Physics Today

Published by AIP Publishing

Online ISSN: 1945-0699


Print ISSN: 0031-9228


Mass Extinctions Caused by Large Bolide Impacts
  • Article

August 1987


146 Reads

A wealth of evidence has forced my colleagues and me to conclude that 65 million years ago a mountain‐sized object hit Earth and caused the extinction of most of the then existing species, bringing a close to the Cretaceous period of geological history and opening the Tertiary period. Much of the evidence for this lies in the unusual layer of clay that separates those periods in the geological record, shown in figure 1. For example, this stratum contains anomalously high concentrations of iridium, an element whose abundance in the crust of the Earth is only one ten‐thousandth that in meteorites and, presumably, in other “bolides,” or large pieces of Solar System debris. Evidence indicates that the collision of Earth and a large piece of Solar System debris such as a meteoroid, asteroid or comet caused the great extinctions of 65 million years ago, leading to the transition from the age of the dinosaurs to the age of the mammals.

Handedness, Origin of Life and Evolution

August 1991


62 Reads

At first glance the nucleic acids and proteins that are the basis of life do not stand out in any way among all the possible polymericstructures. If we look at their functions, however, we find one unique feature of these biological polymers: self‐replication, the distinctive property of living systems. What is self‐replication, and how could this biologically primordial property have originated in an unorganized medium? The solution to the problem of life's origin lies in resolving the paradox of how polymers of rather common structure can exhibit such a distinctive function. Biological polymers have a preferred chirality and con replicate themselves. Physical arguments provide insight into which of these unique and apparently related properties evolved first, and by what mechanism.

FIG. 1: The cumulative citation distribution C(k) versus the number of citations k for all papers published from July 1893 through June 2003 in the Physical Review journals. Circles indicate the data. The curve is the log-normal fit C(k) = A e −b ln k−c(ln k) 2 , with A = 0.15, b = 0.40, and c = 0.16.
FIG. 2: The attachment rate A k is a nearly linear function of the number of citations k, especially for k less than 150 (inset). The different colors indicate different year ranges for establishing k: 1990-1999 (blue squares), 1980-1999 (green ▽), 1970-1999 (red △), and 1893-1999 (black •). The rate A k is determined from citations in the year 2000. Data have been averaged over a range of ±2.5%. Other time ranges for existing and new citations give similar behavior.
FIG. 3: Five revived classics of relevance to colossal magnetoresistance have similar citation histories. Table II gives more information about these papers.
FIG. 4: Six classic, highly-cited publications have varied citation histories. The abbreviations are defined in the text.
Citation Statistics from 110 Years of Physical Review
  • Article
  • Full-text available

June 2005


271 Reads

The long term systematic features about citation statistics that are revealed by publicly available data are presented. The age of citation is the difference between the year when a citation occurs and the publication year of the cited paper. The citation data provides a lots of quantitative information. It can be used to identify influential research, new trends in research, unanticipated connections across fields and downturns in subfields that are exhausted. The data also reveals the idiosyncratic features in the citation histories of individual articles.

Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference

September 2006


557 Reads

We reconsider the crucial 1927 Solvay conference in the context of current research in the foundations of quantum theory. Contrary to folklore, the interpretation question was not settled at this conference and no consensus was reached; instead, a range of sharply conflicting views were presented and extensively discussed. Today, there is no longer an established or dominant interpretation of quantum theory, so it is important to re-evaluate the historical sources and keep the interpretation debate open. In this spirit, we provide a complete English translation of the original proceedings (lectures and discussions), and give background essays on the three main interpretations presented: de Broglie's pilot-wave theory, Born and Heisenberg's quantum mechanics, and Schroedinger's wave mechanics. We provide an extensive analysis of the lectures and discussions that took place, in the light of current debates about the meaning of quantum theory. The proceedings contain much unexpected material, including extensive discussions of de Broglie's pilot-wave theory (which de Broglie presented for a many-body system), and a "quantum mechanics" apparently lacking in wave function collapse or fundamental time evolution. We hope that the book will contribute to the ongoing revival of research in quantum foundations, as well as stimulate a reconsideration of the historical development of quantum physics. A more detailed description of the book may be found in the Preface. (Copyright by Cambridge University Press (ISBN: 9780521814218).) Comment: 553 pages, 33 figures. Draft of a book (as of Sept. 2006, same as v1). Published in Oct. 2009, with corrections and an appendix, by Cambridge University Press (available at

Adaptive Optics and Interferometry

May 1991


20 Reads

Adaptive optics and interferometry, two techniques that will improve the limiting resolution of optical and infrared observations by factors of tens or even thousands, are discussed. The real-time adjustment of optical surfaces to compensate for wavefront distortions will improve image quality and increase sensitivity. The phased operation of multiple telescopes separated by large distances will make it possible to achieve very high angular resolution and precise positional measurements. Infrared and optical interferometers that will manipulate light beams and measure interference directly are considered. Angular resolutions of single telescopes will be limited to around 10 milliarcseconds even using the adaptive optics techniques. Interferometry would surpass this limit by a factor of 100 or more. Future telescope arrays with 100-m baselines (resolution of 2.5 milliarcseconds at a 1-micron wavelength) are also discussed.

Nonequilibrium Hypersonic Aerothermodynamics

February 1989


650 Reads

Nonequilibrium phenomena in hypersonic flows are examined on the basis of theoretical models and selected experimental data, in an introduction intended for second-year graduate students of aerospace engineering. Chapters are devoted to the physical nature of gas atoms and molecules, transitions of internal states, the formulation of the master equation of aerothermodynamics, the conservation equations, chemical reactions in CFD, the behavior of air flows in nonequilibrium, experimental aspects of nonequilibrium flow, a review of experimental results, and gas-solid interaction. Diagrams, graphs, and tables of numerical data are provided.

Alternatives to Cosmological Inflation

March 2009


242 Reads

The inflationary universe scenario is the current paradigm of early-universe cosmology. Inflation has been successful phenomenologically: It not only provides answers to several questions that cannot be addressed in the standard Big Bang (SBB) cosmology, but it also led to the first quantitative and predictive theory for the origin of structure in the universe, a theory whose predictions have been verified to great accuracy by cosmic microwave background (CMB) anisotropy experiments. at present, however, models of inflation suffer from serious conceptual problems that have driven attempts to find alternative paradigms.

Earth Flyby Anomalies

October 2009


67 Reads

In a reference frame fixed to the solar system's center of mass, a satellite's energy will change as it is deflected by a planet. But a number of satellites flying by Earth have also experienced energy changes in the Earth-centered frame -- and that's a mystery.

Satellite Observed Changes in the Arctic

July 2004


68 Reads

The Arctic is currently considered an area in transformation. Glaciers have been retreating, permafrost has been diminishing, snow covered areas have been decreasing, and sea ice and ice sheets have been thinning. This paper provides an overview of the unique role that satellite sensors have contributed in the detection of changes in the Arctic and demonstrates that many of the changes are not just local but a pan-Arctic phenomenon. Changes from the upper atmosphere to the surface are discussed and it is apparent that the magnitude of the trends tends to vary from region to region and from season to season. Previous reports of a warming Arctic and a retreating perennial ice cover have also been updated, and results show that changes are ongoing. Feedback effects that can lead to amplification of the signals and the role of satellite data in enhancing global circulation models are also discussed.

The Gum Nebula - A new kind of astronomical object

October 1971


11 Reads

The Gum Nebula is seen on photographs of the southern skies as an extensive region of ionized hydrogen that surrounds the well known Vela X supernova remnant. One of the largest objects in our galaxy, the Nebula was first recognized as a single emission complex by Colin S. Gum during the 1950's. Until recently, astronomers assumed that it was a “Strömgren sphere,” excited by ultraviolet light from the hot stars gamma Velorum and zeta Puppis within it. This excitation process accounts for the Orion Nebula and other H‐II regions. The synchrotron and thermal bremsstrahlung mechanisms account for the radiation from supernova remnants, such as the Crab Nebula and Vela X; these objects consist of rapidly expanding matter ejected by the supernovaexplosions. Did radiation from a supernova explosion ionize this huge mass of hydrogen? Four theories propose ways that the Nebula could have been created by energy from the supernova.

Cosmic rays - Astronomy with energetic particles

November 1974


30 Reads

All nuclei in the periodic table of the elements, as well as electrons and positrons, are present in the stream of cosmic-ray particles. The cosmic-ray particles constitute the only sample of matter from outside the solar system which reaches the earth. Some of the most accurate knowledge of the extrasolar-element abundance distribution is based on the study of these particles. Observational data concerning the cosmic rays are discussed along with cosmic-ray sources, questions of particle interactions and propagation, the electron spectrum, and the significance of the positron component. The directions of cosmic ray research in the immediate future are also considered, giving attention to some fundamental questions which have not yet been answered.

High-energy gamma-ray astronomy

October 1975


13 Reads

The subject of gamma-ray astronomy is discussed with emphasis on celestial gamma rays with energies in excess of 10 MeV. Early observations of such gamma rays are reviewed, a gamma-ray spark-chamber telescope is described together with a gas Cerenkov-counter telescope, and the gamma-ray sky is delineated. It is shown that the diffuse high-energy gamma radiation from the galactic plane probably results primarily from cosmic-ray interactions with interstellar matter. Mechanisms for gamma-ray production are identified, and it is noted that the general galactic radiation may prove to be of great value in studies of galactic structure. Possible sources are considered for the diffuse celestial radiation, and discrete sources are described, including the Crab pulsar, the Vela remnant, the Cygnus region, and Gould's Belt. Future developments in gamma-ray astronomy are considered.

Figure 1: The open economy trilemma  
Figure 2: Phasing of reform
Beyond the Ivory Tower. Interview with Stanley Fischer

January 2004


289 Reads

Stanley Fischer had a long and distinguished career as an academic economist at MIT, and was Vice President, Development Economics and Chief Economist at the World Bank, before becoming First Deputy Managing Director of the International Monetary Fund in 1994. He is now President of Citigroup International and Vice Chairman of Citigroup. In this interview, Brian Snowdon discusses with Stanley Fischer several important issues relating to the contemporary world economy, including problems of stabilisation, inflation and growth, the economics and politics of transition, exchange rate regimes, the IMF, the East Asian crisis, and globalisation and economic development.

From black holes to strange metals

March 2010


169 Reads

Thomas Faulkner


Nabil Iqbal





David Vegh
Since the mid-eighties there has been an accumulation of metallic materials whose thermodynamic and transport properties differ significantly from those predicted by Fermi liquid theory. Examples of these so-called non-Fermi liquids include the strange metal phase of high transition temperature cuprates, and heavy fermion systems near a quantum phase transition. We report on a class of non-Fermi liquids discovered using gauge/gravity duality. The low energy behavior of these non-Fermi liquids is shown to be governed by a nontrivial infrared (IR) fixed point which exhibits nonanalytic scaling behavior only in the temporal direction. Within this class we find examples whose single-particle spectral function and transport behavior resemble those of strange metals. In particular, the contribution from the Fermi surface to the conductivity is inversely proportional to the temperature. In our treatment these properties can be understood as being controlled by the scaling dimension of the fermion operator in the emergent IR fixed point. Comment: 26 pages, 9 figures

Sonic Booms

March 1968


15 Reads

SONIC BOOMS are explosive sounds that occur without warning. Sometimes annoying because of their startle effects and their ability to shake buildings, these sounds pose a unique problem for the orderly development of high‐speed air transport. Although booms from military aircraft are widely observed around the world, the real concern is for proposed commercial airtransport operations that will cause repeated booms over very large areas. What are the effects of sonic booms and what steps can be taken to minimize the exposure to a suitable level? How are sonic booms created? What do they do to people and buildings? Will the future be dominated by the boom?

Bose-Einstein Condensation

July 2009


141 Reads

Bose-Einstein condensation (BEC) is a phenomenon that occurs in a macroscopic system of bosons (particles obeying ► Bose-Einstein statistics) at low temperatures: a nonzero fraction of all the particles in the system (thus a macroscopic number of particles) occupy a singleM one-particle state. This would, of course, happen for a system of distinguishable, noninteracting particles at zero temperature, but in this case the phenomenon disappears as soon as the temperature becomes comparable to the energy splitting between the single-particle groundstate and the first excited state — a quantity which tends to zero with the size of the system. By contrast, in BEC the macroscopic occupation occurs at all temperatures below a transition temperature, usually denoted T c , which while a function of intensive parameters such as density and interaction strength is constant in the thermodynamic limit. The fundamental reason for the occurrence of BEC lies in the requirement, which follows from considerations of quantum field theory, that the ► wave function of a system of identical bosons should be symmetric under the exchange of any two particles. This has the consequence that states that differ only by such an exchange must be counted as identical, i.e. counted only once. Thus, for example, while for a system of N distinguishable objects, which must be partitioned between two boxes, the number of ways of putting M of them into one box is given by the familiar binomial formula N!/(M!N — M!), for bosons there is exactly one way for each M. The effect is to remove the “entropic” factor, which for distinguishable objects militates against putting a large fraction of them in a single one-particle state.

Climate and the Earth's Radiation Budget

June 1989


42 Reads

Data from NASA's Earth Radiation Budget (ERB) Experiment instruments carried by the ERB Satellite and by NOAA-9 and -10 are presently evaluated, with a view to the role played by clouds in the global radiation energy balance. While an individual water droplet scatters 85 percent of incident energy in the forward direction, a cloud of such drops can scatter 75 percent or more of the energy backward. The resulting enhancement of surface-atmosphere albedo reduces the solar radiation absorbed by the atmospheric column. Clouds also significantly enhance the long-wave opacity of the atmosphere; like gaseous absorption, this reduces the radiation emitted to space.

Quantum Calorimetry

September 1999


72 Reads

Your opponent's serve was almost perfect, but you vigorously returned it beyond his outstretched racquet to win the point. Now the tennis ball sits wedged in the chain-link fence around the court. What happened to the ball's kinetic energy? It has gone to heat the fence, of course, and you realize that if the fence were quite colder, you might be able to measure that heat and determine just how energetic your swing really was. Calorimetry has been a standard measurement technique since James Joule and Julius von Mayer independently concluded, about 150 years ago, that heat is a form of energy. But only in the past 15 years or so has calorimetry been applied, at millikelvin temperatures, to the measurement of the energy of individual photons and particles with exquisite sensitivity. In this article, we have tried to show that continuing research in low-temperature physics leads to a greater understanding of high-temperature astrophysics. Adaptations of the resulting spectrometers will be useful tool for fields of research beyond astrophysics.

Citation Statistics From More Than a Century of Physical Review

August 2004


673 Reads

We study the statistics of citations from all Physical Review journals for the 110-year period 1893 until 2003. In addition to characterizing the citation distribution and identifying publications with the highest citation impact, we investigate how citations evolve with time. There is a positive correlation between the number of citations to a paper and the average age of citations. Citations from a publication have an exponentially decaying age distribution; that is, old papers tend to not get cited. In contrast, the citations to a publication are consistent with a power-law age distribution, with an exponent close to -1 over a time range of 2 -- 20 years. We also identify a number of strongly-correlated citation bursts and other dramatic features in the time history of citations to individual publications.

Top-cited authors