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The Talk of the Lab Bowling balls The detectors that particle physicists build to peer deep into matter are enormous, multiton devices fitted with delicate systems of vacuum tubes, chips, wires, relays, and switches, all intent on converting the energies and paths of particles into digital data. In the KTeV detector, alongside these state-of-the-art electronic tools, are some unusual and decidedly mundane parts: four bright-red bowling balls, purchased at the local bowling lanes in North Aurora. Hogan Nguyen, now a Wilson Fellow physicist at Fermilab and the person who was responsible for introducing the bowling balls, believes that no other detector in the world has anything like them. He got the idea of using them from Donald Goloskie, now in the Particle Physics Division working on the DZero upgrade. Of course, the bowling balls don’t look much like bowling balls anymore. KTeV collaborator Ed Blucher says they look more like porcupines, though they function like power strips. The bowling balls are part of the calibration system in the KTeV detector for the photomultiplier tubes that convert light to current and for the electronics that convert the current to digital signals. The calibration system is distinctly high-end. "It’s not an easy job to calibrate a 17-bit device," said Nguyen, referring to the highly sensitive cesium iodide crystals that make up the heart of the KTeV detector. These crystals can pick up a wide range of signals–whether from a transverse muon from a cosmic ray (depositing an energy of only about 15 to 30 MeV) or from a photon (60 to 80 GeV). To fit the bowling balls to their new role, their north and south poles were shaved off; their insides were hollowed out; and each was pierced with 800 one-millimeter holes, using a pneumatic drill. To each bowling ball, a powerful laser sends pulses of light, whose wavelength is shifted and redistributed by a fluorescent dye inside a vial in the bowling ball. From there, fiber optic cables threaded through the one-millimeter holes send the light to the 3,100 cesium iodide crystals. Nguyen said that when designing the calibration system, he realized he needed each of the fiber optic cables to "see" the same light, and to sit at the same distance from the light source. Clearly, a symmetrically shaped vessel to hold and position the cables would work, but what sort of vessel? Nguyen thought of aluminum–it was hard enough to be machined–but he couldn’t find any available in a perfectly round shape. The bowling ball idea met his criteria, so long as the balls were not the cheaper variety with soft-foam cores, but the $100 kind that are nearly solid throughout. When Nguyen picked them up at the bowling lanes and said he wasn’t headed for the championships, he got a few quizzical looks. But what better use for a top-of-the-line bowling ball? –Sharon Butler
Infinity as an illusion Is the universe actually finite, tricking us into thoughts of infinity with multiple images of distant stars and galaxies? Does light make multiple passes through some as-yet-unimagined (and possibly unimaginable) geometry of space? In an article in Scientific American ("Is Space Finite?" April 1999), the team of Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks proposes to use data from the Sloan Digital Sky Survey to study whether there are patterns in galaxy-to-galaxy distances. They theorize that peaks in such patterns could represent the true size of a finite universe that is tricking us with multiple images. In commenting on the article, astrophysicist and SDSS collaborator Rich Kron, of Fermilab and the University of Chicago, indicated he is maintaining some healthy skepticism on the question. "In some cosmologies, or models of the universe," Kron said, "light rays can follow multiple paths to the observer–so that you see more than one image of the same galaxy, for example. This phenomenon is already seen in the case of ‘gravitational lenses,’ associated with mass concentrations bending the light from distant galaxies and quasars. "The cosmology application," he continued, "has the whole universe acting as a gravitational lens. The notion that we are seeing several versions of the same thing when we look at the distant universe is, I think, pretty speculative. The conventional wisdom is that the geometry is not that bizarre. Still, it is a question worth asking." The SDSS will create a three-dimensional map of one-quarter of the entire sky, determining the positions and absolute brightnesses of more than 100 million celestial objects. It will also measure the distances to more than a million galaxies and quasars. It has already observed three of the four most distant quasars ever detected. Kron said that testing the multiple-image theory would require first, a large total sample ("to get good statistics," he explained); and second, large distances measured in the data ("since the effect, if it exists, is likely to manifest itself only over very large distances"). In terms of data and distances, Kron said, "the SDSS is in a class by itself in terms of fulfilling both of these desiderata." But, Kron concluded: "While the SDSS could do this project, I don’t think we should think of this as a featured goal of the SDSS–since it is speculative." Finite double exposure In the same issue of Scientific American, Fermilab theoretical physicist Chris Quigg reviews "The Elegant Universe: Superstrings, Hidden Dimensions and the Quest for the Ultimate Theory," by Brian Greene (W.W. Norton & Company, 1999). Quigg describes string theory as the attempt by "an intrepid band of theoretical physicists and mathematicians" to resolve the fundamental incompatibility between general relativity and quantum mechanics: "It (string theory) holds that the fundamental constituents of the universe are not the elementary particles that we idealize as having no size, like geometric points, but tiny strings. The resonant patterns of vibrations of the strings are the microscopic origin of the masses of what we perceive as particles and the strengths we assign to the fundamental forces. Because strings have a finite, though fantastically tiny, size, there is a limit to how finely we can dissect nature. That limit–set by the size of the strings–comes into play before we encounter the devastating quantum fluctuations that rend space-time. Thus, the conflict between quantum mechanics and general relativity is resolved." –Mike Perricone |
| last modified 4/2/1999 email Fermilab |
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