and that inertial frame is completely negligible.]   
      
   > So an inertial frame cannot actually   
   > exist in 3D space.   
      
   Approximately-inertial frames are aspects of (4D) spacetime, not (3D)   
   space.   
      
   Tom Roberts   
      
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