🚀 go-pugleaf

In article <mcirvin.713582409@husc8> mcirvin@husc8.harvard.edu (Mcirvin) writes:
>
>>   The retarded solutions usually refer
>>   to the sequence cause -> effect , while the advanced solutions are  often
>>   referred  to  in  terms  of a backward sequence in time, effect -> cause;
>>   which solutions, if applied, make it appear that the sequence in question
>>   violates the law of causality. Again ,to avoid this apparent conflict  in
>>   the  context of Maxwell's equations we must reverse the names emitter and
>>   absorber.
>
>Actually, it's a matter of boundary conditions.  When properly
>considered, advanced solutions don't violate causality at all; they
>just make assumptions about past boundary conditions rather than
>future ones.  Emitters do become absorbers in the process, but
>it's necessary to remember how the situation is set up.

    However, one is always left with explaining why one doesn't
    see solutions that do violate causality or the second law.
    This always seemed to me a 'have your cake and eat it 
    too philosophy' in that we wish to maintain the time-symmetry
    of the underlying equations while retaining the effects
    of having only retarded solutions (i.e. the effects of the special boundary
    conditions).

[stuff deleted]

>continuous-wave situation.  But you need very special boundary
>conditions set up in the past for that to happen; assuming
>that they're there is like expecting eggs to unscramble.  The fact
>that we don't expect such boundary conditions in the past is
>a manifestation of the arrow of time, and opens up a whole
>other can of worms-- though I suspect that most arrows of
>time have something to do with *cosmological* boundary
>conditions.

     Probably, since the primary difficulty here would 
     be waves coming in from infinity.

[very good in-depth explanation deleted]

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