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4.3 Quantum Ontologies Now we come to problems in the philosophy of nature. There are five main ways of describing the nature of substances as revealed by quantum physics (assuming, as we did in chapter 1, that this is not a meaningless question).       1. We could follow Whitehead, or Bertrand Russell, and declare that there are no continuing substances, and that the only things in nature that definitely exist are events or processes. The world is not composed of definite material substances, we could then say, but is only `patterns of activity', or `energy in certain forms'.   2. A second view is to hold that substances are still really Newtonian corpuscles, but that they behave in rather peculiar ways which we just have to accept as `facts of nature'.   3. A third way is similar to the second, but holds that substances are really waves, albeit rather peculiar waves.   4. Copenhagen Interpretation with wave-particle complementarity.   5. Another approach is to take some of Aristotle's ideas more seriously, and build potentialities and/or dispositions into the very nature of substance itself.     6. Born's Statistical Interpretation of quantum mechanics (Born [1926]) does not allow us to apply the quantum theory to individual systems at all. All these different ways describe different ontologies: different possibilities for the individual things which exist in the quantum world. To the first approximation they all lead to the same experimental predictions, so we cannot distinguish between them empirically. Testable differences may arise later when they have been formulated sufficiently precisely, so in the meanwhile they can only be considered from the points of view of completeness, consistency, what could be called `naturalness'. By this term, I mean whether the subsequent features of the theories follow `naturally' from the natures of the entities hypothesised to exist.     1. Ontology of Events According to the first option, all that is in nature are events, such as collisions, particle decays, measurements etc. These events are at particular places and times, or else take place in some definite volume of space for some stretch of time. This gets around the problem of the nature of substances by not having any! It may seem a farfetched solution to the problem of substance, but this option is at least consistent. It does explain all the measurements and observations we may make, because they are all events of some kind. Where we might normally look for substances to which the events are changes, this option says that events just occur, without being changes in anything. The difficulty with this ontology is that it is not clear how events cause succeeding events. We could always postulate `physical laws' sufficient for the purpose, but physical laws talk about such properties as `energy', `momentum' and `mass', etc., and these have no counterpart in a world that is made only of events.   Whitehead [1929] originally had the link between events to be some process of `prehension' modelled on perception.   Stapp [1977] has tried to adapt Whitehead's ideas for quantum physics, but he has had to assume that there are `geodesics' between events. `Each geodesic is associated with real mass m , and also with a momentum-energy vector p=mv, where v is the four-velocity defined by the direction of the geodesic.'4.2 This is to be regarded `as a mathematical condition on the overall spacetime structure of what emerges from the process of creation', but seems to admit ideas from classical physics in an ad hoc manner that is not compatible with the original ontology.     Cramer [1980, 1983, 1986] has proposed what he calls the `transactional interpretation' of quantum mechanics. This uses primarily an ontology of events, but again supplemented by links (here, waves) that propagate between events and carry energy and momentum. What is novel in this interpretation is that the waves propagate both forwards and backwards in time, and set up `transactions' between cause and effect events. However, these transactions cannot be set up in any temporal sequence, but must be `in place' before any single event can actually occur.   This requires a `block universe' theory of time (see section 2.3), in which all future events are definite even at the beginning of the universe, and according to which nothing new becomes true when events (appear to) `happen'.     2. Ontology of Particles The third way of dealing with the problem is that taken by many scientists today.   As Feynman puts it4.3, `quantum [theory] ``resolves'' this [problem] by saying that light is made of particles (as Newton originally thought), but the price of this great advancement of science is a retreat by physics to the position of being able to calculate only the probability that a photon will hit a detector, without offering a good model of how it actually happens'. That is, partly because of the intuitive power of using images of corpuscles, quantum entities are assumed still be be particles, but with rather unusual properties (to put it mildly). The laws governing these particles must predict the interference phenomena and non-local correlations described above. The present laws of quantum mechanics would have to be regarded either as purely statistical laws, or as laws using some new kind of logic to be called `quantum logic'. If Born's statistical interpretation were adopted, then quantum mechanics cannot say anything about individual systems.     The way is now open to imagine some new `sub-quantum' level, containing (as yet) hidden variables, so that quantum probabilities are the result of fluctuations at this new level.   Belifante [1973] surveys some of these theories, and Bohm4.4 [1980] gives more details of a proposed sub-quantum level that is essentially classical in detail.   If a new `quantum logic' were adopted, then it is possible to keep an ontology of particles like corpuscles with definite properties, even though we are limited as to the inferences that we can draw. Quantum logic is usually `non-distributive', in that the distributive law of Boolean logic is now no longer valid.   Gibbins [1987] surveys the methods and scope of quantum logic. The logic has the disadvantages (or features) that truth tables cannot be used to check deductions, and that there is no scale (such as a Planck's constant) to make a gradual transition to the approximately classical world of macroscopic bodies.     3. Ontology of Waves The fourth approach, which takes waves to be the ultimate substance, is also followed by a number of physicists. They like the idea of a `universal field' of which all particles are simply localised concentrations.   The earliest proposal along these lines was that of Schrödinger, using the wave function $\Psi$ of his formulation of quantum physics. In his view, physical reality consisted of waves and waves only, and he denied categorically the existence of discrete energy levels and quantum jumps. What appeared to be localised particles are really moving `wave packets', or localised concentrations of waves. This view has the difficulty, however, that the wave packets do not remain localised after any length of time. In rectilinear unperturbed motion, for example, the different frequencies in the wave packet move at slightly different velocities, so the wave packet will eventually become more and more spread out.   After any kind of interaction, moreover, Schrödinger's equation requires that the wave packet can be split into parts which are moving in completely different directions.     These alternatives remain as a superposition, leading to the question of whether there is ever a `reduction of the wave packet' that brings back a localised wave packet again.   There is also the difficulty that the wave function for an n-particle system is defined over a `configuration space' of 3n dimensions, not over real (three dimensional) space. This allows interacting particles to be correlated with each other because of previous interactions,   and prefigures the nonlocalities shown by Einstein, Podolsky and Rosen. It is now difficult to interpret the wave functions as real entities in physical space. One modern variant of this `wave ontology' solves the problem of the `reduction of wave packet', by assuming that it never occurs!     This is the `many worlds interpretation' of quantum mechanics, as proposed by Everett, Wheeler and Graham4.5. According to this interpretation, the Schrödinger wave function describes everything that there is in the world. If it splits up into two or more alternatives, then the whole universe (or at least some part of it4.6) is duplicated as many times as is necessary. And this happens at every interaction: the theory is generous on universes! The `benefit' of believing in this extravagant production of universes, is that there is then no need for any `reduction of the wave packet', and there are then fewer physical laws in the quantum world (only, there are more worlds!). Even if these difficulties were solved (or ignored), there are still problems in the philosophy of nature, as it is not clear however what exactly is `waving' when a Schrödinger wave or a field goes by. This is the problem of substance again. It cannot be that a `wave function' makes up the physical world, since a `function' is a mathematical rather than a physical entity.       4. Ontology of Wave-particle Complementarity There are troubles with both the third and fourth approaches, as it turns out nature does not behave as we would expect if it were only particles, or as it would if it were only waves. Niels Bohr therefore proposed his idea of `wave-particle complementarity', which is that nature alternates between being like particles and being like waves, according to what the experimentalist chooses to measure. Both particles and waves are definite things that Newton knew about, but neither of these concepts by itself is adequate to describe the nature of quantum substances. Bohr, and Heisenberg later, want us to take what amounts to a strange and unpalatable mixture of the two concepts.         5. Ontology of Propensities The second choice has that the ultimate substances are really forms of potentiality, or `forms of propensity' to give a more commonly used description. Instead of a corpuscle we have a `packet of propensity', or `propensiton', following N. Maxwell [1982, 1988]. This is a extension of Aristotle's approach, if we take `propensity' to be his underlying matter, so physical objects are propensity in various forms. With this choice it is then not surprising that in the quantum world there are very few properties that always have perfectly definite values, and that there is no such thing as a definite corpuscle. I will describe Maxwell's ideas in more detail, because they are not as widely known as those above, and because they parallel many of the ideas I will be presenting later. According to Maxwell [1988], quantum substances are discrete propensitons. These `only evolve probabilistically intermittently in time, when relevant physical conditions arise, the values of propensities (or the states of propensitons) otherwise evolving deterministically'4.7. Specifying the nature of a propensiton amounts to specifying the laws governing these two kinds of time evolution: The evolution of genuinely (discrete) propensiton die would have to be conceived of in something like the following terms. The propensiton die is tossed. As the die flies through the air it is gradually into six potential, virtual, ghostly dice, each with a different face uppermost, each with a different (probability) density (all equal in the case of unbiasedness), which may very well vary with time. When the six potential dice hit the table top, five vanish and one solid die remains. If the die is tossed repeatedly, the statistical outcomes are determined by the probability densities of the six virtual dice just before contact with the table top.4.8 The position probability density can easily be postulated to be variable in space -- even in a wave-like way [e.g. according to Schrödinger's equation]. If the conditions for probabilitistc events to occur are modified, it would even be possible to create a possible kind of propensiton which is such that an ensemble of such propensitons, passed through a two-slitted screen, creates an interference pattern of the kind created by electrons or photons.4.9 The `relevant physical conditions' for probabilistic events to occur are that the wave function splits up into two or more alternatives, and that there is an inelastic energy difference for some system between the alternatives. This will be discussed again in chapter 12, and amounts to one solution of the problem of measurement in quantum physics.

    

Author: I.J. Thompson (except as stated)

Email: IJT@generativescience.org