🚀 go-pugleaf

In article <mcirvin.713483129@husc8> mcirvin@husc8.harvard.edu (Mcirvin) writes:
>crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>
>>In article <1992Aug10.184700.3204@sbcs.sunysb.edu> rscott@csws1.ic.sunysb.edu (Robert Scott) writes:
>>> [claim that SR follows from signal delays] 
>
>>      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).
>
>The interesting thing about this idea is that even though the 
>space as modeled in the program has an "ether" (coordinates are
>presumably stored internally in some absolute frame), objects
>made entirely of formations of robots exchanging signals behave
>in a Lorentz-invariant way, and the speed of light appears to
>be the same in all frames, provided that you use measuring
>equipment available within the program's physics: measure time
>with a canonical pair of robots bouncing a signal, etcetera.

     An 'ether' is not ruled out by SR, of course, it just seems
     superfluous since the relevant reference frame is certainly not
     unique.

>>     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?
>
>Reread his post:  he's putting it in by denying his robots, which
>are the only particles in the model world's physics, any 
>knowledge of quantities that aren't Lorentz invariant.  The
>*kinematics* of SR will indeed emerge if you only use systems of
>robots as clocks, meter sticks, etcetera.  The only real problem
>is that this is a model of insufficient generality to show how
>SR intrudes into real-world phenomena, without the added
>specification that c should look the same in all frames no matter
>what measuring devices you use-- but that's more an issue of
>interpreting the program than of writing it.  The model also
>doesn't really address the full complexity of SR *dynamics*, but
>one can model such things as collisions by giving the robots
>appropriate instructions.

     Upon re-reading, this is still not clear to me from his posting.
   
     I still do not see how he imposes this, without actually
     imposing it.  In other words, how does he insure that appropriate
     quantities will obey the lorentz transform.  Simply saying that
     messages cannot pass at greater than lightspeed begs the question.
     The question is lightspeed with respect to what?   He must make
     it with respect to everything appropriately defined 
     if he wishes correct behavior.  How does one do this in 
     any computational space without using the machinery of 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?
>
>The robot isn't going to go *to* event Y, it's going to have the
>*velocity* corresponding to the interval between X and Y.  This
>is a means of specifying velocities that is completely Lorentz
>invariant.
     
     I did misunderstand his point.  However, how does one do 
     this correctly without using the machinery of SR?  Keep in mind
     that one could do what he says here perfectly well in a 
     galilean universe with lightspeed delay with two events 
     X and Y.  How does the robot achieve the appropriate 'velocity'
     with respect to everything else without using the machinery?
     
>>      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.
>
>Look again, keeping in mind that the only clocks and meter
>sticks you can use are formations of robots.  Don't be fooled
>by the all-caps (note to Robert Scott: on Usenet people will
>read your posts more carefully and take you more seriously if 
>you use lower case).

     One can use metersticks and clocks to ones heart's content to 
     describe a system, but in the words of S. Parrot "At this point
     it is no more than a mathematical tautology which says nothing
     of substance about the world"  (very enjoyable book, by the way,
     "Relativistic Electrodynamics and Differential Geometry", Springer
     1987).   It is still not clear to me how he imposes lightspeed
     delay between robots in relative motion in such a way as to 
     make the 'delay' the same for all the robots without using the
     SR machinery.  I see no simple way to do this, though I would 
     be most edified if someone else does.

     The lines about the 'eye-in-the-sky' especially made me wary
     since that should be essentially no better than any other eye.

>>>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.  
>
>He's not.  He's talking about a symmetry that physics may or may
>not possess, and such things as scaling relations in nature do
>rely intimately on the symmetry.  Suppose dilation symmetry really
>did exist in nature: then a physical evolution enlarged in space
>and time by a constant factor would still be an allowable physical
>evolution, and lots of things having to do with objects would
>be different.  Matter of a given composition could possess any
>density, and objects could be structurally sound at any size
>provided that the matter were scaled appropriately.  There
>could be tiny civilizations made of incredibly dense versions of
>ordinary matter.  The nonexistence of dilation symmetry in
>real physics is what prohibits all this.

     The existence of a length scale certainly does not constrain the
     evolution equations from having dilational symmetry.  Length
     scales can simply arise from the boundary/initial conditions.
    
>Personally I'd be all for giving the robots proper-time clocks.
>That would both get rid of the extra symmetry and also imply
>some Lorentz-invariant physics independent of the exchange of
>light-speed signals, such as we have in real life.

     I still do not see how one imposes signaling requirements of SR
     without using the machinery.  The poster also seems to indicate
     that there is no time scale whatsoever.  My question before, as now,
     is:  If there is no determination of a 'time' quantity, how
     does one impose lightspeed delays at all.

>Many SR-kinematic phenomena can indeed be made to follow from
>the kind of model Robert Scott is talking about.  It's a little
>misleading to say that it follows entirely from signal delays,
>since you additionally have to base all physics upon the 
>transmission of the signals.  It's better to say that it
>follows from the intrinsically Lorentz-invariant nature of
>the model he's constructed.

     How is it constructed without using Lorentz covariance, as he 
     seems to indicate?  It seems to me that this is reading a bit much into
     what he says.

>I would also argue that the treatment of SR from a geometric
>viewpoint is ultimately simpler than thinking in terms of
>a model where all physics consists of signal transmission.
>Nevertheless, a program like the one he's described would
>illustrate relativistic kinematics in a novel way.

     Provided it was based on relativistic kinematics.  I am still uncertain
     how one would do what the previous poster was alluding to in the
     manner that he seems to indicate.

                               dale bass

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