Jacob Bekenstein

“Jacob was the first physicist to hear from me about MOND. It was 1982; Jacob was still at Ben Gurion University in Beer Sheba. I went there from Rehovot with my initial MOND trilogy of preprints in my bag to tell Jacob and benefit from his advice. I had not prepared him for what was in these preprints. Jacob was immediately captured, but he also warned me — as I vividly remember — that this is going to encounter much opposition, but also that I should not heed such opposition. He was drawing on his own experience with black-hole entropy and on how his ideas had been received a decade earlier.”

“Bekenstein incorporated black hole entropy into a generalized second law—that the sum of the entropy outside black holes plus the newly proposed entropy of black holes must never decrease—and carefully considered processes that might violate it. To avoid violations, he found that he had to assume limitations on how close to the black hole’s horizon one could lower matter. Those ideas eventually evolved into a proposal for a bound on the entropy-to-energy ratio of matter confined to a region of given size.”

“There is a prescription that works well, MOND, but the reason it works so well is not known. You may say that MOND tells us how the real theory of gravity should look.”

“Within the DM paradigm the Tully-Fisher law must arise from galaxy formation since it connects luminosity of baryonic matter with a dynamical property, rotation, which is seen as dominated by the DM halo. But it has not been easy to derive Tully-Fisher from any natural connection between the two components. And as R. H. Sanders has pointed out, the messiness of galaxy formation is hardly the natural backdrop for such a sharp correlation between galaxy properties. The sharpness needs a dynamical reason as opposed to an evolutionary one.”

“The DM halo hypothesis is evidently an attempt to resolve the acceleration discrepancy within orthodox gravitation theory. But in coming to terms with this discrepancy, suspicion fell on Newtonian gravity already early in the game. Zwicky, who had exposed the acceleration discrepancy in clusters of galaxies ..., opined much later that the discrepancy may reflect a failure of conventional physics ...”

“Black holes set a limitation on the number of species of elementary particles—quarks, leptons, neutrinos—which may exist. And black holes lead to a fundamental limitation on the rate at which information can be transferred for given message energy by any communication system.”