# Jacob Bekenstein

Jacob Bekenstein (May 1, 1947 – August 16, 2015) was a Mexican-born Israeli-American theoretical physicist, winner of the Wolf Prize in Physics (2012) and the Einstein Prize (2015).

## Quotes

* In a previous paper we showed that a static (nonrotating) black hole cannot be endowed with exterior scalar-meson or massive vector-meson fields. Here we show that the same is true for massive spin-2 meson fields. We also extend the above results to the case of a rotating stationary black hole. We conclude from our results that a black hole in its final (static or stationary) state cannot interact with the exterior world via the strong interactions which are mediated by meson fields such as the π (scalar), ρ (vector), and ƒ (spin-2). A direct consequence of this is the impossibility of determining the baryon number of the black hole by means of exterior measurements alone. This results in the transcendence of the law of baryon-number conservation as originally predicted by Wheeler.
** (15 May 1972)"Nonexistence of baryon number for black holes. II". Physical Review D 5 (10): 2403–2412. DOI:10.1103/PhysRevD.5.2403.
* We show that it is natural to introduce the concept of black-hole entropy as the measure of information about a black-hole interior which is inaccessible to an exterior observer. Considerations of simplicity and consistency, and dimensional arguments indicate that the black-hole entropy is equal to the ratio of the black-hole area to the square of the Planck length times a dimensionless constant of order unity. A different approach making use of the specific properties of Kerr black holes and of concepts from information theory leads to the same conclusion, and suggests a definite value for the constant.
** (15 April 1973)"Black Holes and Entropy". Phys. Rev. D 7 (8): 2333–2346. DOI:10.1103/PhysRevD.7.2333.
* 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.
** (April 1982)"Black holes and everyday physics". General relativity and gravitation 14 (4): 355–359. DOI:10.1007/BF00756269.
* 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 ...
** (27 March 2007)"The modified Newtonian dynamics—MOND and its implications for new physics". arXiv preprint astro-ph/0701848. (p. 2)
* 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.
** (21 January 2010)"Alternatives to dark matter: Modified gravity as an alternative to dark matter". arXiv preprint arXiv:1001.3876.
* 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.
** as quoted in: Milgrom, Mordehai (2018). "MOND from a brane-world picture". arXiv preprint arXiv:1804.05840. (p. 2)

## Quotes about Bekenstein

* 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.
** Robert M. Wald: (1 December 2015)"Jacob David Bekenstein". Physics Today 68 (12). DOI:10.1063/PT.3.3029. (p. 68)
* 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.
** Mordehai Milgrom: "MOND from a brane-world picture." arXiv preprint arXiv:1804.05840 (2018), p. 2.