Scientific American July 1993 [antikvár]

_ LETTERS TO THE EDITORS More Black Hole Paradoxes I enjoyed "Black Holes and the Centrifugal Force Paradox," by Marek Artúr Abramowicz [SciENTinc American, March), very much but was left somewhat puzzled. As we all know, centrifugal force is a fictitious force that appears to exist when a reference frame is rotated. For example, when a car turns a comer, unrestrained passengers tend to continue to move along a straight tangential line, in keeping with Newton's first law. Could the explanation of the paradox be made in terms of real centripetal forces? Stanley R. Drake Shawnee, Kan. The author writes that two astronauts Inside a tube surrounding a black hole "know that the axis of the tube is circular because Bob has measured the curvature of the walls using straight rulers." Is this not impossible, since linearity is defined by the path of Ught? Would not the ruler be unable to measure any curvature because there is no curvature along the axis of the mbe? RalfPhiupp Smdent, grade 9 Hackley School Tarrytown, N.Y. On page 29, the author states that "in any space-time, with or without a gravitational field, light always moves along geodesies, and therefore it always traces the geometry of space-time. In a space warped by a gravitational field, however, the light rays are curved and in general do not coincide with geodesies" (emphasis added). Is it left to the reader to choose? Gaston Fischer Observatolre Cantonal Neuchatel, Switzerland Every so often you publish an article that reminds me of why 1 subscribe. The subject matter of Abramowicz's article is fascinating, but what is particularly pleasing is that it is one of the best-written scientific articles I've ever read. Frankly, it reads like a Borges short story. Davtd N. Schwartz London, England Abramowicz replies: Astronomers smdy rotating stars by looking at the rotating reference frame. They consider both the gravitational and centrifugal forces acting on the stellar material because the introduction of those fictitious forces makes the problem much easier. My discussion could have been In terms of free-falling frames and centripetal forces, but that would have obscured the subject. One can tell whether two Identical rulers are straight without referring to the path of light as the standard. The method is used by professional draftsmen: they simply lay the rulers beside one another. If the left and right sides of each ruler match, they are straight Of course, the straight rulers will not appear as straight in a curved space! Perhaps an analogy will explain why light trajectories are geodesies in fotu--dimensional space-time but generally not in three-dimensional space. Each great circle on a globe is a geodesic line on the two-dimensional surface, yet, being a circle, it is not a geodesic line in the three-dimensional Euclidean space in which the globe rests. Inspecting Bridges In "Why America's Bridges Are Crum-bltag" [Scientific American, March], Kenneth F. Dunker and Basile G. Rab-bat state that "The Silver Bridge disaster [at Point Pleasant, W.Va., in 1967] happened in part because of poor inspection by local authorities." I am surprised to see that statement in Scientif ic American because there is not the slightest facwal basis for it. I was closely associated with the investigation of the collapse, begirming in January 1968 when I identified the fracture in eyebar 330 as the cause. As a metallurgical smdy by the National Bureau of Standards showed, the eyebar had fractured suddenly because of a stress corrosion crack less than one eighth of an inch deep that had started on the surface of the hole in the eye. The hole was almost completely filled by the pin that coupled successive links in the eyebar chain. The end of the pin and die hole in the eye were also covered by a plate that prevented visual inspection. At the time of the coUapse of the Point Pleasant bridge, an idenUcal bridge was in service a few miles upstream. Nam- rally, there was great interest in determining whether its eyebars could be inspected. The best brains in the nondestructive inspection field concluded unanimously that it could not be done. Consequently, the bridge was removed. John A. Bennett Bethesda, Md. Dunker and Rabbat reply: We thank Bennett for his clarificatiorL Ironically, lax inspection noted at the time of the Silver Bridge collapse helped to trigger a massive federal bridge inspection program, and yet state-of-the-art nondestructive testing would not have detected the hidden defect. X-cluded from Credit Regarding "Spot Marks the X," by John Rennie ["Science and the Citizen," Scientific American, April], concerning the putative role of the Xist gene in mammalian X chromosome inactivaUon, I wish to make the following clarifications. First, the human Xist gene was discovered during my postdoctoral fellowship at the International Institute of Genetics and Biophysics in Naples, Italy, and subsequently characterized in a collaboration with Huntington F. Wil-lard's group and mine. Second, the murine Xist gene was discovered independently and reported simultaneously by Sohaila Rastan's group and mine. Andrea Ballabio Instimte for Molecular Genetics Baylor College of Medicine errata The photograph on page 47 of "How Parasitic Wasps Find Their Hosts" [March] shows a potter wasp, which carries prey to its yotmg, and not, as suggested, a parasitic wasp. On page 94 of "DNA's New Twists" [March], reference is made to the "linkage of methyl groups to cysteine." The DNA base in question is acwally cytosine. Because of the volume of mail, letters to the editor cannot be acknowledged Letters selected for publication may be edited for length and clarity. Scientific American 7wly j 993 5