From: hitlong@yahoo.com   
      
   On Tuesday, September 5, 2017 at 11:55:37 PM UTC-6, Nicolaas Vroom wrote:   
   >   
   > On Sunday, 3 September 2017 16:31:18 UTC+2, Gary Harnagel  wrote:   
   > >   
   > > On Saturday, September 2, 2017 at 8:33:52 AM UTC-6, Nicolaas Vroom wrote:   
   > > >   
   > > > For GPS clocks you can do a simular experiment. One GPS clock you keep   
   > > > on earth and one other you bring in orbit for 1 year and you bring it   
   > > > back.   
   > >   
   > > A proper test would have to be done with both clocks in space.  They would   
   > > be synchronized when both are together and then one would be accelerated   
   > > (gently) to a different trajectory and then brought back later and   
   compared.   
   > >   
   > > > Also here the question is: is the duration the same?   
   > > > I doubt this.   
   > >   
   > > So does every scientist who understands relativity.   
   >   
   > That means that all scientist agree that moving clocks (once synchronised)   
   > run slower?   
      
   Nope.  It means that moving clocks (once synchronized) become unsynchronized.   
      
   > At the same time this also means that these moving clocks continuously   
   > require some form of synchronisation to be 'usefull' ?   
      
   I guess that depends on what you mean by "useful."   
      
   > The issue is that you can explain that just by studying the inner workings   
   > of a clock and based on the assumption that the speed of light in one   
   > coordinate system is the same in all directions.   
      
   Also that it's the same in all inertial frames.   
      
   > That means if you have clocks with move all in the x direction with   
   > different speeds, these speeds are important for the ticking rate and   
   > not the speed of light.   
      
   APPARENT ticking rate as observed from different inertial frames.   
      
   > > > In fact you should do one experiment to test both.   
   > > > 1) One clock stays for one year on earth,   
   > > > 2) One clock travels for half a year away and for half a year back (fast)   
   > > > 3) One clock which travels for one year around the earth (stays in orbit)   
   > > > when they meet which clock shows longest duration (highest # of ticks)   
   > > > and which the shortest?   
   > >   
   > > It most likely won't be done because   
   > > (1) every knowledgeable person knows what will happen,   
   > > (2) satisfying the whims of doubters is a waste of time and money, and   
   > > (3) spacecraft are better used discovering what we DON'T know.   
   >   
   > What I try to compare is the behaviour of a twin type experiment with an   
   > as much as possible identical GPS type of experiment.   
   > That means one clock stays at home and the two other two clocks move under   
   > different circumstances.   
   > For any of these clocks, if they run behind, then IMO this is a physical   
   > issue and require continuous synchronisation.   
   > What I'm supposed to understand that all these clocks run normal.   
      
   You seem to be choking at that.   
      
   > > > Specific what type of experiment do you have in mind to demonstrate   
   > > > "time dilation"? Is it one above?   
   > >   
   > > (1) The postulates of special relativity are well-established   
   > > experimentally,   
   >   
   > IMO we should start from the experiments and try to explain them   
   > (as simple as possible)   
      
   SR and GR do that quite well.   
      
   > > (2) the derivation of the LT is logically correct and predicts   
   > > TD and the twin paradox,   
   > > and (3) there is abundant experimental evidence to support   
   > > various aspects of both SR and GR and none to refute them.   
   >   
   > I'm not claiming that SR and GR are wrong. I have my doubts if you need   
   > all of them to explain the trajectories of the stars in our Galaxy or   
   > the planets in our solar system.   
      
   Generally speaking, these motions are too slow to require relativistic   
   corrections.   
      
   > > So why would you continue to doubt?   
   >   
   > Part of the problem is the more I try to read, the more I try to understand   
   > and discuss the issues involved are becoming less clear.   
   > Sometimes the responds indicate that Einstein had it wrong.   
   > Also many books have it wrong.   
      
   Listen to Tom Roberts and others in this group who have been around a long   
   time.  They have it right.   
      
   > As such I have problems with this sentence:   
   > > > "Length contraction", like "time dilation" is purely a geometrical   
   > > > projection, in both SR and GR. So the object being observed is not   
   > > > physically affected. But still, such projections can have physical   
   > > > consequences (e.g. a ladder fits through a narrow doorway) etc.   
   > What is "Length contraction"? Is it physical? yes or no?   
      
   That depends on what you mean by "physical."  It's what measurement shows.   
      
   > What is "Time dilation"? Is it physical?   
      
   That depends on what you mean by "physical."  It's what measurement shows.   
      
   > What are twin type experiments? Are they physical?   
      
   Of course.   
      
   > Part of the problem is what and how do you measure something.   
   > If you use a rod (example) but the length changes during the   
   > measuring process then you have to be careful.   
   > To measure the rate of a clock with an other clock you also   
   > have to be careful.   
   >   
   > In some sense Newton's Law is very straight forward.   
   > The book Newton's Prinicpia emphasizes this.   
   > Newton's Law starts in some sense that at each instant for the whole of   
   > the Universe there is a now. At that same instant you can calulate   
   > the sum of all the forces active for each object considered.   
   > Is the sum is zero the particle will continue to move in the same   
   > direction. If it's not the direction will change. Using that concept   
   > you can calculate the trajectories of the planets around the sun.   
   > The most important issue that the forces don't act instantaneous   
   > as Newton assumed.   
   >   
   > IMO when you compare Newton with SR and you consider our solar system   
   > the Newton considers only one coordinate system with the Sun at rest.   
   > In SR you can consider an observer at rest on the surface on earth,   
   > or an observer at rest at the center of earth, or an observer at jupiter   
   > etc. All these observer are at rest in their frames and clocks moving in   
   > these frames run slower.   
      
   That's a bit simplistic.  The "anyone can consider himself at rest" rule   
   is valid in inertial frames.  But there is no truly infinite inertial frame.   
      
   > IMO such an approach is not very practical.   
   > For our galaxy Newton also would assume one coordinate system with a BH   
   > at its center.   
      
   That depends on how big a volume and how short a time is under consideration.   
      
   > IMO anything related to SR Newton would not consider i.e. no moving   
   > clocks and no worldlines.   
   >   
   > The more I read about worldline the less I understand why 'you' call   
   > it a worldline. It is not a line like a string, it is something else   
   > with the underlying issue if a worldline IS something.   
   >   
   > Nicolaas Vroom.   
   >   
   > [[Mod. note -- We now have direct experimental tests of intercomparison   
   > of moving clocks.  Some early classic experiments involving flying atomic   
   > clocks in around the world are described in   
   >   
   > @article{Hafele-Keating-1972,   
   >   author = "J. C. Hafele and Richard E. Keating",   
   >   title  = "Around-the-World Atomic Clocks: Predicted Relativistic Time   
   > Gains",   
   >   journal = "Science",   
   >   year   = 1972, month = "14 July",   
      
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