From: jos.bergervoet@xs4all.nl   
      
   On 8/8/2017 7:54 PM, fortunati.luigi@gmail.com wrote:   
   > [Moderator's note: If you already changed your way to post to newsgroups   
   > it hasn't helped. I had to adjust the posting again. Use plain ASCII and   
   > a true news client (e.g., Thunderbird), not a web interface or something   
   > the like.]   
   >   
   > Jos Bergervoet alle ore 13:43:18 del 08/08/2017 ha scritto:   
   >> [Moderator's note: The idiosyncratic characters result from not using a   
   >> proper news client to post to sci.physics.research. In fact in my   
   >> moderation process I always try to repair this as far this is possible   
   >> in an automized way. hvh.]   
   >>   
   >> On 8/8/2017 9:36 AM, LuigiFortunati wrote:   
   >>    ...   
   >>> =A0=A0   
   >>> =A0=A0 I ask you:   
   >>   
   >> Luigi, can you please post more readable text? I do not know what   
   >> you mean with "=A0=A0" and similar insertions.   
   >   
   > I am not the one who puts these curses "=A0=A0", now I will try to   
   > change my news client (MESNEWS) as the moderator suggests (I will use   
   > GOOGLE GROUPS).   
   >   
   >>> can the transition from one reference to another only be   
   >>> achieved with acceleration (as in this case) or is there any other way   
   >>> to achieve it?   
   >>   
   >> There is no prescribed reference frame to use. Everyone is free   
   >> to use any reference system, *also* those in which he himself is   
   >> not at rest. So the transition to another reference frame can be   
   >> achieved simply by a decision: "let me now use reference frame   
   >> Y instead of X for my calculations".   
   >>   
   >> If, for whatever reason, you insist (unnecessarily) on using only   
   >> the reference frame in which you are at rest, then (by your own   
   >> choice) you need to change your velocity to be able to use another   
   >> reference frame. But then this is just your own chosen rule.   
   >>   
   >> A reference frame is just a grid of coordinates. There is no law   
   >> in physics that forces you to use any specific grid. You can even   
   >> use arbitrarily curved coordinate lines (which were not yet   
   >> considered in this discussion until now).   
   >>   
   >> Your questions are only about your own superficial choices of   
   >> choosing some specific reference frame. You maybe have the idea   
   >> that there is only one choice allowed, from popular descriptions   
   >> of the theory of relativity. But those descriptions are wrong!   
   >>   
   >> It is *not true* that if you are moving at some speed, you're only   
   >> allowed to use one reference frame. Please change your beliefs.   
   >   
   > I did not ask if there is a prescribed reference frame to use for   
   > calculations.   
      
   OK, I see. It was very difficult to read! (Because of all those   
   =A0=A0 insertions). I thought you asked about the transition "to   
   a reference frame" without specifying what you meant with a   
   transition. Reference frames are coordinate systems. They exist.   
   Anyone can make his pick which one to use! But as I speculated   
   already, you maybe meant the transition of being at rest in one   
   reference frame to being at rest in another frame.   
      
   > I asked if anyone who is at rest in a certain K can go to being at rest   
   > no more than K but to another K' (at gamma=8 than K) without accelering   
   > some "spaceship".   
      
   Of course the spaceship is not needed. To be at rest in the other   
   frame you only need to change your speed (as defined in the first   
   frame, by dx/dt). If there is a heavy star or black hole you just   
   can make a "sling shot" around it. Our traveling twin (T, we all   
   know him well now!) could do this to make his turn around (a   
   parabolic orbit around a black hole, I mean). He would never have   
   to use any rocket engine and would remain weightless during the   
   maneuver. Still he would end up being at rest in a different frame   
   (related by a gamma=8 Lorentz transform to the old frame, if   
   required).   
      
   Obviously it is important here to define 'acceleration' as the   
   locally perceived (by T) acceleration. Incidentally, the other   
   twin, T0, who is still on Earth, perceives an acceleration of   
   9.8m/s^2 all the time, whereas T, using the sling-shot return,   
   perceives no acceleration at all during his journey.   
      
   If you want to use another definition of 'acceleration' (or of   
   'transition' or of 'reference frame') the answers to your   
   questions might well be different. These things are actually   
   very difficult to discuss in words, without mathematical   
   definitions.   
      
   --   
   Jos   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)