The handbook provides a comprehensive description of phosphors with an emphasis on practical phosphors and their uses in various kinds of technological applications. It covers the fundamentals, namely the basic principles of luminescence, the principle phosphor materials, and their optical properties. The authors describe phosphors used in lamps, cathode-ray tubes, x-ray, and ionizing radiation detection. They cover common measurement methodology used to characterize phosphor properties, discuss a number of related items, and conclude with the history of phosphor technology and industry.
The book discusses research and development currently under way on phosphors with potential for practical usage and touches briefly on phosphors that have played a historical role, but are no longer of practical use. It provides a comprehensive treatment of applications including lamps and cathode-ray tubes, x-ray and ionizing radiation, and for vacuum fluorescent and field emission displays and covers inorganic and organic electroluminescence materials. The book also covers phosphors for plasma displays, organic fluorescent pigments, and phosphors used in a variety of other practical applications. Emphasizing the practical and cutting-edge nature of the material included, the editors round out their coverage with a discussion of solid-state and organic laser materials.
The book begins with an introduction to the principles, semiconductor/phosphor materials, and characterizations of solid-state lighting and white light-emitting diodes (LEDs). It then discusses the optical and luminescence processes occurring in optically active centers of solid materials and presents the photoluminescence properties of traditional phosphors for white LEDs, including garnets, aluminates, silicates, sulfides, oxysulfides, phosphates, and scheelites.
The remainder of the text focuses on newly developed nitride phosphors. The authors describe the crystal chemistry of general nitride compounds, the crystal structure and photoluminescence properties of new nitride phosphors, and synthetic methods for preparing nitride phosphors. They detail the structural analysis of nitride phosphors and present experimental and computational results of typical nitride phosphors. The authors also examine key issues, such as excitation and emission spectra, thermal quenching, and quantum efficiency. The final chapter explores applications of nitride phosphors in white LEDs for general lighting and LCD backlight purposes.
Covering novel luminescent materials, this book brings you up to date on the evolving field of solid-state lighting. It illustrates the fundamentals, synthesis, properties, and applications of the latest nitride phosphor materials.
In concise, high-def videos, various skills and techniques are demonstrated and explained. These cover topics for the novice, such as mounting and cleaning of optics, as well as for the more advanced learner, such as balanced detection, and lock-in amplifiers.
Various interactive widgets let you simulate the experience of aligning a laser beam to an optical system, aligning an interferometer to get fringes, or adjust a Fabry-Perot cavity while observing the mode spectrum. Other tools help you quickly find the Gaussian beam parameters of your laser from measured beam radii, and to calculate the position of a lens or pair of lenses to mode match a laser to a cavity.
The Future of the Mind brings a topic that once belonged solely to the province of science fiction into a startling new reality. This scientific tour de force unveils the astonishing research being done in top laboratories around the world—all based on the latest advancements in neuroscience and physics—including recent experiments in telepathy, mind control, avatars, telekinesis, and recording memories and dreams. The Future of the Mind is an extraordinary, mind-boggling exploration of the frontiers of neuroscience. Dr. Kaku looks toward the day when we may achieve the ability to upload the human brain to a computer, neuron for neuron; project thoughts and emotions around the world on a brain-net; take a “smart pill” to enhance cognition; send our consciousness across the universe; and push the very limits of immortality.
From the Trade Paperback edition.
The book begins at the simplest level, develops the basics, and reinforces fundamentals, ensuring a solid foundation in the principles and methods of physics. It provides an ideal introduction for college-level students of physics, chemistry, and engineering; for motivated AP Physics students; and for general readers interested in advances in the sciences.
Kaku skillfully guides us through the latest innovations in string theory and its latest iteration, M-theory, which posits that our universe may be just one in an endless multiverse, a singular bubble floating in a sea of infinite bubble universes. If M-theory is proven correct, we may perhaps finally find answer to the question, “What happened before the big bang?” This is an exciting and unforgettable introduction into the new cutting-edge theories of physics and cosmology from one of the pre-eminent voices in the field.
From the Trade Paperback edition.
Richard Feynman, winner of the Nobel Prize in physics, thrived on outrageous adventures. Here he recounts in his inimitable voice his experience trading ideas on atomic physics with Einstein and Bohr and ideas on gambling with Nick the Greek; cracking the uncrackable safes guarding the most deeply held nuclear secrets; accompanying a ballet on his bongo drums; painting a naked female toreador. In short, here is Feynman's life in all its eccentric—a combustible mixture of high intelligence, unlimited curiosity, and raging chutzpah.
"Now a new laboratory bible for optics researchers has joined the list: it is Phil Hobbs's Building Electro-Optical Systems: Making It All Work."
—Tony Siegman, Optics & Photonics News
Building a modern electro-optical instrument may be the most interdisciplinary job in all of engineering. Be it a DVD player or a laboratory one-off, it involves physics, electrical engineering, optical engineering, and computer science interacting in complex ways. This book will help all kinds of technical people sort through the complexity and build electro-optical systems that just work, with maximum insight and minimum trial and error.
Written in an engaging and conversational style, this Second Edition has been updated and expanded over the previous edition to reflect technical advances and a great many conversations with working designers. Key features of this new edition include:Expanded coverage of detectors, lasers, photon budgets, signal processing scheme planning, and front ends Coverage of everything from basic theory and measurement principles to design debugging and integration of optical and electronic systems Supplementary material is available on an ftp site, including an additional chapter on thermal Control and Chapter problems highly relevant to real-world design Extensive coverage of high performance optical detection and laser noise cancellation
Each chapter is full of useful lore from the author's years of experience building advanced instruments. For more background, an appendix lists 100 good books in all relevant areas, introductory as well as advanced. Building Electro-Optical Systems: Making It All Work, Second Edition is essential reading for researchers, students, and professionals who have systems to build.
"Vivid . . . impressive. . . . Splendidly informative."—The New York Times
"A tour de force."—Salon
Already internationally acclaimed for his elegant, lucid writing on the most challenging notions in modern physics, Sean Carroll is emerging as one of the greatest humanist thinkers of his generation as he brings his extraordinary intellect to bear not only on Higgs bosons and extra dimensions but now also on our deepest personal questions: Where are we? Who are we? Are our emotions, our beliefs, and our hopes and dreams ultimately meaningless out there in the void? Do human purpose and meaning fit into a scientific worldview?
In short chapters filled with intriguing historical anecdotes, personal asides, and rigorous exposition, readers learn the difference between how the world works at the quantum level, the cosmic level, and the human level—and then how each connects to the other. Carroll's presentation of the principles that have guided the scientific revolution from Darwin and Einstein to the origins of life, consciousness, and the universe is dazzlingly unique.
Carroll shows how an avalanche of discoveries in the past few hundred years has changed our world and what really matters to us. Our lives are dwarfed like never before by the immensity of space and time, but they are redeemed by our capacity to comprehend it and give it meaning.
The Big Picture is an unprecedented scientific worldview, a tour de force that will sit on shelves alongside the works of Stephen Hawking, Carl Sagan, Daniel Dennett, and E. O. Wilson for years to come.
For more than thirty years as a beloved professor at the Massachusetts Institute of Technology, Lewin honed his singular craft of making physics not only accessible but truly fun, whether putting his head in the path of a wrecking ball, supercharging himself with three hundred thousand volts of electricity, or demonstrating why the sky is blue and why clouds are white. Now, as Carl Sagan did for astronomy and Brian Green did for cosmology, Lewin takes readers on a marvelous journey in For the Love of Physics, opening our eyes as never before to the amazing beauty and power with which physics can reveal the hidden workings of the world all around us. “I introduce people to their own world,” writes Lewin, “the world they live in and are familiar with but don’t approach like a physicist—yet.”
Could it be true that we are shorter standing up than lying down? Why can we snorkel no deeper than about one foot below the surface? Why are the colors of a rainbow always in the same order, and would it be possible to put our hand out and touch one? Whether introducing why the air smells so fresh after a lightning storm, why we briefly lose (and gain) weight when we ride in an elevator, or what the big bang would have sounded like had anyone existed to hear it, Lewin never ceases to surprise and delight with the extraordinary ability of physics to answer even the most elusive questions.
Recounting his own exciting discoveries as a pioneer in the field of X-ray astronomy—arriving at MIT right at the start of an astonishing revolution in astronomy—he also brings to life the power of physics to reach into the vastness of space and unveil exotic uncharted territories, from the marvels of a supernova explosion in the Large Magellanic Cloud to the unseeable depths of black holes.
“For me,” Lewin writes, “physics is a way of seeing—the spectacular and the mundane, the immense and the minute—as a beautiful, thrillingly interwoven whole.” His wonderfully inventive and vivid ways of introducing us to the revelations of physics impart to us a new appreciation of the remarkable beauty and intricate harmonies of the forces that govern our lives.
This book covers a variety of statistical problems in optics, including both theory and applications. The text covers the necessary background in statistics, statistical properties of light waves of various types, the theory of partial coherence and its applications, imaging with partially coherent light, atmospheric degradations of images, and noise limitations in the detection of light. New topics have been introduced in the second edition, including:Analysis of the Vander Pol oscillator model of laser light Coverage on coherence tomography and coherence multiplexing of fiber sensors An expansion of the chapter on imaging with partially coherent light, including several new examples An expanded section on speckle and its properties New sections on the cross-spectrum and bispectrum techniques for obtaining images free from atmospheric distortions A new section on imaging through atmospheric turbulence using coherent light The addition of the effects of “read noise” to the discussions of limitations encountered in detecting very weak optical signals A number of new problems and many new references have been added Statistical Optics, Second Edition is written for researchers and engineering students interested in optics, physicists and chemists, as well as graduate level courses in a University Engineering or Physics Department.
Divided into four sections, the book covers fundaments of IR detection, IR thermal detectors, IR photon detectors, and focal plane arrays. It begins with a tutorial introduction to essential of different types of IR detectors and systems. The author explores the theory and technology of different thermal detectors and then moves on to the theory and technology of photon detectors. He concludes his treatment with a discussion of IR focal plane arrays where relations between performance of detector array and infrared system quality are considered.
New to the Second Edition:
Fundamentals of IR detection, radiometry, and flux-transfer issues needed for IR detector and system analysis Major achievements and trends in the development of IR detectors Novel uncooled detectors such as cantilever, antenna, and optically coupled detectors Type II superlattice detectors Quantum dot IR detectors Terahertz (THz) arrays and new generation of IR detectors, so-called third generation detectors
The author accomplishes the difficult task of making the information accessible to a wide readership. A comprehensive analysis of the latest developments in IR detector technology and basic insight into the fundamental processes important to evolving detection techniques, the book provides the most complete and up-to-date resource of its kind, including a summary of useful data, guide to the literature, and overview of applications.
Dr. Kock's lucid introduction to lasers and holography has now been revised and updated for a second edition. It begins with a clear discussion of wave patterns and coherence. Then the development of lasers is summarized, along with the phenomenon of wave diffraction. Finally, the important subjects of zone plates and the properties of holograms are skillfully described. A new, concluding chapter brings the story up to the present, with a survey of recent advances in such areas as viewing holograms, hologram computer memories, liquid surface holography, synthetic-aperture radar and sonar, large new lasers, fiber optics, etc.
Using language that can be readily understood by high school and junior high school students, Dr. Kock has written a brief, yet authoritative volume that should satisfy anyone's curiosity about this burgeoning field. The remarkable discoveries that have already occurred are only a prelude to an even more remarkable future. 84 illustrations, including 8 new to this edition. New preface. Suggested (1981) additional reading. Index.
Amateur astronomers are always contemplating the "next telescope up" and this will point the way to the most suitable instrument to which they should aspire. Similarly, those who are buying their first telescope – and these days not necessarily a low-cost one – will be able to compare and contrast different types and makes.
Jim Mullaney is an astronomy writer, lecturer and consultant who has published more than 500 articles and five books: he has also been an editor for Sky & Telescope, Astronomy, and Star & Sky magazines. One of the contributors to Carl Sagan’s award-winning Cosmos PBS-Television series, his work has received recognition from such notables as Sir Arthur Clarke, Johnny Carson, Ray Bradbury, Dr. Wernher von Braun, and one of his former students – NASA scientist/astronaut Dr. Jay Apt. He is probably the ideal person to write this book, containing as it does a directory of instruments offered by all the major manufacturers.
This exciting, upbeat new guide provides an extensive overview of binoculars and telescopes. It includes detailed up-to-date information on sources, selection and use of virtually every major type, brand and model of such instruments on today’s market – truly an invaluable treasure-trove of information and helpful advice for all amateur astronomers. Also includes details on the the latest released telescope lines, e.g., the 10-, 12-, 14- and 16-inch aperture models of the Meade LX-R series.
This book will be mandatory reading for anyone working on the foundations of modern devices such as free electron lasers, plasma accelerators, synchroton sources and other modern sources of bright, coherent radiation with high spectral density.
In this work Einstein intended, as far as possible, to give an exact insight into the theory of relativity to those readers who, from a general and scientific philosophical point of view, are interested in the theory, but who are not conversant with the mathematical apparatus of theoretical physics. The theory of relativity enriched physics and astronomy during the 20th century.(Relativity: The Special and the General Theory by Albert Einstein, 9789380914220)
Laser Fabrication and Machining of Materials introduces readers to the fundamental properties and physical phenomena of laser machining and its applications on the macro, micro, and nano scales. Examples of existing, emerging, and potential techniques are discussed, giving students and practicing engineers alike the opportunity to expand their use of lasers in such disciplines as mechanical processes, electronics, materials, and manufacturing.
Each topic is carefully introduced at three levels to aid student understanding. First, theoretical ideas and background information are discussed, then explanations of mathematical solutions follow and finally practical solutions are presented using MATLAB. The content includes:A compendium of equations and numerical data required by the modern colour and imaging scientist. Numerous examples of solutions and algorithms for a wide-range of computational problems in colour science. Example scripts using the MATLAB programming language.
This 2nd edition contains substantial new and revised material, including three innovative chapters on colour imaging, psychophysical methods, and physiological colour spaces; the MATLAB toolbox has been extended with a professional, optimized, toolbox to go alongside the current teaching toolbox; and a java toolbox has been added which will interest users who are writing web applications and/or applets or mobile phone applications.
Computational Colour Science Using MATLAB 2nd Edition is an invaluable resource for students taking courses in colour science, colour chemistry and colour physics as well as technicians and researchers working in the area. In addition, it acts a useful reference for professionals and researchers working in colour dependent industries such as textiles, paints, print & electronic imaging.
Review from First Edition:
“…highly recommended as a concise introduction to the practicalities of colour science…” (Color Technology, 2004)
Reflecting the big developments in research, this new edition includes major new content: slow light effects, which offers a reduction in noise and power consumption and more ordered network traffic-stimulated Brillouin scattering; vectorial treatment of highly nonlinear fibers; and a brand new chapter on supercontinuum generation in optical fibers.Continues to be industry bestseller providing unique source of comprehensive coverage on the subject of nonlinear fiber optics Updated coverage of intrapulse Raman scattering, four-wave mixing, and Harmonic GenerationIncludes a new chapter excusively devoted to supercontinuum generation in optical fibers
An adaptive lens is a lens whose shape has been changed to a different focal length by an external stimulus such as pressure, electric field, magnetic field, or temperature. Introduction to Adaptive Lenses is the first book ever to address all of the fundamental operation principles, device characteristics, and potential applications of various types of adaptive lenses.
This comprehensive book covers basic material properties, device structures and performance, image processing and zooming, optical communications, and biomedical imaging. Readers will find homework problems and solutions included at the end of each chapter—and based on the described device structures, they will have the knowledge to fabricate adaptive lenses for practical applications or develop new adaptive devices or concepts for advanced investigation.
Introduction to Adaptive Lenses includes chapters on:
Elastomeric membrane lenses
Liquid crystal lenses
This is an important reference for optical engineers, research scientists, graduate students, and undergraduate seniors.
Nonlinear optics is essentially the study of the interaction of strong laser light with matter. It lies at the basis of the field of photonics, the use of light fields to control other light fields and to perform logical operations. Some of the topics of this book include the fundamentals and applications of optical systems based on the nonlinear interaction of light with matter. Topics to be treated include: mechanisms of optical nonlinearity, second-harmonic and sum- and difference-frequency generation, photonics and optical logic, optical self-action effects including self-focusing and optical soliton formation, optical phase conjugation, stimulated Brillouin and stimulated Raman scattering, and selection criteria of nonlinear optical materials.
· Covers all the latest topics and technology in this ever-evolving area of study that forms the backbone of the major applications of optical technology
· Offers first-rate instructive style making it ideal for self-study
· Emphasizes the fundamentals of non-linear optics rather than focus on particular applications that are constantly changing
This book explains the basic principles of spectroscopy, including the fundamental optical constraints and all mathematical aspects needed to understand the working principles in detail. It covers the complete theoretical and practical design of standard and Echelle spectrographs. Readers are guided through all necessary calculations, enabling them to engage in spectrograph design. The book also examines data acquisition with CCD cameras and fiber optics, as well as the constraints of specific data reduction and possible sources of error. In closing it briefly highlights some main aspects of the research on massive stars and spectropolarimetry as an extension of spectroscopy. The book offers a comprehensive introduction to spectroscopy for students of physics and astronomy, as well as a valuable resource for amateur astronomers interested in learning the principles of spectroscopy and spectrograph design.
Features include:Comprehensive and authoritative treatments of the latest research in this area of nanophotonics, written by the leading researchersAccounts of numerous microfluidics, nanofabrication, quantum informatics and optical manipulation applicationsCoverage that fully spans the subject area, from fundamental theory and simulations to experimental methods and results
Graduate students and established researchers in academia, national laboratories and industry will find this book an invaluable guide to the latest technologies in this rapidly developing field.Comprehensive and definitive source of the latest research in nanotechnology written by the leading people in the fieldFrom theory to applications - all is presented in detailEditor is Chair of the SPIE Nanotechnology Technical Group and is leading the way in generation and manipulation of complex beams
The book covers topics such as Fourier spectrometry and field-widened interferometers; the applications of Fourier transform spectrometry; transmission studies on the different phases of matter; asymmetric interferometry; and the measurement of complex indices of refraction. Also included in the book are the areas of cryogenic instrumentation; Hadamard-Transform spectrometry and its related theory; measurement of infrared transient phenomena; and calibration techniques.
The text is applicable for both novices and practitioners in the field of spectrometry, as it can serve as a guide and handbook for the former and reference for the latter.
The book begins with a brief history of the effort to enable the device that generates light through modern organic LEDs and reviews the fundamentals and principles of light prior to a detailed explanation of how LEDs generate different colors. After forming this basic foundation, the book examines the key LEDs in lighting and communications. It then discusses the latest opportunities and advancements in high brightness (HB) LED technology, solid state lighting, and handheld electronic applications.
As we approach a new decade the role of LEDs is literally set to explode, with organic light emitting diodes emerging as a leading next generation technology for electronic displays and lighting. Challenges still exist, including light extraction, luminosity, and white light generation, not to mention non-technical obstacles such as IP disputes and the lack of standards. This book provides a foundation for resolving these issues and developing new applications for LEDs in the promising general illumination market.
This book guides the reader through the mathematics, physics and practical techniques needed to use telescopes (from small amateur models to the larger instruments installed in many colleges) and to observe objects in the sky. Mathematics to around Advanced Placement standard (US) or A level (UK) is assumed, although High School Diploma (US) or GCSE-level (UK) mathematics plus some basic trigonometry will suffice most of the time. Most of the physics and engineering involved is described fully and requires no prior knowledge or experience.
This is a ‘how to’ book that provides the knowledge and background required to understand how and why telescopes work. Equipped with the techniques discussed in this book, the observer will be able to operate with confidence his or her telescope and to optimize its performance for a particular purpose. In principle the observer could calculate his or her own predictions of planetary positions (ephemerides), but more realistically the observer will be able to understand the published data lists properly instead of just treating them as ‘recipes.’ When the observer has obtained measurements, he/she will be able to analyze them in a scientific manner and to understand the significance and meaning of the results.
“Telescopes and Techniques, 3rd Edition” fills a niche at the start of an undergraduate astronomer’s university studies, as shown by it having been widely adopted as a set textbook. This third edition is now needed to update its material with the many new observing developments and study areas that have come into prominence since it was published. The book concentrates on the knowledge needed to understand how small(ish) optical telescopes function, their main designs and how to set them up, plus introducing the reader to the many ways in which objects in the sky change their positions and how they may be observed. Both visual and electronic imaging techniques are covered, together with an introduction to how data (measurements) should be processed and analyzed. A simple introduction to radio telescopes is also included. Brief coverage of the most advanced topics of photometry and spectroscopy are included, but mainly to enable the reader to see some of the developments possible from the basic observing techniques covered in the main parts of the book.
This comprehensive reference provides up-to-date information on component and subsystem technologies, fundamental limitations, and approaches to reach those limits. It covers basic concepts and state-of-the-art technologies, emphasizing device technology, implementation techniques, and system trades. The authors discuss hardware technologies and their applications, and also explore ongoing research activities and those planned for the near future.
The analytical aspects of laser communication have been covered to a great extent in several books. However, a detailed approach to system design and development, including trades on subsystem choices and implications of the hardware selection for satellite and aircraft telecommunications, is missing. Highlighting key design variations and critical differences between them, this book distills decades’ worth of experience into a practical resource on hardware technologies.
In his Atlas of Astronomical Discoveries, astronomy journalist Govert Schilling tells the story of 400 years of telescopic astronomy. He looks at the 100 most important discoveries since the invention of the telescope. Doing what Schilling does best, he takes the reader on an adventure through both space and time. Photographs and amazing pictures line the pages of this book, offering the reader an escape from this world and an invitation to a world far beyond what the unaided human eye can detect.
This book is comprised of five chapters and begins with an overview of the fundamentals of holography, with particular reference to spatial and temporal coherence, diffraction at finite and infinite distances, and filtering of spatial frequencies. The reader is then introduced to the principles and applications of holography, focusing on the geometric optics and aberrations of holograms as well as interferometry and microscopy. The following chapters explore how images are produced and reconstructed using a hologram and explain how to detect the differences between two images. Three main types of synthetic holograms are also considered: binary holograms, holograms with several intensity levels, and the holograms called kinoforms. The last chapter is devoted to optical filtering and pattern recognition.
This monograph will be of value to physicists and researchers as well as to those interested in how holography works.
One of the nineteenth century's most significant papers, "A Dynamical Theory of the Electromagnetic Field," appears here, along with similarly influential expositions of Maxwell's dynamical theory of gases. The author's extensive range of interests is well represented, from his discussions of color blindness and the composition of Saturn's rings to his essays on geometrical optics, ether, and protecting buildings from lightning. His less technical writings are featured as well, including items written for the Encyclopedia Britannica and Nature magazine, book reviews, and popular lectures. Striking in their originality, these papers offer a wealth of stimulating and inspiring reading to modern students of mathematics and physics.
The book is based on the materials taught by S. Fujita for several courses in Quantum Theory of Solids, Advanced Topics in Modern Physics, and Quantum Statistical Mechanics.
If you sit as still as you can in a quiet room, you might be able to convince yourself that nothing is moving. But air currents are still wafting around you. Blood rushes through your veins. The atoms in your chair jiggle furiously. In fact, the planet you are sitting on is whizzing through space thirty-five times faster than the speed of sound.
Natural motion dominates our lives and the intricate mechanics of the world around us. In ZOOM, Bob Berman explores how motion shapes every aspect of the universe, literally from the ground up. With an entertaining style and a gift for distilling the wondrous, Berman spans astronomy, geology, biology, meteorology, and the history of science, uncovering how clouds stay aloft, how the Earth's rotation curves a home run's flight, and why a mosquito's familiar whine resembles a telephone's dial tone.
For readers who love to get smarter without realizing it, ZOOM bursts with science writing at its best.
Parallel universes are a staple of science fiction, and it's no wonder. They allow us to explore the question, "what if?" in a way that lets us step completely outside of the world we know, rather than question how that world might have turned out differently. For cosmologists, the question isn't "what if the South won the Civil War?" but "what if the constants that make up the fundamental building blocks of physics were different?" Physicists argue that any slight change to the laws of physics would mean a disruption in the evolution of the universe, and thus our existence. Take gravity, for example: too strong and stars would burn through their fuel far more quickly. If the universe expanded too fast, matter would spread out too thin for galaxies to form. The list of examples goes on – to the point where the laws of physics might seem finely tuned to make our existence possible. Short of a supernatural or divine explanation, one possibility is that our universe isn't the only one. That's the idea explored in this eBook, Possibilities in Parallel: Seeking the Multiverse. In Section 1, we explore why scientists think other universes could exist. After that, we get a look at the implications. Is it possible to have life in a universe with different physical laws? It would seem so. In "Cracking Open a Window," George Musser discusses the possibility that our universe has more than three spatial dimensions – the others happen to be very small. Other articles, including "The Universe's Unseen Dimensions," analyze the idea that our universe is one of many "branes" – three-dimensional structures stretched out over a higher-dimensional space. The concept of a parallel universe also touches time travel, and then there's the question of what the term "parallel universe" actually means. It's a triumph of the sciences that the very question of why the universe looks as it does can be asked at all. There are currently several possibilities for a multiverse, if it exists. Time and a lot of scientific spadework will reveal which one is right – and get us closer to answering those metaphysical questions: what if, why us, why now?
The authors outline how their positions have further diverged on a number of key issues, including the spatial geometry of the universe, inflationary versus cyclic theories of the cosmos, and the black-hole information-loss paradox. Though much progress has been made, Hawking and Penrose stress that physicists still have further to go in their quest for a quantum theory of gravity.