ISSN:0031-9228    

DOI:10.1063/1.2405441

出版商:AIP    

年代:1900

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882926

出版商:AIP    

年代:1900

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882927

出版商:AIP    

年代:1900

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882928

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We often speak of the Hubble Space Telescope as a unique window on the most distant and ancient galaxies. But the HST can also tell us things we never knew about our most intimate interstellar surroundings. The 10 JanuaryAstrophysical Journalbrings us the first of two articles, by Jeffrey Linsky, Seth Redfield, and colleagues at the University of Colorado, that offer a three‐dimensional map of the “local interstellar cloud,” derived mostly from ultraviolet absorption spectra recorded by instruments aboard the HST. The LIC, they report, is a rather uniform, egg‐shaped cloud of warm atomic hydrogen, only 20 light‐years long, in which the Solar System and its surrounding heliosphere of solar wind and magnetic field sit like a tiny, offcenter yolk. (See the figure below.)

ISSN:0031-9228    

DOI:10.1063/1.882930

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Ongoing monitoring of Earth's north polar region has been turning up numerous signs of change in its atmosphere, waters, and ice pack (see PHYSICSTODAY, November 1998, page 17). Such studies have revealed that the Arctic ice mass is shrinking, but now, it seems, the rates of decline are much more rapid than previously thought. Such changes are of concern because the polar region is expected to amplify any change in Earth's climate.

ISSN:0031-9228    

DOI:10.1063/1.882931

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Quantum computers have been predicted to be exponentially faster than their classical counterparts for some computations, such as the factoring of large numbers. (See the article by Charles Bennett in PHYSICSTODAY, October 1995, page 24.) Whereas classical computers perform operations on information stored as classical bits, which can be in one of two discrete states, quantum computers perform operations on quantum bits, or “qubits,” which can be put into any superposition of two quantum states.

ISSN:0031-9228    

DOI:10.1063/1.882932

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The Relativistic Heavy Ion Collider (RHIC) recently completed at Brookhaven National Laboratory is expected to produce a plasma of quarks and gluons—the state in which matter is thought to have existed in the first microseconds of the Big Bang, before matter coalesced into the neutrons and protons that characterize our universe today (see PHYSICSTODAY, October 1999, page 20). Before entering any such new territory, explorers naturally speculate on what unknowns might be encountered. In this case, the speculations have included a few scenarios—recognized to be extremely remote even by those physicists who have raised them—that could possibly spell danger. Concerns over those scenarios were voiced last summer in the general press. To examine their likelihood, Brookhaven director John Marburger appointed a panel of particle physicists consisting of Wit Busza (MIT), Robert Jaffe (MIT), Jack Sandweiss (Yale University), and Frank Wilczek (Institute for Advanced Study). Their report concludes that “the candidate mechanisms for catastrophe scenarios at RHIC are firmly excluded by existing empirical evidence, compelling theoretical arguments, or both.” (www.bnl.gov/bnlweb/ rhicreport.html)

ISSN:0031-9228    

DOI:10.1063/1.882933

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Escherichia coliis a single‐celled organism that lives in your gut. It is equipped with a set of rotary motors only 45 nm in diameter. Each motor drives a long, thin, helical filament that extends several cell body lengths out into the external medium. The assemblage of motor and filament is called a flagellum. The concerted motion of several flagella enables a cell to swim. A cell can move toward regions that it deems more favorable by measuring changes in the concentrations of certain chemicals in its environment (mostly nutrients), deciding whether life is getting better or worse, and then modulating the direction of rotation of its flagella. Thus, in addition to rotary engines and propellers,E. coli's standard accessories include particle counters, rate meters, and gear boxes. This microorganism is a nanotechnologist's dream. I will discuss the features that make it so, from the perspectives of several scientific disciplines: anatomy, genetics, chemistry, and physics.

ISSN:0031-9228    

DOI:10.1063/1.882934

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Vision is a most acute human sense, so it is rather surprising that the very first step in the visual process—the formation of an image on the retina—is often defective. One reason is that the human eye has significant optical defects, aberrations, that distort the passing optical wavefront, blur the retinal image, and degrade our visual experience. Diffraction, which is caused by the finite size of the eye's pupil, is the other reason for blurri‐ness. Together, aberrations and diffraction limit not only what the eye sees looking out, but also determine the smallest internal structures that can be observed when looking into the eye with a microscope (see box 1 on page 33). Spectacles and contact lenses can correct the eye's major aberrations, but if all the aberrations could be quantified and corrected while, at the same time, minimizing diffraction, high‐resolution retinal microscopy could become routinely feasible—and we might eventually achieve supernormal vision.

ISSN:0031-9228    

DOI:10.1063/1.882935

出版商:AIP    

年代:1900

数据来源: AIP