From: eaglesondouglas@gmail.com   
      
   On Tuesday, October 3, 2017 at 11:30:42 AM UTC-4, Tom Roberts wrote:   
   > On 10/1/17 10/1/17   12:07 PM, Nicolaas Vroom wrote:   
   > > On Monday, 25 September 2017 01:59:52 UTC+2, Tom Roberts  wrote:   
   > >> On 9/24/17 9/24/17   3:56 PM, Phillip Helbig (undress to reply) wrote:   
   > >>> [...] Length contraction here is obviously an illusion.   
   > >>   
   > >> Not so. An "illusion" could not have physical consequences, but   
   > >> "length contraction" does. For instance (1) the magnetic forces from   
   > >> current-carrying wires, (2) the correspondence between fixed-target and   
   > >> intersecting-beam cross-sections, and (3) the frequency/wavelength of   
   > >> free-electron lasers.   
   > >   
   > > Is there "length contraction" involved in these 3 examples?   
   > > If yes then please explain one.   
   >   
   > (1) For a wire carrying a current involving moving electrons, in the rest   
   frame   
   > of the wire it remains electrically neutral (the power supply generating the   
   > current ensures this is so). So a nearby charged particle at rest in that   
   frame   
   > experiences no EM force from the wire and its current. But a charged particle   
   > moving parallel to the wire at the same speed as the electrons sees the ions   
   of   
   > the wire (i.e. the atomic nuclei) as "length contracted", and the electrons   
   as   
   > not, so in its frame there is a net positive charge on the wire, and it   
   feels an   
   > EM force. In the wire rest frame we call this "magnetic force", while in the   
   > moving frame it is "electrostatic force". This is much more general that my   
   > simple description, and when worked out numerically it is correct; I believe   
   > that Perkins's book on E&M goes into this in detail.   
   >   
   > (2) scattering an unpolarized particle beam from an unpolarized target is   
   > cylindrically symmetric, and we measure the differential cross-section as a   
   > function of polar angle. For a fixed-target experiment the target is at rest   
   in   
   > the lab; for a colliding-beam experiment the center-of-mass is at rest in the   
   > lab. To reconcile these two measurements at a given center-of-mass energy,   
   one   
   > must invoke "length contraction".   
   >   
   > (3) a free-electron laser consists of an energetic electron beam traveling   
   > through a magnetic field that alternates transverse directions in space,   
   > typically every 10-20 cm over a length of several meters (the magnets are at   
   > rest in the lab). As the beam is "wiggled" by the magnetic field, it   
   oscillates   
   > with the frequency it sees the field alternate -- this generates radiation of   
   > that frequency, and for quantum reasons this can be a laser. In the lab this   
   > radiation is measured, and to account for the observed frequency, that 10-20   
   cm   
   > alternation must be reduced by the "length contraction" formula in the beam   
   rest   
   > frame.   
   >   
   > Tom Roberts   
      
   I hope this does not sound to speculative.  From my look at the subject   
   of reference frame effect on emission frequency, the concept of   
   conservation of light power appears.  Some people say this another way   
   and require length contraction to explain.  In my case it is total   
   4pi power conserved.  Is beam power contraction the same as spherical?   
      
   My dilemma being the cause to coherency. Is length contraction the   
   relation that makes a spherical emission change to beam?  A stationary   
   wiggler will emit, but will it display a focusing, i.e. coherency if the   
   particle moves?   
      
   The pure electron emission is my model here.  Coherency as a solid state   
   channel effect is a con-founder for me. Here a mass causes resonation   
   making mass cause focus.  A common relation for coherency would be nice   
   to find.   
      
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