Customer Reviews

Schrödinger's Rabbits: The Many Worlds of Quantum

5 star:		(3)
4 star:		(1)
3 star:		(4)
2 star:		(0)
1 star:		(0)

 

 

Every few years I look for a good book to bring me up to date on the latest musings of the quantum mechanics to see if they've figured things out yet and Colin Bruce's Schrödinger's Rabbits admirably filled the bill. Bruce does the obligatory opening chapters on quantum mechanics in general and the weirdness that arises when mere humans [like me] try to fathom the subject. The book then launches into a thorough accounting of what the author refers to as the Oxford interpretation, giving us the agreements and disagreements, warts and all. Bruce is a great writer and keeps the tough material engaging. I should confess that I'm a very willing participant and have been fascinated with the many worlds interpretation ever since I read the Larry Niven short stories All The Myriad Ways and For A Foggy Night. I highly recommend Schrödinger's Rabbits to anyone interested in quantum mechanics.

First, let me say that I was both surprised and gratified to learn that this book was written in a world in which I won the Fields medal rather than the Nevanlinna prize. Second, the first few chapters of this book comprise one of the best and most entertaining expositions I've seen on the weirdnesses of quantum mechanics. The sections on the many-worlds interpretation (or interpretations, since not all many-worlders agree) of quantum mechanics are very good, although they haven't changed my views. (I'm still firmly agnostic about the interpretation of quantum mechanics.) This book explains the two most mainstream interpretations of quantum mechanics quite clearly, and compares them reasonably fairly. The main disappointment I had in the book was its treatment of Bohm's and other alternative interpretations of quantum mechanics (i.e., not Copenhagen or many-worlds). They are dismissed quite cavalierly, and I would have liked to see them treated more seriously and at greater length, and dismissed with better justifications. (Just to set the record straight, let me say that I think there are good reasons to dismiss Bohm's interpretation and excellent reasons for dismissing all the other alternative interpretations. It's fairly hard to articulate these reasons, as doing it right would involve learning more than I really want to about these alternative interpretations, and I would have really liked Colin Bruce to do this work for me. On the other hand, maybe I'm just being selfish here and hoping somebody else will do work that I'm too lazy to do myself. Even without a comprehensive treatment of these other interpretations, this is a really entertaining and instructive book, which deserves every one of the five stars I'm giving it.)

The book pushes what the author calls the Oxford Interpretation of Quantum Mechanics, i.e. the many-worlds interpretation, as the "correct" interpretation of quantum mechanics. However, the argument in favor for it is only supported by a cavalier and arbitrary application of Occam's Razor, and dismissive (and sometimes ridiculing) statements against all alternative interpretations. I couldn't really find any strong argument supporting the Oxford Interpretation at all. I enjoyed the author's other books very much but this one was a bit of a disappointment. The author must be spending too much time in Oxford and allowed himself to be brain-washed by the many-worlders.

I felt that the introduction Colin Bruce does in Chapter 1 through his `lottery card' to entanglement and its associated non-locality phenomena is quite simple, entertaining, revealing, and faithfully depicting the crux of the matter in quantum physics. In spite of his famous "God does not play dice", Einstein was willing to accept the non-deterministic character of the Universe but, he could not renounce to the principle of locality.
While writing the book in Spanish entitled "¿Qué es la Realidad? - Einstein, Física Cuántica, y Folclore" ("What is Reality? - Einstein, Quantum Physics, and Folklore"), I am particularly interested in reading popular science books like Bruce's where the philosophical implications of different ontologies for Quantum Mechanics are discussed -- even though it is clear Bruce has already taken a position based more on gut feelings than hard evidence. Sometimes (I agree with other reviewers) he goes a little overboard in suggesting the reality of his many-worlds interpretation but, in many others occasions, he appears objective, acknowledging there is a long way to go before a particular ontology (perhaps a different one not yet imagined) is universally accepted by the scientific community.
Chapter 4, where he tries to make the reader "feel in her bones" that sending information faster than the speed of light is equivalent to send it backwards in time, is obscure and hard to understand by a person not imbued of the epistemological intricacies behind the concept of simultaneity in Special Relativity. Those epistemological issues are thoroughly treated for the non-professional in Cabalgando con la Luz - Einstein, Relatividad, y Folclore. Chapter 6, introducing the Hilbert space for the non-professional, is well done. The concepts of collapse and decoherence throughout the book are difficult to grasp in their intrinsic significance and I felt the author should have spent more time explaining them and emphasizing their importance in making the strange world of quantum look sensible. The second half of the book is dedicated to explaining the Many-Worlds ontology and is quite interesting and abundant in the good, the bad, and the ugly of such an interpretation. It is clear his position but it is clear as well he cannot convince the reader.
Overall, I give four stars to this book and recommend its reading without hesitation. Felix Alba Juez.

In the nineteenth and early twentieth centuries, many natural philosophers believed that the laws of the Universe could be derived from Reason. For instance, Galilean invariance gave us Newton's Laws, and similar invariance conditions give rise to special and general relativity. Quantum mechanics changed all that because many quantum mechanical phenomena violate one or more 'reasonable' assumptions, such as the impossibility of action at a distance. This is really why Einstein never accepted quantum mechanics as more than a provisional theory.

It is tempting to be dismissive of quantum mechanics because it only deals with phenomena at extremely sub-microscopic dimensions, and classical mechanics and electrodynamics can get alone without it perfectly well at 'human' dimensions. Both reasons are faulty. First, quantum mechanics explains quantum tunneling, which is central to solid state devices such as transistors, and more generally, quantum effects can be amplified without difficulty to the human level (this is the famous Schrödinger's Cat problem, to which the title of the book alludes. Second, there are deep antinomies in classical electromagnetic theory, including the prediction that the energy in blackbody radiation goes to infinity as the wavelength of the energy goes to zero.

There are many strange and beautiful quantum phenomena. Perhaps the two most famous are wave/particle duality and entanglement. Wave-particle duality is well illustrated by the famous two-slit experiment. Photons are directed towards an absorbing wall with two vertical slits some macroscopic distant apart. The few photons that go through one of the slits hit a clear photographic film that records the absorption of the photon by coloring the impact point black. If photons are particles, there should be two widely separated masses of black dots whose location can be calculated using simple geometry. If photons are waves (like waves in a pond), it is not hard to show that there should be alternating bands of light and dark on the photographic plate, corresponding the interference pattern of the two parts of the wave that pass through the distinct slits. If you do this experiment, photons act wave-like. However, if you measure the passage of the photon through a slit, you always find it went through either one or the other, but not partly through both. This shows the photon is a particle. Moreover, if you can measure which slit the photon went through, the interference patter on the plate disappears, and the photon leave a black spot on the plate.

Early interpreters of this weir phenomenon argued that by measuring the photon, you interfered with its normal path, as explained by classical signaling theory, so the phenomenon is just poor experimental design. For a variety of reasons that you can read about, this explanation is faulty. The widely accepted explanation (the so-called Copenhagen theory, named after Neils Bohr) is that the photon is indeed a wave, as described by Schrödinger's famous equation or Planck's relativistic version. These equations describe not where the photon is, but rather a probability distribution over its possible locations. When observed, the wave 'collapses' to a particular location, the location being proportion to its relative probability.

This experiment and its interpretation calls into question the nature of objectivity and subjectivity in a highly radical form. What does it mean to be 'observed'? By whom? What if it is 'observed' by a machine and you do not have access to the machine's memory? The two-slit and related experiments, as explained by the Copenhagen school, simply divides reality in to nature plus observer, where observer means a human or an instrument that can be read and recorded, accessible to a human. This is fine for all practical purposes, because that's how we humans do science. But its ontological status is highly compromised.

A second weird phenomenon involves 'entanglement.' Electrons have 'spin,' which when measured in any direction, is either 'up' or 'down.' When electrons are generated in pairs, they may be entangled in the sense that after they are separated, if one measures 'up' in a particular direction, the other must measure 'down.' However the state of spin is a probability distribution that collapses to a particular up or down when you measure it. Suppose the entangled electrons move apart until they are a light-second apart, and then the spin of one is measured in some direction. Then immediately, not a second later, the other measures the opposite direction. This is not a relativistic time effect, and it has been repeatedly verified in the laboratory. It is called a violation of locality.

Einstein and his colleagues Podolsky and Rosen (1935) used a similar argument to show that there must be something wrong with quantum mechanics, but it turns out that that is just the way the world works, like it or not.

Most physicists do not care why quantum mechanics works the way it does, but some do, and many of those think that there must be a better model that the Copenhagen collapse theory. One is the Many Worlds Interpretation (MMI), which Bruce explains and defends in this book. The MMI says that whenever there appears to be a collapse of the quantum wave, the Universe really branches into a large number of alternative Universes, side-by-side, each of which captures one of the possible states. So, for instance, if you set up an apparatus that kills you if a certain photon is measured as having spin up, and does nothing if the spin is down. Then the whole Universe splits into two Universes, in the first of which you are alive and the other of which you are dead.

This theory does explain most of the queerness of quantum mechanics, and it could be true. But there is no evidence that it is true. Nor has anyone ever suggested an experiment that would determine its truth value (Bruce suggests some possibilities, but they are far-fetched). Moreover, the theory itself is completely outlandish, far weirder than the phenomenon it is supposed to explain. The fact is that reading this and other attempts to explain quantum weirdness are fun and challenging intellectually, but their real value is virtually zero. When someone tells me he believes a version of the MWI, I treat the person the same as if they told me they believed in transubstantiation or the tooth fairy.

Nevertheless, quantum mechanics is certainly not a "true" theory, if only because it makes no room for gravity. The string and quantum gravity theorists attempt to repair this fault, but they produce theories that are as yet untested, although possibly testable.

But I think there is a deeper problem. Quantum mechanics represents a particle (e.g., a photon) as a point in Hilbert space, which is an infinite dimensional vector space. I don't believe there are `real' infinities, and hence this representation must be an approximation of the true, finite, model. We should be looking at finite or countable models of particle physics, not hugely overpopulated continuous models. Richard Feynman registered his discomfort with nonconstructible, continuous models, in the following words (quoted by Bruce, p. 242): "It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time..." I have often make the hypothesis that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple, like the checkerboard with all its apparent complexities."

I realize many readers will not share my skepticism concerning infinite models. But they should, because it is the only reasonable position. Countable models are fine, because we can attain any element of a countable model in finite time. But uncountable models are just a useful human construct, probably not corresponding to anything real in the world. At any rate, reading Bruce's book conjures up all sorts of cute ideas that are fun to think about while taking a break from the real world.

This book is somewhat harder to read because the subject does not flow well from chapter to chapter, but on the positive side, it is written for a general reader which requires only basic knowledge of quantum physics. The author states that his main focus is the recent advances at Oxford University, due to Roger Penrose and David Deutsch, with emphasis on the many worlds interpretation of quantum physics. But as you read through the chapters his lack of focus becomes evident.

The summary of the book is as follows: Schrodinger's equations for an electron in an atom are described by the "time-inde¬pendent" equation, which does not answer the question of where the particle is located at any given instant. The time-dependent equation predicts that as long as it is not interacting with anything, the wave will flatten and spread out to infinity. Yet even a tiny observation-like interaction with this wave somewhere in the universe can bring an extremely point-like electron springing into view, with dimensions that remain too small to mea¬sure and this happens in less time than it would take light to cross the region of space where it was until a moment ago. It is hard to imagine any reasonable physi¬cal mechanism that could bring about quantum collapse (faster than speed of light).

The quantum phenomenon is interpreted by two major schools of thought; the Copenhagen interpretation due to Niels Bohr, and the many worlds interpretation due to Hugh Everett. According to the former, that unmodified Schrodinger wave equation gives rise to a collapsed single reality when perceived by a conscious observer, but it does not provide a mechanism for quantum collapse. Roger Penrose proposes that the Schrodinger wave equation must be modified to include some physical collapse mechanism that gives rise to a single-valued reality. Exponents of many-worlds interpretation propose that collapse never hap¬pens and the universe continues with all outcomes which are equal and real. Just believe what the equations (math) are telling us, that the universe is tracing out all possible histories, rather than just one privileged one expounded by Copenhagen school. There is one problem at the heart of the many worlds concept, which is how do you treat relative probabilities of different outcomes and the world lines that follow them? There is a third interpretation due to David Bohm (Bohmian mechanics) called hidden variables interpretation. The conceptual difficulties of the quantum world such as, the two-slit experiment, Heisenberg uncertainty, and the quantum col¬lapse is explained by postulating some fine struc¬ture to space that is too delicate to measure directly. This hypothetical fine structure also called hidden local variables. Waves that can influence the motion of both photons and matter particles and make small objects judder about so as to complicate the measurement of their positions and motions. Abrupt collision jolt particles from the waves they are associated with resulting in localization of matter (quantum collapse).

The spacetime in which the probability waves move is also quantized. If gravitational force (space-time curvature) were subjected to quantum fluctuations in the way that other fields and energies are, mathematical infinities would arise. In physical terms, the structure of space-time would be very un¬stable. A quantum fluctuation in a tiny region of space-time would very rapidly grow, perhaps spawning exotic entities such as black holes at a colossal rate. Roger Penrose offers an explanation by suggesting that such uncertainties in gravitational field energy tend to cancel themselves out producing quantum collapse as a side effect. Even two atomic nuclei attract each other gravitationally and produce tiny curvature in spacetime. Penrose's calculations for quantum collapse as expected is fast for larger objects (for a cat it is 10(e-37) seconds, and for a beryllium ion, 100 years).

The first half of the book is sometimes irrelevant and boring but the second half is interesting. I found the last three chapters are particularly interesting; it is here the author discusses the quantized spacetime and the information stored in space-time in relation to the rest of the universe. There are many books on quantum physics and physical reality written for general readers, and I recommend readers to look for other sources to strengthen their knowledge in quantum reality and philosophy of existence.

1. Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity
2. The Tao of Physics: An Exploration of the Parallels between Modern Physics and Eastern Mysticism (25th Anniversary Edition)
3. The Physics of Consciousness: The Quantum Mind and the Meaning of Life
4. What Is Life?: with "Mind and Matter" and "Autobiographical Sketches"
5. Schrödinger: Life and Thought
6. Physics and Philosophy: The Revolution in Modern Science
7. Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science
8. Niels Bohr's Times,: In Physics, Philosophy, and Polity
9. Schrodinger's Kittens and the Search for Reality: Solving the Quantum Mysteries Tag: Author of In Search of Schrod. Cat
10. The Self-Aware Universe

This book made my brain hurt; but in a good way-much like my thighs hurt after doing squats at the gym. While aimed at the intelligent layman, this book still contains quite a bit that might go over the reader's head. Readers would do well to have at least an amateur grasp of some of the main concepts involved in Relativity, classical mechanics, particle physics, and mathematics these days (don't worry, you don't actually need to understand the equations these fields employ). I felt it was a wonderful overview of the many-worlds approach to the problems of quantum physics, and Dr. Bruce certainly fleshes out the terrifying problems the many-worlders themselves must account for.
However, I agree with the reviewer who suggests that the "non-many-worlders" are given short shrift in this book; one gets the sense that the great physicists who cannot support many-worlds are a motley crew of cranks on the fringes of theoretical physics. This is hard to believe, given the seemingly ludicrous implications of many-worlds and its frightening "decoherence" with the human experience. Only Roger Penrose seems to be given due credit, and his theories are really only alluded to.
Finally, the author's "solution" for reconciling the problems introduced by many-worlds relies upon a somewhat flimsy elucidation of the concept of subinformation, which this reader found incomprehensible. Perhaps seasoned physicist readers will understand and appreciate this book more than I did, but I would still recommend it highly to anyone wanting a rigorous intellectual workout.

This is without a doubt the one book since Douglas Adams stopped writing that made me glad English was my native language.