From: tjroberts137@sbcglobal.net   
      
   Note: the current limit on the sum of neutrino masses is below 1 eV. The   
   neutrinos we detect have energies much greater than that, so they have   
   speeds greater than 0.99 c. I will discuss only neutrinos that have some   
   prospect of being detected.   
      
   On 10/16/17 6:45 AM, toadastronomer@gmail.com wrote:   
   > Could neutrinos be lost behind black hole horizons   
      
   Certainly.   
      
   > or get trapped in accretion around them?   
      
   Not likely. The phase space for orbits with speeds > 0.99 c is extremely   
   small [#], and there's no plausible mechanism to trap neutrinos in such   
   orbits.   
      
   	[#] Light orbits a Schwarzschild black hole at r=3M; these   
   	orbits would be slightly outside that. Light orbits are   
   	exactly circular; these are very nearly so.   
      
   > Could neutrinos interact strongly with neutron star matter or perhaps get   
   > trapped in low neutron star orbit?   
      
   Note that "strongly interacting" has a technical meaning in physics, and   
   neutrinos don't interact strongly; they are neutral leptons which   
   interact only weakly (also a technical term) and gravitationally   
   (ditto). Once formed, a neutron star is nearly transparent to neutrinos,   
   due to the tiny neutrino cross-section and the colossal degeneracy of   
   the neutron-star constituents.   
      
   For an object that originates far away from a mass to enter an orbit   
   requires the object to interact non-gravitationally near the mass, such   
   that its speed is reduced to that of an orbit there. There is no   
   plausible way to do that for a neutrino.   
      
   > Might the neutrino flux in the direction of the galactic center (or   
   > line-of-sight to any compact object) be slightly lower than background, due   
   > to such gravitational interactions?   
      
   Unmeasurably so. The only missing neutrinos would be those that   
   intersect the black hole's horizon, and that subtends an extremely tiny   
   solid angle from earth. Its gravitation will deflect neutrinos over a   
   much larger area, which is in principle observable, but in practice our   
   neutrino detectors cannot point back with anywhere close to enough   
   accuracy to see it.   
      
   Tom Roberts   
      
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