🚀 go-pugleaf

In article <1992Aug10.184700.3204@sbcs.sunysb.edu> rscott@csws1.ic.sunysb.edu (Robert Scott) writes:
>
>WELL, MAYBE I DIDN'T MAKE MYSELF CLEAR.  I WASN'T TRYING TO SAY THAT
>THE ORTHODOX VERSION OF SPECIAL RELATIVITY IS WRONG AND THAT IT NEEDS
>TO BE REPLACED BY A COMMON-SENSE VERSION IN WHICH EVERYTING IS
>EXPLAINED AS A SIMPLE RESULT OF SPEED-OF-LIGHT DELAYS.  RATHER, I WAS
>TRYING TO SAY THAT THE ORTHODOX VERSION OF SPECIAL RELATIVITY _IS_ A
>COMMON-SENSE VERSION IN WHICH EVERYTHING IS EXPLAINED AS A SIMPLE
>RESULT OF SPEED-OF-LIGHT DELAYS, REGARDLESS OF HOW MUCH SOME TEACHERS
>AND TEXTBOOKS TRY TO OBFUSCATE THIS POINT.  AS FOR THE "GALILIEAN
>ELECTRODYNAMICS" PEOPLE, I REALLY DON'T KNOW VERY MUCH ABOUT THEM, BUT
>IF THEY ARE TRYING TO ARGUE THAT GALILEAN INVARIANCE IS A MORE USEFUL
>ORGANIZING PRINCIPLE IN PHYSICS THAN LORENTZIAN INVARIANCE IS, THEN
>THAT SOUNDS LIKE A VERY FOOLHARDY THING TO ATTEMPT.  (ACTUALLY I
>SUSPECT THAT THEY ARE TRYING TO DO SOMETHING ELSE ALTOGETHER, BUT I
>HAVE VERY LITTLE INTEREST IN FINDING OUT WHAT IT IS.)

      It is not this simple.  It is not just that there are 'speed of 
      light delays'.  That was perfectly understandible in terms of 
      a fixed propagational ether.  It is that the speed of light
      appears to be the same no matter what your motion (or relative motion
      with respect).  And no matter how you express this, it *is*
      a bit complicated when one starts trying to determine then consequences
      of this.

>THIS VIRTUAL WORLD WOULD BE SPECIAL RELATIVISTIC, NOT BY MASSIVE
>CONTRIVANCE, BUT SIMPLY BY VIRTUE OF RESTRICTING ALL SIGNALS TO TRAVEL
>AT LIGHT-SPEED (AND DOING NOTHING ELSE TO BREAK THE SYMMETRY ANY
>FURTHER).  THEN STUDENTS WOULD LEARN BY BUTTING THEIR HEADS UP AGAINST
>MATHEMATICAL REALITY THAT EVEN THOUGH A FLEET OF ROBOTS CAN BE
>PROGRAMMED TO EXECUTE VERY INTRICATE AND INTERESTING FORMATIONS, THERE
>IS, FOR EXAMPLE, NO WAY TO PROGRAM THEM SO THAT, STARTING FROM RANDOM
>POSITIONS IN SPACE-TIME AND RANDOM SUB-LIGHT-SPEED VELOCITIES, THEY ARE
>ABLE TO ORIENT THEMSELVES RELATIVE TO EACH OTHER AND THEN ACHIEVE
>DISTANT SYNCHRONIZATION OF ACTION.  SIMILARLY THEY WOULD LEARN THAT ANY
>ATTEMPT TO PROGRAM THE ROBOTS TO ASSUME A FORMATION OF A PARTICULAR
>RIGID SHAPE WOULD BE VULNERABLE TO CONTRACTION OF THE SHAPE IN THE
>DIRECTION OF MOTION.  THUS THEY WOULD LEARN ABOUT SPECIAL RELATIVITY IN
>A VERY VISCERAL BUT ALSO VERY INTELLECTUAL WAY.
[stuff deleted]

     Restrict signals to the speed of light all you want, but you need
     the Lorentz covariance to actually model relativity.  How do
     you suggest programming this without using the SR framework?
     
     In your example, you stated that all messages travel at one fixed speed.
     Fixed with respect to what?  Unless it is everything, you have 
     not appropriately modeled SR.

>4.  SELECT A NEW VELOCITY, SPECIFIED AS THE VELOCITY THAT WOULD BE
>NEEDED TO TRAVEL FROM EVENT X TO EVENT Y, WHERE X AND Y ARE AFFINE
>COMBINATIONS OF THE POINTS OF ORIGIN OF PREVIOUSLY RECEIVED MESSAGES.  
>IF X=Y OR IF THE SPECIFIED VELOCITY IS LIGHT-SPEED OR GREATER, THEN THE
>COMMAND IS IGNORED.

     Event Y already occurred?  How would one reach an event that
     had already happened?

>AS FOR YOUR COMPLAINT THAT LIGHT-SPEED DELAYS CAN NOT EXPLAIN WHY
>TIME-DILATION OCCURS, I THINK THAT YOU GOT IT EXACTLY BACKWARDS.  THAT
>IS, IT'S NOT THAT THE ROBOTS IN THE GAME DON'T EXHIBIT TIME-DILATION;
>RATHER IT'S THAT THERE ISN'T EVEN ANY STANDARD AGAINST WHICH YOU CAN
>ASK WHETHER THEIR TIME IS DILATED.  THAT IS, THE SYMMETRY GROUP
>OPERATING HERE, WHICH IS THE INTERSECTION OF THE GROUP OF AFFINE
>TRANSFORMATIONS (PRESERVING THE STRUCTURE RELATED TO INERTIAL MOTION
>AND THE SPECIFICATION OF SPACE-TIME POINTS IN TERMS OF AFFINE
>COMBINATIONS) AND THE GROUP OF "LORENTZIAN CONFORMAL" TRANSFORMATIONS
>(PRESERVING THE STRUCTURE RELATED TO THE LIGHT-SPEED SIGNALLING
>PROTOCOL) IS ALMOST, BUT NOT QUITE, JUST THE POINCARE GROUP OF SPECIAL
>RELATIVITY.  

     What?   If there is no relation between the time standards, there
     is complete anarchy and you cannot describe the kinematics of the 
     system at all.

     If you have Lorentz covarience of all frames then you will get
     time dilation.  If you have no time dilation, you cannot have
     SR.  What you have here sounds very much (though in a confused
     fashion) like galilean relativity with lightspeed delays.  I think
     that you will find, to your misfortune, that you either have
     a preferred frame or light (or messages) does not travel
     at a single speed with respect to all observers.

>IT HAS ONE EXTRA DIMENSION, CONSISTING OF THE GROUP OF
>UNIFORM EXPANSIONS OF SPACE-TIME ABOUT AN ARBITRARY REFERENCE POINT. 
>NEVERTHELESS, I STILL THINK THAT THE EXPERIENCE OF LIVING IN THIS SORT
>OF VIRTUAL WORLD CONVEYS ALMOST ALL OF THE FLAVOR OF WHAT SPECIAL
>RELATIVITY IS LIKE.  THE EXTRA EXPANSION SYMMETRY IS VERY EASY TO
>UNDERSTAND (DESPITE THE RARITY OF TRUE EXPANSION SYMMETRY IN NATURE, AS
>ILLUSTRATED FOR EXAMPLE BY GALILEO'S ANALYSIS OF WHY IT IS UNHEALTHY
>FOR AN ANIMAL FOR IT TO GROW TOO BIG), AND ONCE YOU'VE GOTTEN USED TO
   
     You are confusing the existence of a symmetry of the underlying
     equations (space, etc.) with a symmetry of objects that exist 
     in the space.  

>LIVING WITH [POINCARE+EXPANSION] SYMMETRY IT'S ALSO FAIRLY EASY TO
>UNDERSTAND HOW JUST THE EXPANSION SYMMETRY CAN BE BROKEN BY THE
>INTRODUCTION OF CLOCKS WHICH KEEP A STANDARD PROPER TIME (AS CONTRASTED
>TO A STANDARD TIME VALID IN ALL REFERENCE FRAMES, WHICH WOULD OF COURSE
>BREAK THE SYMMETRY MUCH FURTHER).

     I'd like to see exactly how you quantify this.  For example, 
     how do you measure velocities?  How do you know that signals
     'go' at the speed of light?

                           dale bass

-- 
C. R. Bass                                           crb7q@virginia.edu
Department of Mechanical and Aerospace Engineering
University of Virginia
Charlottesville, Virginia                            (804) 924-7926