ISSN:0031-9228    

DOI:10.1063/1.2808008

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808009

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

What a difference a year can W make—especially when almost a thousand physicists spend that year in single‐minded pursuit of a goal. The goal in this case was the discovery of the top quark, which one year ago sat at the threshold of statistical respectability. (SeePHYSICS TODAY, June 1994, page 17.) Since then, the Collider Detector Facility group and the DO group at Fermilab's Tevatron have steadily improved their statistics and their analyses until the question of whether they have in fact seen the top quark is no longer a subject of controversy. CDF sees 56 top candidates over a predicted background of 23.1, for a statistical significance of 4.8 standard deviations. DO sees 17 events over a predicted background of 3.8, for a statistical significance of 4.6 standard deviations. Perhaps equally significant, all subsequent analyses of the kinematic, production and decay properties of the top samples are consistent between the two experiments and support the hypothesis that the excess events over background are indeed due to top production. The focus of debate now seems to have passed from whether the top has been discovered to how significant the discovery will be for particle physics. On the subject of significance, there seem to be two main camps.

ISSN:0031-9228    

DOI:10.1063/1.2808010

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Just as the study of seismic waves lets us look into the bowels of the Earth, helioseismology has for more than 20 years been a rich source of information about the interior of the Sun. Apparently driven by internal convective motion, the Sun rings like a great spherical bell. Many thousands of resonant pressure‐wave modes with periods on the order of 5 minutes have been painstakingly decoded from complex surface motions of dauntingly small amplitude. From the pattern of resonant frequencies one learns much about the composition, density and structure of the Sun. We now know, for example, that the Sun's convection zone is twice as deep as solar modelers believed before these oscillation modes were measured.

ISSN:0031-9228    

DOI:10.1063/1.2808011

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Clouds have always bedeviled those trying to model Earth's climate, because they are such complex systems, involving parameters whose size and time scales range over many orders of magnitudes. (See the article by Jeffrey Kiehl inPHYSICS TODAY, November 1994, page 36.) While struggling to represent clouds realistically in their models, atmospheric researchers have at least felt that they understood the basic physics of clouds. But now they are not so sure.

ISSN:0031-9228    

DOI:10.1063/1.2808012

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The illumination of the ordinary—of why the sky is blue or why the stars shine—is not the least important role of physics and physicists. Then can't we add to the list of deeper queries some of the questions that seemed so important to me in my youth: How can Babe Ruth hit so many home runs? What makes Carl Hubble's curveball and screwball swerve in their trips to the plate? And if baseball plays no known role in the fundamental structure of the universe (seeThe Iowa Baseball Confederacyby W. P. Kinsella for a contrary position), it is not of trivial importance in the perception and appreciation of that universe by some of its inhabitants. Although not quite so important now, in the period between the Civil War and World War II baseball was a significant part of what defined the United States. Forty years ago, Jacques Barzun, a preeminent student of American culture and a native of France, said, “Whoever wants to know… America had better learn baseball.” But, even as the game itself is subtle and complex, I have found subtleties and complexities in my attempts to know the physical bases of this American game.

ISSN:0031-9228    

DOI:10.1063/1.881460

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Scientists in many fields are recognizing that the systems they study often exhibit a type of time evolution known as chaos. Its hallmark is wild, unpredictable behavior, a state often perplexing and unwelcome to those who encounter it. Indeed this highly structured and deterministic phenomenon was in the past frequently mistaken for noise and viewed as something to be avoided in most applications. Recently researchers have realized that chaos can actually be advantageous in many situations and that when it is unavoidably present, it can often be controlled to obtain desired results. In this article, we present some of the basic ideas behind the feedback control of chaos, review a few illustrative experimental results and assess the status and future promise of the field.

ISSN:0031-9228    

DOI:10.1063/1.881461

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The unaided eye has an angular resolution of about 1 arcminute. From the invention of the telescope in the 17th century to the middle of the 1970s, astronomers improved on this resolution by two orders of magnitude by building bigger telescopes and putting them at good sites. Even at good sites, however, atmospheric turbulence limits the resolution at visible and infrared wavelengths to 1 arcsec or a little better. In the past 20 years, a further factor‐of‐ten improvement has come with two developments that deal with the atmosphere: “speckle interferometry,” in which the blurred image is frozen in a short exposure and the image is reconstructed from many exposures, and adaptive optics, in which the effects of the atmosphere are sensed, then corrected with a defbrmable mirror, before the image is recorded. (See Laird A. Thompson's article inPHYSICS TODAY, December 1994, page 24.)

ISSN:0031-9228    

DOI:10.1063/1.881462

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808013

出版商:AIP    

年代:1995

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808016

出版商:AIP    

年代:1995

数据来源: AIP