Re: simplier is more difficult

[twayburn@juno.com (Thomas L Wayburn)]

On Wed, 08 Oct 1997 16:32:58 EDT David Bowman
<dbowman@tiger.gtc.georgetown.ky.us> writes:
> James Wheeler wrote:
>
> > I didn't think that a uniform magnetic field was possible (except as 
> an
> > approximation).
>
> This is true, but it is not a problem in principle.  In physics we use
> idealizations all the time to simplify the things when considering 
> problems
> of principle. It's also not possible to make a perfectly ideal gas, 
> simple
> harmonic oscillator, frictionless plane, or a host of other 
> idealizations.
> If a given region of arbitrary size is required to have a B field of a
> specified degree of uniformity, then it is possible in principle to 
> build an
> appropriate coil external to that region such that it will produce the
> required field to the uniformity specifications desired.  The only 
> problem in
> practice is one of practicality, cost and technological prowess -- not 
>
> principle.
*****************************************************************************
But, Dave, physics is concerned with ideals which exist but in the Ideals
rather than in the Universe.
******************************************************************************
 
> > Three thoughts (not-coherent)
> > 1.  The assymetry is that electric charges exist, and magnetic 
> monopoles
> >     do not. (Cabrerra notwithstanding) This means that all
> >     real electric and magnetic fields are the superposition of
> >     relativistically transformed electric fields. (N.B. the 
> superposition
> >     of transformed fields need not be the transform of a single 
> field.)
>
> This is not true.  Superposing separately transformed the static 
> electric
> fields from charged particles will never produce propagating 
> electromagnetic 
> waves.  E&M is *not* just electrostatics supplemented with the Lorentz 
>
> transformation rules for the sources and the E fields.  The EM field 
> is its
> own dynamic entity and is not just a mathematical artifact left over 
> from a
> creative way (using a field description) of treating electrostatic 
> coulomb
> interactions between charges.
>
> > 2.  What about the intrinsic magnetic moments of elementary 
> particles.
> >     These are relativistic in origin, but need not represent moving
> >     charge.
>
> This statement contradicts the point 1. above.  Clearly, the magnetic 
> field
> from the intrinsic magnetic moment of an elementary particle is *not* 
> the
> result of a relativistic transform of that particle's electric field 
> (esp.
> since it exists when that particle is at rest).  Your example in point 
> 2. is
> a counterexample which invalidates your point 1.  Whether or not an 
> intrinsic
> magnetic moment is relativistic in origin or not is a somewhat 
> debatable
> issue which depends to some extent on just how the term 'relativistic' 
> is
> used.  What is more certain, however, is that intrinsic magnetic 
> moments are
> *quantum* effects.
>
> > 3.  Since charge cannot be created, perhaps we should believe that
> >     current rather than charge is intrinsic.  Maybe all electric 
> fields
> >     are transforms of magnetic fields!  Isn't duality fun! Pick your
> >     basis.
>
> This is not true either.  It simply is not possible to get along with 
> just
> E-fields or just B-fields and treat the other field type as a 
> transform (or a
> superposition of separate transforms over the separate sources) of the 
> other
> type of field.
>
> David Bowman
> dbowman@gtc.georgetown.ky.us