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A**N
Science now tells us we must seriously consider we live in a multiverse
Following up his two previous bestsellers, The Elegant Universe and The Fabric of the Cosmos, Columbia University Professor of Physics and Mathematics Brian Greene has carried on in his attempt to enlighten the lay reader to leading edge developments in scientific research into the deepest mysteries of both the micro-world and the macro-world of not only our own universe, but other universes that may exist beyond our current ability to contact them.Including 30 pages of notes, mostly for the more technically inclined, an extensive index, and using metaphor, analogy, historical anecdotes, and a touch of humour, Professor Greene looks at the latest theoretical thinking and experimental analyses to give, as he says in the Preface, "...a broadly accessible account of some of the strangest and, should they prove correct, most revealing insights of modern physics. Many of the concepts require the reader to abandon comfortable modes of thought and to embrace unanticipated realms of reality."What spurred Greene to give us an up-to-date account of developments in fundamental theoretical physics is that they have led investigators to the serious consideration of different types of parallel universes. In the book, he identifies 9 varieties of what are called "multiverses." And, what is so amazing to Greene and to his colleagues around the world is that "...all of the parallel-universe proposals that we will take seriously (in the book) emerge unbidden from the mathematics of theories developed to explain conventional data and observation."Proceeding like a class in the conceptual, non-technical overview of contemporary physics, displaying his knack for making difficult concepts easy to understand by relating them to common, everyday examples from life, and ascribing nomenclature generally used in the field of cosmology, Dr. Greene first examines what is known as the Quilted Multiverse. He begins the discussion with a review of the Big Bang and Einsteinian relativity to illustrate "...that basic physical principles establish that if the cosmos is infinitely large, it is home to infinitely many parallel worlds - some identical to ours, some differing from ours, many bearing no resemblance to our world at all."Because astronomers have calculated that we can only see out from earth about 41 billion light-years (called the cosmic horizon - an enormous distance, but certainly less than infinite), if the universe itself is infinite, there must be an infinite amount of other regions of space that have their own cosmic horizons. If those regions are sufficiently distant from each other, such an array would look like a patchwork quilt of an infinite number of finite regions that are individual universes themselves. Such a scenario is called a Patchwork or Quilted Multiverse. In such a multiverse, there would be endless doppelgangers - exact, repetitive reproductions of everything we experience, even ourselves.Greene's second type of multiverse is called the Inflationary Multiverse. This one is based on the continual expansion of our universe that would eternally produce bubble universes, and only one of those would be the one we see. We can think of the bubble universes in the Inflationary Multiverse as the holes in an ever-expanding Swiss cheese cosmos.Professor Greene's third and fourth types of multiverses derive from String theory and from the braneworlds of M-theory. In Chapter 4, he reiterates from his previous books a clear explanation of various aspects of quantum mechanics resulting in String theory, extra spatial dimensions, singularities, and black holes that lead us into Chapter 5's discussion of the Brane Multiverse and the Cyclic Multiverse. The former consists of three-dimensional branes (don't worry - Greene explains what branes are) that float in higher dimensions with other branes, and the latter derives from the collisions of those braneworlds that result in new universes with their own big bangs.We are next introduced to something called the Landscape Multiverse. It derives from a combination of the Inflationary Multiverse and String theory.In 1998, two separate teams of astronomers measured a positive but tiny number for Einstein's cosmological constant - a value that gives us the amount of dark, invisible energy thought to be existing uniformly throughout space. Dark energy governs the repulsive gravitational force that drives our universe's inflation. Contrary to what we would expect - that after the Big Bang the inflation of the universe would gradually slow down - the measurements indicated that for approximately the past half of our universe's life its rate of expansion has been accelerating.String theory tells us that each of the ever-increasing number of bubble universes in an Inflationary Multiverse contains a different configuration of extra dimensions "...providing a cosmological framework that realises all possibilities." It also tells us that different values of the cosmological constant in each bubble universe give rise to "...bubbles inside of bubbles inside of bubbles..." When combined with accelerating expansion, this bubble tunnelling process provides an entire "landscape" of different universes. Hence, the totality is called a Landscape Multiverse.50-page long, Chapter 8 describes what's called the Quantum Multiverse - a multiverse that emerges directly from quantum mechanics. Greene reminds us of the double-slit experiment and its consequential interference pattern in order to guide us through his explanations of a particle's probability wave and Niels Bohr's Copenhagen Interpretation which dictate that the act of measurement/observation results in the collapse of the wave function locating only one position for a given particle - one definite, observed reality outcome.Professor Greene then takes us to the realm of the Holographic Multiverse where reality takes place on a universe's distant boundary surface and projects its information into the 3D world we know and experience as a kind of holographic movie. We can think of this as we would think of the information in an architect's blueprints being translated into the actual physical realisation of a building. In other words, the boundary surface of a universe can be thought of as a physically equivalent parallel universe.Greene adds, "That familiar reality may be mirrored, or perhaps even produced, by phenomena taking place on a faraway, lower-dimensional surface ranks among the most unexpected developments in all of theoretical physics.... Looking to the future, I suspect that the holographic principle will be a beacon for physicists well into the twenty-first century."The 8th and 9th multiverses identified in the book involve both actual and computer simulations. The 8th variety is called a Simulated Multiverse. Here, Greene takes a bold step in contemplating universe creation by future humans in the possession of very advanced technologies. There are two types of these we can think of: (1) usual, physical universes, and (2) virtual, computer-generated universes. The first involves artificially producing a white hole that spews out matter. The second is akin to the conceptual presentation in movies like The Matrix, The Thirteenth Floor, and Vanilla Sky.The 9th and final multiverse discussed is what Professor Greene calls the Ultimate Multiverse. It is his own rationalisation for the existence of a multiverse, independent of being a by-product of quantum mechanics, inflationary cosmology, String theory, or any other such applications that led indirectly to the previous 8 types of multiverses. He surmises, "Maybe math is more than just a description of reality. Maybe math is reality." Perhaps, "Different collections of mathematical equations are different universes. The Ultimate Multiverse is thus the by-product of this perspective on mathematics." (M.I.T.'s Max Tegmark calls this the Mathematical Universe Hypothesis.)Greene further posits, "Mathematical existence is synonymous with physical existence. And since this would be true for any and all math, this provides another road leading us to the Ultimate Multiverse." It's another way of saying that every possible universe we can imagine, and therefore describe with a mathematical equation, is, somewhere and at some time, a real universe.Greene ends with questions like, "Can scientific theories that invoke a multiverse be tested?" And, "Should we believe mathematics?" In fact, he admits that math is central to all he discusses. The multiverse theories examined in his book "rely on a belief that mathematics is tightly stitched into the fabric of reality."He adds in conclusion, "It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality."- This review first appeared in New Dawn magazine issue #132
P**E
Lucid and impressive survey of multiverse models
This is an impressive survey of 9 multiverse models by Greene featured in lucid, engaging and effortless prose. This should be a go to book for the public as well as physics students for a conceptual understanding of the multiverse models available.He began with the "Quilted multiverse" model. This model sets up the most basic bare bone structure of the rationale of multiverse theory. Consider our own visible universe's cosmic horizon of a 14 billion year old universe. The cosmic horizon visible to us is only 41 billion light years (distance based on the light coming from objects receding from us). Our cosmic horizon or patch is all there is ever visible to us. The entire universe in the infinite space can contains infinite number of patches each with it's own cosmic horizon, constituting a quilted universe. Each patch is out of reach of the other patches. This is a basic multiverse model showing each multiverse in its own cosmic horizon patch.The "inflationary model" from Alan Guth and Andrei Linde offers further mechanism to account for how the vast expanse of the multiverse can be built out. The idea is an inflation that is triggered by quantum jitter in high energy inflation field level causing it to expand and bubble out into different multiverses. This inflationary mechanism is also commonly used in other multiverse models.Greene, as a String theorist, offers the String Theory model of multiverse by considering our 3 dimensional expanse universe on a 3-brane sheet. Another multiverse would be on another 3-brane sheet, and there can be as many 3-brane sheets on which each multiverse exists. This multiverse model is the "Brane Multiverse" model.Another String Theory model is the "Landscape" model offered by Suskind. In this model, an inflationary mechanism for bubbling into multiverse is augmented with a further feature of quantum tunneling. A multiverse suffused with higher energy level or cosmological constant can expand via repulsion and inflate, but tunnels or drops down to a lower energy level for bubbling into another multiverse. The multitude of landscape with different cosmological constants represented by unique Calabi Yau manifolds can continue and repeat this tunneling process for proliferating further multiverses.Greene also offers a thorough discussion of the Everttian "multiple-world quantum" model. In this model, the Everttian interpretation of quantum mechanics by branching the possible outcomes of quantum state into different worlds is treated as another multiverse model. Instead of collapsing probability amplitudes as in Copenhagen interpretation, Evertt suggested to let probability outcomes to branch out into different worlds such that each outcome constitute its own world. Greene highlighted that Everttian rationale is that multi-world interpretation actually stays faithful to Schrodinger equation and let the equation results speak for itself instead of ad hoc adding probability amplitudes together which are not reflected in the equation. Greene also discussed if such a multi-world interpretation takes quantum probability seriously, which he thinks each multi-world is still a probabilistic outcome.Another model Greene discussed in this work is the conceptually challenging "Holographic multiverse" model. In this model, our universe is a mere holographic phenomena taking place on a distant bounding surface, a physically equivalent parallel universe. In the holographic principle, open strings movement on 3-branes is described by quantum field particle theory in four dimensional space-time. The physics described is the same as the closed loop strings on 10 dimensional black branes as long as the strings at low energy are closed to the event horizon surface. Hence the holographic model reveals universes as holograms of equivalent parallel universes.Three other models discussed are the Cyclic multiverse model and the mathematical models of "Simulated Multiverse" and "Ultimate Multiverse" models which consider mathematical multiverses to be as real as physical multiverses. There is also a chapter devoted to methodological issues such as experimental accessibility, predictions, and the limits of mathematical applicability to physics.
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