100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

144 R. H. Priceviscous heating of the gas which requires radiation for cooling. The detailsof the disk’s structure, temperature profile, luminous signature, andso forth, all depend on the microphysics of the viscosity, and there is nosecure model for this. The viscosity may be based on the highly conductingplasma being threaded by tangled magnetic fields. In any case, somefeatures of the models are relatively insensitive to the details; one of theseis the nature of the characteristic radiation. Again it follows from a simpleorder-of-magnitude calculation that the temperature must be on theorder of 10 9 K, and the radiation will be predominately in the form ofX-rays.When we move from stellar mass holes to supermassive holes, we movefrom physical models that miss details, to physical models themselves thatare still uncertain. But we also move to astrophysical black holes whoseexistence is essentially certain (or at least establishment dogma). Somegalaxies have active galactic nuclei (AGNs), tiny central regions that emitluminosity comparable to galactic luminosity, but are incredibly small onan astrophysical scale 13 . From a number of arguments, it is known thatthe engine – the source of energy – in these AGNs consists of a mass socompact that it must involve a supermassive black hole. The mechanism forproducing the radiation must certainly be different from that of a black holeaccretion disk, since AGNs are characterized by radio jets, narrow regionsof strong radio emission emerging from a central source. Theorists believethat the mechanism for these complicated sources probably involves a largescale magnetic field anchored in a rapidly rotating supermassive hole. It isbelieved that this large scale magnetic field is created by the accretion ofplasma carrying magnetic flux. The magnetic field then links the horizonto distant regions of the surrounding plasma. With the horizon playing therole of a rapidly rotating conducting membrane, the hole and magneticfield act like a kind of dynamo. The EMFs on rotating field lines createstrong currents that flow to the distant plasma linked by the field lines.The production of these currents, in effect, converts the rotational energyof the black hole into radio and other forms of radiation.There had been a distinct difference between the charactersitics andmodels of stellar mass holes and of supermassive holes, but a few yearsago this difference faded. A class of objects called “microquasars” 14 wasdiscovered that seemed to be stellar mass X-ray black holes that also havethe radio jets of AGNs. The tentative understanding of these microquasarsis that they are accretion disk objects that can build up large scale magneticfields and behave like AGNs scaled down by a linear factor of 10 8 or so.