ISSN:0031-9228    

DOI:10.1063/1.2805829

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882239

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882240

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Since the discovery of the integer quantum Hall effect in 1980, many physicists have been intrigued by this special state, in which electrons confined in a two‐dimensional world execute tiny orbits about magnetic flux lines. Much as we may want to visualize its microscopic structure, however, most of what we have learned to date about the quantum Hall state is based on measurements of bulk properties such as the Hall conductance. Microscopic measurements of more local properties have eluded us because the electrons that form the quantum Hall state reside at an interface between semiconducting layers that are buried tens of nanometers below the surface. And the quantum Hall behavior shows up only when the sample is cooled to temperatures below a few kelvin and placed in a high magnetic field, on the order of several teslas.

ISSN:0031-9228    

DOI:10.1063/1.882254

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

To researchers who use Earth's normal modes of oscillation to map the threedimensional structure of the planet's interior, the good fortune of having the largest‐ever deep‐focus earthquake occur just after the deployment of global broadband seismographic networks seemed like a dream come true. It was certainly no dream, though, and computers have been crunching data ever since. Barbara Romanowicz, Xiang‐Dong Li and Joseph Durek (University of California, Berkeley) used normal modes excited by recent large deep‐focus quakes in Bolivia and elsewhere to characterize the anisotropy of Earth's inner core. Michael Ritzwoller and Joseph Resovsky (University of Colorado at Boulder) and Jeroen Tromp and Xiong He (Harvard University) used data from several recent deep‐focus earthquakes to show that normal‐mode studies can yield improvements in three‐dimensional models of Earth, especially for the planet's mantle

ISSN:0031-9228    

DOI:10.1063/1.2805830

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Although deep‐focus earthquakes occur deep in the mantle, where high temperatures and pressures should make impossible brittle failure (the machanism of shallow quakes), seismically, deep quakes look very much like their shallow counterparts. For this reason, researchers have sought mechanisms that resemble shallow faulting, but that involve processes occurring in regions prone to deep earthquakes—that is, in slabs of oceanic plates descending into the mantle. The figure below depicts a cross section of such a slab (dark green and dark blue) as it descends into the mantle (light green and light blue) as it descends into the mantle (light green and ligh blue). According to one theory, the dehydration of magnisium/iron silicated, occurs from 100 to 125 km beneath the surface and liberates water the could weaken and reactivate a preexisting fault plane. This dehydration mechanism probably accounts for earthquaks occuring at depths below 75 km down to 300 km, and possibly down to 670 km.

ISSN:0031-9228    

DOI:10.1063/1.2805831

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Imagine that a pair of coins are tossed in a black box. The box reports only one of the following three results at random: (1) the outcome of the first coin (heads or tails), (2) the outcome of the second coin (heads or tails), or (3) whether the outcomes of the two coins matched or were different. Our task is to construct a joint probability distribution of the four possible outcomes of the coins (HH, TT, HT, TH) based on many observations of the black box outputs. Now suppose that after many trials, the black box reports that each coin comes up heads two‐thirds of the time when measured individually, yet the coinsnevermatch when they are compared. (Clearly the results of the coin tosses have been correlated—perhaps a joker in the black box flips the coins and then changes the outcomes appropriately). We seek a distribution that both reflects this correlation and obeys the marginal distributions of each coin as two‐thirds heads, one‐third tails (see the three tables on page 23). The only way to satisfy both requirements is to force the joint probabilityP(TT)of getting two tails to be negative! Mathematically, this is becauseP(HH)+P(TT)is observed to be zero, yet we expectP(HH)to be greater thanP(TT),because the individual coins are weighted toward heads.

ISSN:0031-9228    

DOI:10.1063/1.882256

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The origin of cosmic rays has been a major mystery in astrophysics for nearly a century. However, any lingering doubt about whether the bulk of the cosmic rays (those with energies below about1015 eV) are Galactic or extragalactic has been removed in the 1990s in favor of a Galactic origin. The question has been settled by gamma‐ray observations made by the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory. The EGRET observations showed that the cosmic‐ray energy density in a nearby galaxy—the Small Magellanic Cloud—is much lower than that found locally in our own Galaxy and is thus inconsistent with a uniform extragalactic density. This discovery, of course, does not preclude an extragalactic origin for the very highest energy cosmic rays, which are observed above about1019 eV.(SeePHYSICS TODAY, January 1998, page 31). The power of about1041 ergs/srequired to maintain the cosmic rays throughout the Galaxy is most likely supplied by supernovae (figure 1). With a Galactic supernova rate of roughly three per century, the required energy per supernova is about1050 ergs,which is about 10&percent; of the kinetic energy of the expanding supernova ejecta. Shock acceleration in the supernova blast wave driven by the ejecta could impart such a proportion of the available kinetic energy to cosmic rays.

ISSN:0031-9228    

DOI:10.1063/1.882257

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Albert Einstein believed in the possibility of a quantum theory without observers—a version of quan tum theory for which the notions of measurement, observation and observer are not invoked in its very formulation, but rather emerge from an analysis of more fundamental concepts. Niels Bohr believed that such a theory was “in principle” impossible. In part one of this article, I described one approach to such a theory, that of decoherent histories (DH). Although much progress has been made, it could be argued that this approach has not yet yielded a theory that is sufficiently well defined to provide decisive support for Einstein's view. The theories I discuss in this final part of the article are completely well defined and hence provide a conclusive refutation of Bohr's impossibility claims.

ISSN:0031-9228    

DOI:10.1063/1.882241

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882242

出版商:AIP    

年代:1998

数据来源: AIP