Summary
Part I: Reality's Arena
The main focus of Part I is space and time.
Chapter 1, "Roads to Reality", Introduces what is to come later in the book, such as discussions revolving around classical physics, quantum mechanics, and cosmological physics.
Chapter 2, "The Universe and the Bucket", features space as its key point. The question posed by Greene is this: "Is space a human abstraction, or is it a physical entity?" The key thought experiment is a spinning bucket of water, designed to make one think about what creates the force felt inside the bucket when it is spinning. The ideas of Isaac Newton, Ernst Mach, and Gottfried Leibniz on this thought experiment are discussed in detail.
Chapter 3, "Relativity and the Absolute", focuses on spacetime. The question now becomes: "Is spacetime an Einsteinian abstraction or a physical entity?" In this chapter, concepts of both special relativity and general relativity are discussed, as well as their importance to the meaning of spacetime.
In chapter 4, "Entangling Space", Greene explores the revolution of the quantum mechanical era, focusing on what it means for objects to be separate and distinct in a universe dictated by quantum laws. This chapter studies quantum mechanics, including the concepts of probability waves and interference patterns, particle spin, the photon double slit experiment, and Heisenberg's uncertainty principle. The reader is also informed of challenges posed to quantum mechanics by Albert Einstein, Boris Podolsky, and Nathan Rosen.
Part II: Time and Experience
Part II begins by addressing the issue that time is a very familiar concept, yet it is one of humanity's least understood concepts.
Chapter 5, "The Frozen River", deals with the question, "Does time flow?" One key point in this chapter deals with special relativity. Observers moving relative to each other have different conceptions of what exists at a given moment, and hence they have different conceptions of reality. The conclusion is that time does not flow, as all things simultaneously exist at the same time.
Chapter 6, "Chance and the Arrow", asks, "Does time have an arrow?" The reader discovers that the laws of physics apply moving both forward in time and backward in time. Such a law is called time-reversal symmetry. One of the major subjects of this chapter is entropy. Various analogies are given to illustrate how entropy works and its apparent paradoxes. The climax of the chapter is the co-relation between entropy and gravity, and that the beginning of the Universe must be the state of minimum entropy.
In Chapters 5 and 6, time has been explained only in terms of pre-modern physics. Chapter 7, "Time and the Quantum", gives insights into time's nature in the quantum mechanics. Probability plays a major role in this chapter because it is an inescapable part of quantum mechanics. The double slit experiment is revisited to reveal things about the past. Many other experiments are presented in this chapter, such as the delayed-choice quantum eraser experiment. Other major issues are brought to the reader's attention, such as quantum mechanics and experience, as well as quantum mechanics and the measurement problem. Finally, this chapter addresses the important subject of decoherence and its relevance towards the macroscopic world.
Part III: Spacetime and Cosmology
Part III deals with the macroscopic realm of the cosmos.
Chapter 8, "Of Snowflakes and Spacetime", tells the reader that the history of the universe is in fact the history of symmetry. Symmetry and its importance to cosmic evolution becomes the focus of this chapter. Again, general relativity is addressed as a stretching fabric of spacetime. Cosmology, symmetry, and the shape of space are put together in a new way.
Chapter 9, "Vaporizing the Vacuum", introduces the theoretical idea of the Higgs boson. This chapter focuses on the critical first fraction of a second after the Big Bang, when the amount of symmetry in the universe was thought to have changed abruptly by a process known as symmetry breaking. This chapter also brings into play the theory of grand unification and entropy is also revisited.
Chapter 10, "Deconstructing the Bang", makes inflationary cosmology the main point. General relativity and the discovery of dark energy (repulsive gravity) are taken into account, as well as the cosmological constant. Certain problems that arise due to the standard Big Bang theory are addressed, and new answers are given using inflationary cosmology. Such problems include the horizon problem and the flatness problem. Matter distribution throughout the cosmos is also discussed, including the concepts of dark matter and dark energy.
Chapter 11, "Quanta in the Sky with Diamonds", continues with the topic of inflation, and the arrow of time is also discussed again. The chapter addresses three main developments: the formation of structures such as galaxies, the amount of energy required to spawn the universe we now see, and the origin of time's arrow.
Part IV: Origins and Unification
Part IV deals with new theoretical aspects of physics, particularly in the author's field.
Chapter 12, "The World on a String", informs the reader of the structure of the fabric of space according to string theory. New concepts are introduced, including the Planck length and the Planck time, and ideas from The Elegant Universe are revisited. The reader will learn how string theory could fill the gaps between general relativity and quantum mechanics.
Chapter 13, "The Universe on a Brane ", expands on ideas from chapter 12, particularly on M-theory, of which string theory is a branch. This chapter is devoted to speculations on space and time. The insights of a number of physicists, including Edward Witten and Paul Dirac, are presented. The focus of the chapter becomes gravity and its involvement with extra dimensions. Near the end of the chapter, a brief section is devoted to the cyclic model.
Part V: Reality and Imagination
Part V deals with many theoretical concepts, including space and time travel.
Chapter 14, "Up in the Heavens and Down on the Earth", is about various experiments with space and time. Previous theories are brought back from previous chapters, such as Higgs theory, supersymmetry, and string theory. Future planned experiments are described in an attempt to verify many of the theoretical concepts discussed, including the constituents of dark matter and dark energy, the existence of the Higgs boson, and the verification of extra spatial dimensions.
Chapter 15, "Teleporters and Time Machines", is about traveling through space and time using intriguing methods. Quantum mechanics is brought back into the picture when the reader comes across teleportation. Puzzles of time travel are posed, such as the idea of time travel to the past being a possibility. The end of the chapter focuses on wormholes and the theory behind them.
Chapter 16, "The Future of an Allusion", focuses on black holes and their relationship to entropy. The main idea of this chapter is that spacetime may not be the fundamental makeup of the universe's fabric.
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of second order partial differential equations.
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars, as well as in the early stages of the universe moments after the Big Bang.
In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries the gravitational force. Thus, string theory is a theory of quantum gravity.
A theory of everything (TOE), final theory, ultimate theory, unified field theory or master theory is a hypothetical, singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all aspects of the universe. Finding a theory of everything is one of the major unsolved problems in physics.
A Brief History of Time: From the Big Bang to Black Holes is a book on theoretical cosmology by the physicist Stephen Hawking. It was first published in 1988. Hawking wrote the book for readers who had no prior knowledge of physics.
In the philosophy of space and time, eternalism is an approach to the ontological nature of time, which takes the view that all existence in time is equally real, as opposed to presentism or the growing block universe theory of time, in which at least the future is not the same as any other time. Some forms of eternalism give time a similar ontology to that of space, as a dimension, with different times being as real as different places, and future events are "already there" in the same sense other places are already there, and that there is no objective flow of time.
In philosophy, the philosophy of physics deals with conceptual and interpretational issues in modern physics, many of which overlap with research done by certain kinds of theoretical physicists. Historically, philosophers of physics have engaged with questions such as the nature of space, time, matter and the laws that govern their interactions, as well as the epistemological and ontological basis of the theories used by practicing physicists. The discipline draws upon insights from various areas of philosophy, including metaphysics, epistemology, and philosophy of science, while also engaging with the latest developments in theoretical and experimental physics.
Lee Smolin is an American theoretical physicist, a faculty member at the Perimeter Institute for Theoretical Physics, an adjunct professor of physics at the University of Waterloo, and a member of the graduate faculty of the philosophy department at the University of Toronto. Smolin's 2006 book The Trouble with Physics criticized string theory as a viable scientific theory. He has made contributions to quantum gravity theory, in particular the approach known as loop quantum gravity. He advocates that the two primary approaches to quantum gravity, loop quantum gravity and string theory, can be reconciled as different aspects of the same underlying theory. He also advocates an alternative view on space and time that he calls temporal naturalism. His research interests also include cosmology, elementary particle theory, the foundations of quantum mechanics, and theoretical biology.
Brian Randolph Greene is an American physicist. Greene was a physics professor at Cornell University from 1990–1995, and has been a professor at Columbia University since 1996 and chairman of the World Science Festival since co-founding it in 2008. Greene has worked on mirror symmetry, relating two different Calabi–Yau manifolds. He also described the flop transition, a mild form of topology change, showing that topology in string theory can change at the conifold point.
The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory is a book by Brian Greene published in 1999, which introduces string and superstring theory, and provides a comprehensive though non-technical assessment of the theory and some of its shortcomings. In 2000, it won the Royal Society Prize for Science Books and was a finalist for the Pulitzer Prize for General Non-Fiction. A new edition was released in 2003, with an updated preface.
In theoretical physics, the anti-de Sitter/conformal field theory correspondence is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) that are used in theories of quantum gravity, formulated in terms of string theory or M-theory. On the other side of the correspondence are conformal field theories (CFT) that are quantum field theories, including theories similar to the Yang–Mills theories that describe elementary particles.
The Road to Reality: A Complete Guide to the Laws of the Universe is a book on modern physics by the British mathematical physicist Roger Penrose, published in 2004. It covers the basics of the Standard Model of particle physics, discussing general relativity and quantum mechanics, and discusses the possible unification of these two theories.
A physical paradox is an apparent contradiction in physical descriptions of the universe. While many physical paradoxes have accepted resolutions, others defy resolution and may indicate flaws in theory. In physics as in all of science, contradictions and paradoxes are generally assumed to be artifacts of error and incompleteness because reality is assumed to be completely consistent, although this is itself a philosophical assumption. When, as in fields such as quantum physics and relativity theory, existing assumptions about reality have been shown to break down, this has usually been dealt with by changing our understanding of reality to a new one which remains self-consistent in the presence of the new evidence.
In physics, a symmetry of a physical system is a physical or mathematical feature of the system that is preserved or remains unchanged under some transformation.
Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos is a popular science book by Michio Kaku first published in 2004.
Everything, every-thing, or every thing, is all that exists; it is an antithesis of nothing, or its complement. It is the totality of things relevant to some subject matter. Without expressed or implied limits, it may refer to anything. The universe is everything that exists theoretically, though a multiverse may exist according to theoretical cosmology predictions. It may refer to an anthropocentric worldview, or the sum of human experience, history, and the human condition in general. Every object and entity is a part of everything, including all physical bodies and in some cases all abstract objects.
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain, and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena.
In mathematical physics, de Sitter invariant special relativity is the speculative idea that the fundamental symmetry group of spacetime is the indefinite orthogonal group SO(4,1), that of de Sitter space. In the standard theory of general relativity, de Sitter space is a highly symmetrical special vacuum solution, which requires a cosmological constant or the stress–energy of a constant scalar field to sustain.
Physics is a scientific discipline that seeks to construct and experimentally test theories of the physical universe. These theories vary in their scope and can be organized into several distinct branches, which are outlined in this article.
The index of physics articles is split into multiple pages due to its size.
