From: tjroberts137@sbcglobal.net   
      
   [REPOST with word wrapping]   
      
   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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)