From: helbig@asclothestro.multivax.de   
      
   In article , "Phillip Helbig (undress to   
   reply)"  writes:   
      
   > Note that neutrinos from the supernova 1987A in the Large Magellanic   
   > Cloud emitted neutrinos which were detected on Earth.  Since the   
   > difference in emission times of light and neutrinos is only about 3   
   > hours, this is negligible for the travel time of 157,000 years and thus   
   > provides a good lower limit on the speed of neutrinos (i.e. very close   
   > to the speed of light).  This gives an upper limit on the mass of the   
   > electron neutrino of about 26 eV.  Current limits are about 0.11 eV.   
   > However, because of neutrino oscillations, we know that it is non-zero.   
   >   
   > [[Mod. note -- To amplify one point Phillip made: measurements of neutrino   
   > oscillations tell us the *difference* between the *squares* of the masses   
   > of different types of neutrinos, i.e., they tell us something like   
   >    m1**2 - m2**2  .  But if my dim recollection is correct, they don't   
   > easily tell us which of the 3 types of neutrinos  m1  and  m2  refer to.   
   > -- jt]]   
      
   I now see that what I wrote is somewhat confusing.  The only sentence   
   which refers to neutrino oscillations is the very last one, added to   
   emphasize the point that while upper limits have been decreasing, unlike   
   the mass of the photon or graviton it is ruled out that the mass of the   
   neutrino could be exactly zero, because of neutrino oscillations, even   
   there is no lower limit on the mass from more-direct measurements of the   
   mass.  As Jonathan mentioned, they tell us something about the   
   differences of the squares of the masses, not the masses themselves.   
   (I'm not absolutely sure that the case of one massless and two massive   
   neutrinos is not allowed, but I think that the implication is that all   
   three types have a non-zero mass.)   
      
   With regard to the time-of-flight measurements from supernova 1987A,   
   note that massless neutrinos would all have the same time of flight,   
   even with different energies.  With a mass, different energies would   
   lead to differences in the times of flight.  The measured spread on   
   arrival times can put an upper limit on the mass.   
      
   While probably most expect the electron neutrino to be the lightest and   
   the tau neutrino the heaviest, like the corresponding charged particles,   
   apparently an inverted mass hierarchy is possible, with some other   
   ordering.  In other words, we don't know if the electron neutrino is   
   necessarily the lightest one.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)