ISSN:0031-9228    

DOI:10.1063/1.2808398

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2808399

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Ever since the copper oxide superconductors were first discovered seven years ago, any number of experimenters have come across a sample or two that have exhibited traits suggesting superconductivity at temperatures over 200 K. Typically the behavior was fleeting or elusive, disappearing after the material was taken through several thermal cycles or stubbornly refusing to show up in other samples produced in the same way. In most cases the sample consisted of several phases, and the experimenters could not link the high‐Tcbehavior to a particular phase. Robert Dynes of the University of California, San Diego, refers to these teasers as “leprechauns.” Paul Chu of the University of Houston has described them as “unidentified superconducting objects.”

ISSN:0031-9228    

DOI:10.1063/1.2808400

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This issue ofPHYSICS TODAYis devoted to the interface between physics and biology, commonly termed biological physics or biophysics. Physicists tend to consider biological physics as physics inspired by biology and biophysics as biology revealed by physical methods—or to put it colloquially, what biology can do for physics and what physics can do for biology. Biologists take a broader view. Cells and organisms must know some physics as well as biology because they have evolved in the face of daunting physical constraints. So biophysics includes the physics mastered by living things. Some of this physics is understood by physicists, and some is not. In the former case, one is awed by how much physics organisms know. In the latter case, one is intrigued by how much physics they might yet reveal. The topics addressed in this issue ofPHYSICS TODAYdeal with these two domains.

ISSN:0031-9228    

DOI:10.1063/1.881409

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Although we are generally unaware of the fact, our nervous systems constantly monitor a variety of mechanical stimuli. Neurons in the spinal cord and brain stem extend sensory terminals to the body's surface and there provide us with sensitivity to touch. Other such neurons measure the tension in and extension of skeletal muscles. Sensory cells of the autonomic nervous system detect pressures within the body's hollow organs, including blood vessels, the bladder, and the gut. The most sensitive of our mechanical receptors are hair cells, the sensory receptors of the internal ear. Such cells underlie our sensitivities to sound, to linear accelerations (including that due to gravity), and to angular accelerations.

ISSN:0031-9228    

DOI:10.1063/1.881410

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Many aquatic animals have the ability to sense very weak electric fields. This electric sense is found in numerous species of marine and freshwater fish and in several amphibian species. Electrosensory abilities have also been reported in “higher” animals including the platypus and a semiaquatic mole.

ISSN:0031-9228    

DOI:10.1063/1.881411

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The question “How does it work?” is the motivation of many physicists. Condensed matter physics, chemical physics and nuclear physics can all be thought of as descriptions of the relation between structure andproperties. The components of a biological system havefunctionalproperties that are particularly relevant to the operation of the system. Thus it is especially important in biology to understand the relation between structure andfunction. Such understanding can be sought at the level of the molecule, the cell, the organ, the organism or the social group.

ISSN:0031-9228    

DOI:10.1063/1.881412

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Photosynthesis, the process by which plants convert solar energy into chemical energy, results in about 10 billion tons of carbon entering the biosphere annually as carbohydrate—equivalent to about eight times mankind's energy consumption in 1990. The apparatus used by plants to perform this conversion is both complex and highly efficient. Two initial steps of photosynthesis—energy transfer and electron transfer—are essential to its efficiency: Molecules of the light‐harvesting system transfer electronic excitation energy to special chlorophyll molecules, whose role is to initiate the directional transfer of electrons across a biological membrane; the electron transfer, which takes place in a pigment‐protein complex called the reaction center, then creates a potential difference that drives the subsequent biochemical reactions that store the energy. (Higher plants use two different reaction centers, called photosystems I and II, while purple bacteria make do with a single reaction center. The difference is that the bacteria do not generate oxygen in the photosynthetic process.) Both the elementary energy transfer and the primary electron transfer are ultrafast (occurring between10−13and10−12seconds), leading to the trapping of excitation energy at the reaction center (on a 100‐picosecond timescale) and subsequent electron transfer in about 3 picoseconds with almost 100&percent; quantum yield.

ISSN:0031-9228    

DOI:10.1063/1.881413

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Are we moving toward a time when no new and exciting problems appear in physics? Would the vaunted “theory of everything” mean the end of creative physics? A similar scenario was played out at the end of the last century, when some great physicists declared that only minor problems remained to be solved.

ISSN:0031-9228    

DOI:10.1063/1.881414

出版商:AIP    

年代:1994

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.881415

出版商:AIP    

年代:1994

数据来源: AIP