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5.0 out of 5 stars
Excellent Treatment By a Top Theorist, November 16, 2009
The public fascination with modern physics, particularly the strange world of quantum mechanics, seems to be growing and growing. This, like most fads, has both good and bad points - the good being the public is interested in science and seems to want to learn more; the bad being, well, many pop books have arisen exploiting the public's interest in these exotic topics, some of which tend to be written on a casual level - or worse, a quack level- and can therefore be fairly misleading.
That doesn't concern us here. Enter a fairly new book on quantum mechanics geared toward the popular level (superbly translated from the Italian by Gerald Malsbary), but the difference this time is, author Giancarlo Ghirardi is not only a theoretical physicist, he is actively involved in quantum foundational questions. Indeed, Ghirardi is one of the originators of an influential interpretation (more accurately, a "rival" scheme, we'll get into that later) of QM. So here we have the rare treat of an expert in quantum foundations sharing the challenges and struggles of his craft with the public. One couldn't ask for more in this regard. Nonetheless, when I say the book is geared toward the "popular" level, readers should realize the book is demanding. While equations illustrating key points are kept to a minimum and can be ignored to get the gist of his arguments, Ghirardi is very thorough in his description of the microphysics and the epistemological interpretation problems involved, as we might expect coming from an eminent physicist with a keen interest in these areas. Lay folks expecting an easy ride, even assuming some prior familiarity with the concepts, are in for a bumpy journey. One should realize this is a serious treatment of the issues.
I mentioned Ghirardi's "rival" scheme to orthodox QM above...what is it? Briefly, in 1985/86, Ghirardi together with two other Italian physicists (A. Rimini and T. Weber) proposed a theory which introduced two new constants into the standard quantum formalism, proposing a so-called "spontaneous localization" model for fellow specialists to consider. Thus, the trio essentially introduced another "hidden-variable" theory, alongside the existing de Broglie- David Bohm "pilot wave" model. The trio's theory became known throughout the physics literature as the "GRW" model (using the initials of all three physicists). The original motivation arose out of a dissatisfaction, shared by many physicists, with the orthodox so-called "Copenhagen" interpretation...something needed to be done to get around the troublesome "split" or boundary between the quantum world of linear superpositions vs. our classical world of definite outcomes, which is what we actually observe. The standard interpretation of QM left this issue poorly-defined and not succeeding in removing the "split" (although the split was certainly flexible), and hence many physicists interested in quantum foundational problems have attempted to tackle the problem with the goal of eliminating the inherent dualism implied. In the famous words of physicist/philosopher Abner Shimony, the goal is to "close the circle". The "GRW" trio, hence, saw the current orthodox interpretation of QM as merely a set of "recipes" for describing the outcomes of experiments, rather than a truly satisfying explanation for how the objective macroscopic world we observe comes about naturally out of the QM formalism. They wanted to "close the circle".
The GRW proposal, therefore, introduced several modifications to the standard linear Schrodinger state-vector formalism with several goals in mind:
1) the "collapse" of a wavefunction was taken seriously- i.e., there is an actual act of amplification, leading to well-defined individual states- of live cats or dead cats, definite pointer states, objective macroscopic outcomes, etc.. In the GRW model, this "collapse" is configured to come naturally from the dynamics of the microscopic formalism itself. (Hence, the model belongs in a general classification to those theories which accept an "objective collapse" as a real event, which differentiates it from decoherence models...the latter seeing microscopic-type behavior still continuing into the quasi-classical realm);
2) a starting assumption was that any additional terms must nonetheless render the modification equivalent to the standard model of QM, since the latter's predictions for microscopic behavior have been proven over and over to be highly accurate. Hence, any new theory should not contradict standard QM statistical predictions, if it wanted to be taken seriously; and
3) the behavior of a macroscopic system - and this is the strong point of the GRW model - should be shown to arise naturally from its microscopic constituents, and should be consistent with what we observe in the dynamics of the classical world. The virtue of the GRW model is that it does so without any troubling superpositions of macroscopic states. By tweaking the microscopic formalism by a few terms to allow for true collapses of superposition states, subatomic processes lead nicely into what we actually observe in the macroscopic realm- i..e, definite states and well-defined outcomes. This desirable result seems to give the theory somewhat of a conceptual advantage over non-collapse theories such as environmental decoherence- the latter not providing any real explanation for how definite outcomes occur from merely a density-matrix mix. (However, perhaps quantum behavior continues forever! It must be mentioned here that with the success of recent experiments to reproduce quantum-like behavior in larger and larger macroscopic objects, the precise point where quantum behavior leaves off and macroscopic objects take on well-defined properties is still a thorny problem. Indeed, some have speculated that our perception of macroscopic objects with "definite" properties is a result of our own evolutionary development as humans learned to structure the world into familiar patterns...and hence our sense of "definite" objects may not be an objective feature of reality).
Be that as it may, the overall goal of the GRW proposal was to obtain a unified description of all physical processes - microscopic AND macroscopic.
While this is not the place to attempt a detailed explanation of the GRW proposal (not a task most of us are equipped for anyway, including me), let's quickly look at some of the logic behind it. As Ghirardi recounts it, his trio initially set about asking themselves what objectives they wanted to reach and what should be the characteristics of those objectives. As we saw above, any new model should not disagree with the well-tested predictions of the standard theory on microscopic processes, but the GRW trio also wanted to be able to reproduce the dynamic "reduction" processes at the macroscopic level. Since the existing Schrodinger formalism only describes a perfectly deterministic linear evolution, getting beyond this limited state of affairs to get to a truly-collapsed dynamic could be accomplished by looking at various ways to modify it. Somehow, abrupt non-linear stochastic processes (i.e., "collapses") should be allowed and included to alter the smooth evolution of the Schrodinger wavefunction, which would account for the definite outcomes we observe in the world around us. But what is responsible for these "stochastic" dynamics? Our illustrious trio of researchers looked- very logically - at the macroscopic world for clues. It seemed that "position" (spatial location) stood out as a true key. Since sharply-defined positions seem to be a primary characteristic of macroscopic objects, what seemed to be needed on the microscopic level was to view "position" as a truly objective feature (vs. other possible variables), and hence build a theory around position's seemingly privileged role.
Let's quickly look at how the world evolves according to the original GRW model. A quantum state of a system develops according to Schrodinger's equation. At certain randomly selected instants, however, this development is arrested and the quantum state spontaneously collapses into a well-localized state (we won't worry about later refinements here, such as collective density perhaps being a trigger). Again, a particle spontaneously undergoes localization in the sense that it experiences a "collapse" of the linear Schrodinger evolution, a spontaneous abrupt "reduction" takes place, and hence our (previously only approximate) position becomes definite. For a single particle the probability of such a spontaneous collapse is so low that, in practical terms, the predictions of the theory are the same as those of standard quantum mechanics. But for a macroscopic system- i.e., a system consisting of a very large number of particles, this spontaneous collapse becomes a rather frequent event. The definite results are due to a group dynamic of the localized particle being coupled with other particles. When particles couple together to form an object, the small probabilities of spontaneous collapse quickly add up for the system as a whole, since when one particle collapses so does every particle to which that particle is entangled. Naturally, each outcome is unique, just as in our macroscopic world, because each run of events is a unique combination and therefore each collapse-dynamic produces distinct macroscopic results. Thus, the GRW proposal can explain in a mathematically precise way why we often observe superposition interference effects when looking at isolated subatomic particles, but never observe macroscopic objects in superpositions. By appreciating the privileged role played by spatial positions and thus focusing on the possibility of true localizations, the GRW model gives us a logical picture of how the micro-world produces our everyday world of definiteness. GRW obtains an evolution for...
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