From: relativity@paulba.no   
      
   Den 04.02.2026 01:06, skrev Thomas 'PointedEars' Lahn:   
   > Paul B. Andersen wrote:   
   >> Den 03.02.2026 16:14, skrev Thomas 'PointedEars' Lahn:   
   >>> Paul B. Andersen wrote:   
   >>>> Den 03.02.2026 10:07, skrev Maciej Woźniak:   
   >>>>> Anyway, for any context motion is absolute.   
   >>>> Speed is relative, acceleration is absolute.   
   >>> Wrong.   
   >>   
   >> Acceleration is absolute in Newtonian mechanics.   
   >   
   > That is not so.  If two reference frames are non-inertial (in the Newtonian   
   > sense), then it is possible that one is not accelerating relative to the   
   other.   
      
   It was Maciej Woźniak who used the word "absolute".   
      
   In this context, "absolute" means "not relative".   
      
   Speed is relative because the speed of an object depend   
   on in which _inertial_ frame of reference it is measured.   
      
   In Newtonian mechanics the acceleration dv(t)/dt of an object   
   will be the same independent of in which _inertial_ frame of   
   reference the speed v(t) is measured.   
      
   Acceleration is not relative, it is absolute.   
      
   >   
   > That is precisely the case (approximately), when a falling observer is   
   > observing a nearby falling object and we assume approximatively that the   
   > gravitational field is uniform.  Then that object would be observed by thta   
   > observer to be at rest even though it would be accelerating for an inertial   
   > observer.   
      
   You are in free-fall. An apple is right above your head,   
   and will stay there.   
   (If you are falling towards the Earth, the distance between your   
   head and the apple will increase very slowly. We ignore it.)   
      
   You will know that since you are in free-fall your and the apple's   
   proper acceleration is exactly zero. (no air resistance)   
      
   A person who believes that Nature behave according to Newtonian   
   mechanics will see that he is falling towards the Earth, and   
   will know that his speed relative to the Earth is increasing,   
   he is accelerating. Since the apple stays in the same position   
   relative to him, he will conclude that the apple's acceleration   
   is the same as his own. He can however not _measure_  the   
   acceleration without advanced apparatus (like a Doppler radar),   
   but he will know that it is what would be measured in an inertial   
   frame.   
   He will certainly not conclude that the apple's acceleration   
   is zero because it is stationary relative to himself.   
      
   >   
   >> Maciej quote of me is taken from a thread where we> discussed   
   > accelerometres, which are measuring   
   >> the proper acceleration.   
   >   
   > I have read the thread.  If you said anything obviously incorrect there, I   
   > would have corrected you there.   
   >   
   >> Proper acceleration is absolute in SR and GR.(And in NM)   
   >   
   > What do you mean by that?   
      
   The magnitude of the proper acceleration (the value given   
   by an accelerometer) is a Lorentz-invariant scalar.   
      
   It is not relative, it is absolute.   
      
   --   
   Paul   
      
   https://paulba.no/   
      
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