ISSN:0031-9228    

DOI:10.1063/1.2802909

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882200

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2802911

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882204

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

When a beam of 50‐GeV electrons collides head on with a terawatt laser pulse focused down to a few microns, extraordinary things can happen: Almost every electron plowing through the very dense laser field at the focus kicks a low‐energy photon up to multi‐GeV gamma‐ray energy, and lots of these “Compton‐backscattered” gammas then create electron‐positron pairs when they subsequently collide with laser photons coming toward them.

ISSN:0031-9228    

DOI:10.1063/1.882208

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

If you've heard the reports that teleportation has been achieved, and you're anxious about the implications for investments in the transportation sector, you can relax and instead look to physics futures for the payoff. Quantum teleportation as it currently exists involves the delicate dismantlement of an individual photon's quantum state and its reconstruction about a meter away. Although that may sound less exciting than the transport of starship captains from orbit to planet surface it should lead to new tests of the non‐existence of what Albert Einstein, Boris Podolsky and Nathan Rosen called local “elements of reality.”

ISSN:0031-9228    

DOI:10.1063/1.882136

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The most accurate primary frequency standards in the world today reside in the national standards laboratories of several countries: They are an atomic‐fountain clock at the Paris Observatory's Bureau National de Me´trologie—Laboratoire Primaire de Temps et Fre´quences and two atomic‐beam clocks, one run by the US National Institute of Standards and Technology (NIST) in Boulder, Colorado, and another by the German Federal Institute of Physics and Metrology in Braunschweig.

ISSN:0031-9228    

DOI:10.1063/1.882137

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Amajor role of the national standards labs is to keep the world's clocks running on the proper time. That means making a continual comparison of the primary clocks that are used as the standards. But some of the accuracy achieved in a given lab's measurement of time is lost as that information is communicated to another location on Earth's surface. As good as it is, the Global Positioning System (GPS) system (which at its best enables comparison of clocks with accuracies of10−14), is becoming a weak link in this relay of information.

ISSN:0031-9228    

DOI:10.1063/1.2802917

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Perhaps because we are constantly bombarded by sound, it is easy to forget that sound waves actually represent quite small pressure variations. The sound of a jet engine a few meters away measures only about 20 Pa (about 0.0002 atmospheres). As one increases the energy going into a sound wave, nonlinear processes in the gas in which the wave propagates direct more and more energy into harmonics of the drive frequency. The harmonics distort the sound wave and ultimately form shock waves. It is these shocks that limit the amplitudes attainable. Soundwaves’ low energy levels and compression ratios (defined as the ratio of the waveform's peak and minimum pressures) have limited their usefulness in high‐power applications such as compressors and pumps. Accordingly, many researchers have wondered whether the acoustic saturation imposed by shock formation can be circumvented in some special circumstances.

ISSN:0031-9228    

DOI:10.1063/1.882138

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Our understanding of the gamma‐ray sky is being revolutionized. Seven years ago, gamma‐ray astronomers knew of only a scattering of very bright sources. Now, thanks to two international observatories, the gamma‐ray sky appears to be teeming with variety—unstable sources that change violently on short time scales, steady sources that glow radioactively and others whose nature we barely understand.

ISSN:0031-9228    

DOI:10.1063/1.882139

出版商:AIP    

年代:1998

数据来源: AIP