ISSN:0031-9228    

DOI:10.1063/1.2810510

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810511

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In how many ways may a quadrilateral be dissected into agivennumber of smaller quadrilaterals? Or a hexagon into triangles? Or, for that matter, any polygon into given numbers of other polygons? Such questions about enumeration of dissections of a planar surface attracted the attention of mathematicians—most notably, of Leonhard Euler—in the middle of the 18th century. Mathematicians solved many cases of such problems using methods of graph theory in the 1960s. In 1978, however, physicists, in their attempts to enumerate Feynman diagrams in certain theories, independently developed a rather elegant and general method that can be used to enumerate dissections of any surface. Physical insights into the divergences in the solutions obtained by those general methods have now led to exact solutions—the first nontrivial exact solutions to date—of simple models of quantum gravity in two dimensions and of simple string theories. String theorists hope that the solutions will provide much‐needed insights into the structure of those theories, just as solving the Schro¨dinger equation for simple, one‐dimensional potentials yields insights into quantum dynamics: For example, the solutions for one‐dimensional potentials show features, such as the existence of the zero‐point energy and the possibility of tunneling, that distinguish quantum mechanical behavior from the classical one. In the absence of any exact solutions, attempts to understand the content of string theories had been limited to perturbation expansions in the string coupling constant. The new, exact solutions show that string theories have features, not fully understood yet, that no perturbation expansion in the coupling constant could unravel.

ISSN:0031-9228    

DOI:10.1063/1.2810512

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Holograms generally record interference patterns produced by visible light. But one can, in principle, make use of any coherent wave phenomenon. One can, for example, create a hologram by recording on film the interference pattern of an object formed with electron beams. One can then reconstruct the three‐dimensional optical image of the object by illuminating this hologram with a laser beam. This imaging technique, which is called electron holography, transforms an electron wavefront into an optical wavefront. In this way, one can exploit versatile optical techniques to do interesting and useful things that are simply not feasible with pure electron microscopes.

ISSN:0031-9228    

DOI:10.1063/1.881230

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

No other astronomy project has taken so long to develop, proven so technologically challenging or cost so much as the Hubble Space Telescope. At a development cost of about $1.5 billion, the HST is big science by the standards of modern physics. Soon after the HST is launched on 12 April, the physics community will determine if the resources and professional efforts that have gone into the HST will produce the promised scientific returns that drew hundreds of the best scientists from the US and Europe into the project.

ISSN:0031-9228    

DOI:10.1063/1.881231

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810513

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810514

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810515

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2810516

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In the first three decades of this century the modern science of cosmology was forged from general relativity theory and new observing methods and instruments, particularly large optical telescopes perched thousands of feet above sea level in California and Arizona. These radical changes in the theoretical and observational tools used by astronomers, physicists and mathematicians accompanied revolutionary changes in cosmology itself. The new cosmology of the early 1930s included two key cognitive features absent from the cosmology of the turn of the century: first, the existence of galaxies outside our own stellar system that are visible in Earth‐based telescopes, and second, that these galaxies evince the expansion of the universe.

ISSN:0031-9228    

DOI:10.1063/1.881232

出版商:AIP    

年代:1990

数据来源: AIP