Actually, superconductors can have embedded magnetic flux in the form of single flux quanta with each one surrounded by a loop of current. These are call type II superconductors--only type I superconductors completely exclude magnetic fields with the Meissner effect. Most technologically important superconductors are type II and they can support a permanent magnetic field (e.g in an MRI machine or CERN) only if the flux quanta are "pinned" on some defects or imperfections in the materials--usually crystal boundaries. Whether a neutron would be a type I or type II superconductor (a quantum property) is an open question, as is the nature of a "defect" in a neutron star... --Rich H. -----Original Message----- From: math-fun [mailto:math-fun-bounces@mailman.xmission.com] On Behalf Of Warren D Smith Sent: Wednesday, June 25, 2014 5:20 PM To: math-fun@mailman.xmission.com Subject: [math-fun] superconducting magnet confusion As far as I can tell, a chunk of superconductor which is topologically a ball, cannot produce a permanent magnetic field. (This should be clear due to the "Meissner effect.") A chunk which is topologically a solid torus, can (CERN magnets). So for example, if the Earth's interior were superconducting as opposed to normal-conducting, then I presume the Earth would have no magnetic field. Now perhaps there might be some way out of this logic trap for those who believe the neutron stars have superconducting interiors (?).... but at least naively, the fact neutron stars have enormous magnetic fields, seems to contradict the super-theory. "Magnetars" have fields up to 10^11 Tesla, Pulsars 10^8. It seems to me, the onus is on the proponents of the super-theory to explain how to escape this trap. And if you look in the papers I cited which claim "the first direct evidence for superconducting neutron stars" you will see not the teeniest tiniest mention of this massive contradiction albeit it does contain this interesting footnote: "There is, to date, no evidence for the presence of a magnetic field in the Cassiopeia A neutron star." But most neutron stars which have been detected are pulsars and hence have huge fields. Incidentally, the "pressure" associated with a magnetic field B is B*B/(2*mu0) which for B=10^11 tesla is pressure=6*10^27 pascals=6*10^22 atmospheres. Sustained. The pressure inside an H-bomb explosion is a mere 10^15 pascals. _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun