ISSN:0031-9228    

DOI:10.1063/1.2808551

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808552

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808554

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808555

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808558

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

For two years now, three groups of astronomers and refugees from particle physics have been measuring millions of stars night after night in a stellar photometry campaign of unprecedented scale. But in fact they're not really looking for stars at all. What they seek are dark “massive compact halo objects” at least as massive as planets but generally less than a tenth the mass of the Sun. These substellarMACHOs, as they are called, would not be massive enough to ignite the fusion that makes stars shine. Heavier bodies, like neutron stars and old white dwarfs, that are dark because their stellar fuel has run out are also included under theMACHOrubric.

ISSN:0031-9228    

DOI:10.1063/1.2808559

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

In conventional semiconductor lasers, the photons come from the recombination of electrons and holes. But in a new semiconductor laser recently unveiled by a team at AT&T Bell Labs, the photons are emitted as electrons cascade from one energy level down to another in a staircase‐like arrangement of quantum wells. The new laser is thus unipolar, relying only on electrons rather than on both positive and negative charges. While the physics of this “quantum cascade laser” is very exciting, its commercial potential is not. The threshold current is still very high and might limit its practicality. However, the laser could be used as a research tool and should certainly stimulate still more innovations in design.

ISSN:0031-9228    

DOI:10.1063/1.2808560

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

In December the council of the European Southern Observatory approved a revised construction plan for ESO's Very Large Telescope. (See the article “The New Ground‐Based Optical Telescopes,” by Buddy Martin, John M. Hill and Roger Angel inPHYSICS TODAY, March 1991, page 27.) Although the four 8.2‐meter telescopes will be constructed as scheduled, the VLT Interferometer, its subarray, the Coude´ feed and the associated adaptive optics were postponed to reduce the expenditure in the next few years.

ISSN:0031-9228    

DOI:10.1063/1.2808562

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The success of the ideas of chaos has led to attempts to apply them to a great variety of situations. This is in principle a good strategy, but the results are not always up to expectations. In some cases the results are predictably of little interest. Suppose you have concocted a mathematical model in biology or economics; you put this model on your computer and you discover a Feigenbaum period‐doubling cascade, which is often a sign that chaos is present. Is this result interesting? Well, probably not. One reason is that the detailed dynamical properties of your model may not have anything to do with the properties of the real‐life system. Another reason why your discovery may be without interest is that the occurrence of a Feigenbaum cascade need not have any particular biological or economic significance: You still have to address the problem of the relevance of your finding for biology or economics.

ISSN:0031-9228    

DOI:10.1063/1.881395

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

High‐energy accelerators have been physicists’ main tools for exploring the building blocks of matter for more than 60 years. During this time the particle energy has increased exponentially as a result of a combination of improvements in existing machines and the invention of new acceleration techniques. Historically, whenever a given type of accelerator has reached the limit of its performance, an innovative idea for particle manipulation, storage, cooling or acceleration has made possible experiments at ever higher energies. The tremendous increase in the energy of accelerators has not, however, been without an increase in capital costs. The cancellation of the Superconducting Super Collider makes timely an examination of possible alternative concepts for investigating some of the same physics.

ISSN:0031-9228    

DOI:10.1063/1.881396

出版商:AIP    

年代:1994

数据来源: AIP