XPost: sci.electronics.design, sci.physics   
   From: PointedEars@web.de   
      
   Jeroen Belleman wrote:   
   > On 2/19/26 20:27, john larkin wrote:   
   >> On Thu, 19 Feb 2026 20:52:48 +1100, Bill Sloman    
   >> wrote:   
   >>> On 19/02/2026 7:49 am, Ross Finlayson wrote:   
   >>>> On 02/18/2026 12:43 PM, Python wrote:   
   >>>>> Le 18/02/2026 à 20:13, Ross Finlayson a écrit :   
   >>>>> ..   
   >>>>>> and, you know, magnetic monopoles, is widely employed   
   >>>>>> in medical imaging and the like.   
   >>>>>   
   >>>>> No.   
   >>>>   
   >>>> Resonance imaging (NMR) is a thoroughly different mechanism   
   >>>> than Roentgen rays.   
   >>>   
   >>> But as the name implies, it's nuclei of the atoms involved that exhibit   
   >>> the resonance. It's a remarkably low energy effect, and you need   
   >>> remarkably high magnetic fields to get it to give you a detectable signal.   
   >>   
   >> The earth's field is enough. The hydrogen resonance is about 4   
   >> KHz/gauss.   
      
   Resonance is a phenomenon, not a physical quantity.  What is probably meant   
   here is the absolute value of the gyromagnetic ratio γ divided by 2π, from   
   the Larmor frequency   
      
     ω_L = 2π f_L = -γ B_0  <==>  γ/2π = |f_L/B_0|.   
      
      
      
   For the hydrogen nucleus,   
      
               γ = 2.6752218708(11) × 10^8 s^-1 T^-1   
      
     ==> γ/(2pi) =~ 42.6 MHz/T =~ 4.26 kHz/G.   
      
      
      
   That is, if the external magnetic field has a flux density of 1 G (gauss),   
   the Larmor frequency -- the frequency with which the nuclear spin is   
   precessing is ca. 4.26 KHz.   
      
   But   
      
   ,-   
   |   
   | [...] The magnitude of [the flux density of] Earth's magnetic field at   
   | its surface ranges from 25 to 65 μT (0.25 to 0.65 G)   
      
   so the presence of the geomagnetic field causes the nuclear spin of a   
   hydrogen nucleus, i.e. a proton, to precess at a frequency of 1.065 kHz to   
   2.769 kHz (nothing more, nothing less);   
      
   and   
      
      
      
   | A perturbation of nuclear spin orientations from equilibrium will occur   
   | only when an oscillating magnetic field is applied whose frequency ν_rf   
   | sufficiently closely matches the Larmor precession frequency ν_L of the   
   | nuclear magnetization.   
      
   They key word there is *oscillating*.   
      
   > I believe that, due to magnetic field perturbations in urban   
   > areas, the resonance gets scrambled too quickly to be easily   
   > detectable. In rural areas it should be easier.   
      
   If the geomagnetic field, which on short timescales (human lifespan) can be   
   assumed to be *constant* instead, would have any significant influence on   
   the nuclear spin, applications of NMR (on/near the terrestrial surface) such   
   as MRI would not work.   
      
   Also, NMR has nothing to do with magnetic monopoles, on the contrary:   
      
   A nucleus acts (at least) like a rotating *dipole* magnet, it has a   
   quantum-mechanical spin called "nuclear spin".   
      
   There appear to be many misconceptions on the part of the person making   
   those claims as to what NMR is and what it is affected by.  They should read   
   the referred Wikipedia articles very carefully to clarify those misconceptions.   
      
   This has nothing to do with the theories of relativity; F'up2 sci.physics.   
      
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   PointedEars   
      
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