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Transcript of the Philosophical Implications of Quantum Mechanics 22/6/04 This week I want to look at the main interpretations of Quantum Mechanics and the philosophical issues raised by them. This will mainly consist of conceptual and metaphysical problems, but due to popular demand I’ll also say something about the ethical implications of the interpretations (if there area any). But before I do this though it will be necessary to for me to give a brief overview of the emergence of QM as the dominant paradigm within Physics. Historically QM emerges as a response to the suggestion by Max Planck that energy came in packets or quanta. This being the only logical conclusion possible following the observed emission of heat from black boxes. Classically a black box was an idealised thermodynamic object that was supposed to emit energy as continuous electromagnetic radiation, but Plank found that in reality it only emitted energy discontinuously, in certain small, quantifiably distinct emissions or quanta. From this he concluded that energy was released in the form of packets, or particles, in other words as photons (the smallest packet being a single photon of variable energy). This insight led to a Noble prize for Planck and Niels Bohr’s development of the modern theory of the atom, complete with its corresponding electron energy levels. This formal quantification involved something called Planck’s constant, a number that proportionately related the energy and radiant frequencies of all possible energy quanta. From this observation it was possible to conclude that, as Newton had insisted, light itself consisted of particles, an identity that had always been controversial due to its parallel wave-like properties, but now these so called ‘wave descriptions’ could be reduced to particle descriptions via Planck’s equation. De Broglie declared that the wave description was in fact an illusion, produced by the photon’s rapid wave-like trajectory under the restrictions of Planck’s equation. Similarly Schrödinger calculated that the possible orbits of an electron around a nucleus (its energy levels) could be understood in terms of the wave-like path of the electron in its orbit (simply put, only whole numbers of waves fitted into an orbit were allowable. The same restriction being applicable to the path of a photon between source and target). The particle nature of light was further supported by Einstein’s claim that photoelectric effects (the production of electricity from light) could only be explained by photons of the appropriate energy punching electrons out of their orbits, all in accord with Planck’s theory and Bohr’s model atom. For a while everything seemed straight forward. Unfortunately light wasn’t playing ball, and it continued to demonstrate the properties of a wave, with some of these (such as interference and tunnelling) only explicable in terms of a wave theory rather than as particles moving on a wave-like trajectory. Physics was in chaos. It seemed that all the evidence pointed equally to light being both a wave and a particle. For a while the dual existence of a guiding pilot wave and a guided particle was seriously posited, even though this infringed the convention of parsimony (and arguably any kind logic). But soon this explanation was to be superseded. The next puzzling discovery was Heisenberg’s Uncertainty Principle. This was derived from the fact that while it was long known that certain properties of subatomic particles could not always be measured with accuracy, it came as a surprise that certain paired, and arguably mutually exclusive, properties, such as momentum (a dynamic measure) and position (a static measure), or paired up and down particle spin, were closely linked in their uncertainty, and in way oddly proportionate through Planck’s constant. This meant the more we knew about one the less we knew about the other. But what this means is itself uncertain, initially it was thought that it was a physical problem that reflected the fact that photons of various energy bouncing off the particle under measurement effected their position (the Planck correlate being the indication of this). However, despite still being given as an explanation in some text books, this was soon found not to be the case. While this explanation may intuitively hold for position and momentum, it cannot hold for other pairs such as time and energy or the polarization of light. As no other inference seemed possible the only conclusion was that the uncertainty of the properties was not an epistemic or practical problem but was actually a metaphysical or ontological uncertainty, that is the properties didn’t definitely exist until measured, and this measurement made the other property in the pair even more ontologically vague. To many this seemed absurd, but when experimental science achieved the technology to examine the quantum world more closely, and novel experiments such as the triple polarizer were set up, it was found to be almost certainly the case. Though as we shall shortly see die hard realists continue to deny this possibility despite all the apparent evidence for it. The classification of such paired properties included the position and momentum; horizontal and vertical polarisation of light; and temporality and energy state of a single particle, and the up or down spin of paired particles. Neil’s Bohr called these pairs kinematic-dynamic complementaries. Declaring that none of these properties actually existed in any defined way until measured. They were not properties of the object observed, he insisted, but rather properties of the relationship between the measuring device and what was being measured (whatever that meant). Bohr, and others, further claimed that it followed from this, and the theoretical problem with the ontology of light, that another mutually exclusive complementary relationship existed, one between the wave and the particle nature of light. This was thought to be closely related to the kinematic-dynamic complementaries, in that momentum was a wave property and position a particle property, making them both mutually exclusive. As Bohr had also argued that light was not only both a wave and particle phenomena, but that it could not be both simultaneously, as that would result in a direct logical contradiction, and was also demonstratable by the fact that momentum and position could not be measured simultaneously in the same experiment. It followed from this that all quanta, including electrons, also had a dual nature. He thus explained the Uncertainty Principle itself in terms of the dual nature of quantum objects. The actual situation was later shown to be more complex however, for while the idea, of a quanta not having no definite state before measurement, was proven by the triple polarizer experiment, another recent experiment has allegedly demonstrated a simultaneous wave and particle nature for a photon at a single stage of an experiment. But I shall return to that and the problems it raises later. Initially all this was seen by most scientists and philosophers as crazy speculation within a fringe branch of physics, and probably wrong. Particularly as their was no real deterministic theory, with a solid mathematical base, that could describe, and at least partially predict, all of this in a satisfactory fashion. This was to change with the development of Quantum Theory and its mathematical formulation. The maths itself is somewhat complex, and I’m not a mathematician, but I’ll attempt to give an outline of what is involved here. The key to this was the discovery that associated with Heisenberg’s Uncertainty Principle was another factor, non commutability. Non commutability means that a x b is not equal to b x a (contrary to normal mathematics), specifically within Heisenberg’s equation momentum multiplied by position gives a different result to position multiplied by momentum, why is still a total mystery, but gave him a clue in finding the mathematical basis for quantum mechanics. The only form of mathematics Heisenberg knew to be non commutative was Matrix math, the involving the formulation of arrays of numbers. By representing the various states of a quantum system in terms of tables or arrays, as multidimensional vector space matrices, Heisenberg found he could model all the phenomena seen in quantum experiments and formulae, including most importantly their non commutability aspect. There was now a mathematical theory in place that could model and partially predict quantum events which Heisenberg called Matrix Mechanics. The problem was that in incorporating the Uncertainty Principle it predicted several possible outcomes for an experiment in terms of probability, which while in accord with Heisenberg’s discovery in terms of complementary pairs was hard to square with reality as the equation was supposed to be a description of what actually happened in the world. Similarly Schrödinger developed his wave theory to describe the totality of quantum phenomena and discovered something similar. That while a definite state of a quantum system could be described in terms of a wave function, what actually matched experimental results was a superposition of two waves overlaid on each other, with only one of the two waves representing the actual outcome according to certain probability weightings. In other words two quantum states were superimposed on each other, until a measurement was made, meaning for instance a given particle had both mutually incompatible up AND down spin, before it was measured to have down spin (an event with a 50% probability), in which case it only had down spin. This was no surprise to Niels Bohr who had expected this kind of result, though he insisted it was meaningless to talk of a system actually being in both states, and so Schrödinger began to see his waves as not being the physical energy or matter waves postulated by de Broglie but as probability waves. Each mapping out the possible states of a quantum system. Soon after, the mathematician Hilbert was able to demonstrate that Matrix Mechanics and Wave Mechanics were in fact mathematically identical. Later Von Neumann produced an even more influential mathematical model for quantum systems based on Operator algebra. An Operator (mapping a vector or property) was something that caused a change in a mathematical Function (here the Quantum State), and like Matrices were the only other mathematical entities to be non commutative and so suitable as a mapping tool. This system was similar to Matrix Mechanics, in that it mapped states in vector space, and also mathematically equivalent to it. It described quantum systems in terms of potential Eigenvalues each with possible Eigenstates, equivalent to the superposition of wave functions, when a definite Eigenstate was produced from a set of Eigenvalues it was said to have been projected out of the equation by an Operator. It was easier to handle and modelled quantum states more naturally as dynamic states evolving over time, and so became the dominant model of QM. This form of mathematical modelling has proven to be the only scientific model to exactly reflect the behaviour of its corresponding physical reality (other classical models, such as Newton’s equations of motion, being only approximate reflections of the reality they attempt to map). In addition the Quantum Mechanical model has been used in a wide variety of technological applications and proved totally reliable. Therefore, while some claim not all of its predictions have been experimentally demonstrated, the fact that it perfectly describes phenomena that have been observed is taken as evidence for it being the most successful scientific theory to date. This is unfortunate because it is totally counterintuitive, describes bizarre illogical phenomena and cannot be visualised. What was left to complete QM was its interpretation, and this is where the problems begin. Not everyone agreed that interpretation was necessary however, Richard Feynman counselled that QM was totally incomprehensible to everyday reason and so Physicists should give up trying to interpret and stick to calculating. In other words to be scientists rather than philosophers. This very incomprehensibility can be taken as further proof of a Kantian take on science, that is that our ordinary theories and models are conceptual tools rooted in our own cognitive abilities and mental categories rather than mirrors of reality, and reality itself while corresponding in some way to these models was not necessarily knowable to the human mind. Mathematics being the only way to reach it. Despite this Feynman did come up with a very interesting insight into the nature of the Quantum Reality, though he described this in metaphorical terms rather than theoretical ones. Basically he agreed with Bohr that quantum states were in some sense unreal until measured, therefore when a particle was confronted with two slits in a screen it actually went through both simultaneously (Bohr differed slightly on this point arguing it went through both and neither and each one separately, as such talk was meaningless). It followed from this Feynman claimed that if a screen had ten slits in it then the particle would follow all ten paths simultaneously and if there was an infinite number of slits there would be an infinite number of paths taken. In reality in a situation with no slits, a normal distance between two points, there were effectively an infinite number of paths, therefore a particle took every possible path between two points. However most of these paths mutually cancelled out the only one that didn’t being the most direct path. But oddly the particle on this direct path could interact with itself on any of the other paths! What this meant was unclear Feynman simply said it ‘smelled’ out each possible path, taking each before deciding to take whatever path was the most direct, so while only one path was empirically measurable the other paths were equally real. It was Niels Bohr who created the dominant interpretation of QM however, the one which became known as the Copenhagen Interpretation. In its simplest form this states that the wave function with its superpositions is the most useful model and should be seen as a probability wave mapping out all the possible states and trajectories of a particle or wave. Bohr developed his notion of Complementarity to flesh this out further, arguing that it was impossible to measure complementary concept pairs simultaneously (be they position and momentum, or wave and particle) because these were logically incompatible concepts that were both equally necessary to describe the reality of a quantum object, even though neither actually mirrored it. They were he said technically jointly necessary for a complete description but logically incompatible, or in short were mutually exclusive but jointly complete. Some argue that this is a completely contradictory notion, but in fact contradiction was the very thing that Bohr was trying to avoid in his descriptions. He sought two self contained descriptions or acts of measurement that gave totally incompatible results but that were both equally true. Contradiction didn’t arise because these two were never mentioned in the same description. What Bohr seems to be getting at is a Kantian point that these descriptions are mental constructions that imperfectly match reality and that the notion of contradiction is not something that exists in the real world but rather is a feature of our consistent representation of it. So while he saw non-contradiction, and so meaning, as an important feature of our coherent representation of the world, he did not extend this to reality itself. Naturally this was an anathema to Realists who believed non-contradiction to be a feature of the world itself, Bohr in turn may have regarded this as Platonistic. The issue is not clear-cut and perhaps is something we can discuss later. A side effect of this kind of thinking is the possibility of incompatible theories both being true if they complement each other and are supported by evidence. Thus Bohr was not only able to support Ernst Mach’s views on a neutral monism beyond incompatible foundationalist theories of mind and matter (that is idealism and materialism), but to also support the theory of vitalism alongside more conventional theories of mechanistic biology. A deep problem for Bohr today is the fact that experiments have now detected light behaving as both a wave and a particle simultaneously, an apparent descriptive contradiction in the making. Though the interpretation of this experiment is itself open to question. An even more controversial aspect of Bohr’s interpretation was of the nature of measurement, which he insisted defined the actual parameters of a system non-existent before the measurement. This was formulated most strongly by Von Neumann who invented the concept of ‘wave collapse’ or the ‘projection postulate’. What Von Neumann realised was that the wave equation with its superposition of states described all the possible outcomes as equally real, but experience told us that after measurement only one state was actually real. He therefore argued that the wave function didn’t hold after measurement it in some way ‘collapsed’, loosing information and projected the actual result into the world. Therefore physics was incomplete it needed measurement to make it so. For this reason Bohr claimed that a complete physical description was only possible when a measurement was made, therefore science was not describing a set of objects with inherent properties but a set of relations between measured objects and measuring systems and the properties which emerged were properties of those relations not the objects under consideration. Light was neither a wave or a particle these were concepts produced by the way we detected objects and built up concepts from this data, the real objectivity of light was something other. Thus Bohr proposes a kind of inter-reduction in science in which bridging laws, in the form of translation equations, can convert descriptions of light for instance from wave theory talk to particle theory talk, without claiming that either is fundamental or ontologically real, and admitting that under certain circumstances translation may be impossible. Some interpret Bohr as a mild idealist who was claiming that the act of measurement gave reality to light, that the description was the reality, however it seems to me that he was describing a phenomenal reality rather than a noumenal one. Unfortunately Bohr lacked the philosophical training to clearly formulate what metaphysical point he was really making. But either way his views were an anathema to Realists. More problematic than this ideological prejudice is that Bohr inherits all the problems associated with Kant’s philosophy, particularly the connection between phenomena and noumena. Even more heretical to realists were the spin on Bohr’s ideas given to them by the Princeton School around John Wheeler. Wheeler correctly maintained that the term ‘measurement’ was far too vague to be included in a scientific theory, and needed sharpening up. He argued that measurement was inseparable from observation and so it was therefore human consciousness that led to the probability wave to collapsing into a single reality. This meant the physics was incomplete without consciousness of somekind. Naturally this was unacceptable to most Realists as they argued that this was absurd given that consciousness had evolved after the physical world came into being, a physical reality that needed to be complete in order to produce consciousness. Wheeler gets round this by invoking backwards causation and claiming we create our own past through observation in the present. This causes certain problems for our notion of time but is not quite as daft as it sounds. Another factor that supports it is the conclusion of recent philosophers of mind that consciousness is a primitive essential feature of the universe that is not reducible. How this would exactly fit into Wheelers interpretation of QM is uncertain however. Many people reject Wheeler’s interpretation as they think it leads to Idealism and from there to the God hypothesis. But this is far from clear, for one the Observer Effect does not have to create reality it merely collapses a more complex reality into a simpler one, and even id Idealism did underlie quantum ontology, this would not necessarily lead to an external deity, who after all if quantum reality is the norm would have to be seriously disturbed by human standards, but to the almost deification of human consciousness. Though this in itself is as unacceptable to the religious as it is to the materialistic. The Measurement Problem as it is known is not only solvable through calling on consciousness however, and most physicists would reject this interpretation. The real question is what is it about measurement that causes wave collapse. It might be the observation component, but more physicists believe it is something prior to this, found in the act of measurement itself. But given that we have already rejected the disturbance theory of measurement with regard the Uncertainty Principle, what could this be. One theory suggests a disturbance of a more subtle kind, such as decoherence theory. This is a complex notion but basically it says that reality is in fact quantum mechanical and Schrödinger’s cat really is dead and alive in some sense all the time. However when we limit the information available about a quantum system it seems to behave like a classical system. Thus because we cannot know the exact quantum state of a cat (it being too complex) it appears to us in a more simple way as a classical object that is either dead or alive. However a simple quantum system like an electron pair is knowable in its true quantum state of superposition, but when we make a measurement we introduce it to a complex system of which it becomes a part and thus reverts to a classical system. Thus the measurement appears to collapse the probability wave. But the problem here is obvious, the wave isn’t really collapsed so the world is till really in a superposition and the classical world an illusion. This seems absurd, but the only way out of this problem seems to be to claim our perception of reality changes it, which leads us back to the problematic role of consciousness. We can’t decide what aspect of measurement is responsible for the wave collapse however, due to the limitations of QM its actually impossible to find an observable difference that would distinguish between an observer effect and a decoherent measurement effect and so the difference is academic and we are left in a philosophical dead end here. If some evidence of the role of consciousness in the world was demonstratable (such as the alleged effect of consensus belief on the crystallisation of glycerine documented in chemistry journals at the turn of the century) we might be able to conclude in favour of the observer effect, but another problem stands in the way of this, the problem of scale. It is commonly accepted that quantum uncertainty only operates at the microscopic levels and not macroscopic levels. But why this should be is actually a mystery. The orthodox explanation was once the statistical law of scale, while weird non-classical phenomena may happen at the quantum level, such as the brief appearance of a photon (or even an electron) from nowhere, due to the uncertainty relation between energy and time, this was such a rare and isolated occurrence that it is swamped by more common classical behaviour observable at the macro level. However this not now widely accepted, one reason for this is disagreement over the interpretation of probability involved, or the nature of the quantum state. It is quite possible to define the quantum state of an insect given enough computing power, and given that it is not beyond the realms of probability for that insect to exist in two places at once, given it has a quantum state no different to that of a microsystem (it is just very unlikely given classical statistics). This seems too leaky for some, but there seems no easy way to limit the quantum indeterminacy of objects to the micro level, and what’s more even if there were, given complexity theory, it is quite possible to conceive of a situation (not unlike the butterfly effect) in which a quantum fluctuation in a complex macrosystem effects the entire structure (the loophole Roger Penrose uses to import Quantum phenomena into the classical world). It is here that decoherence theory becomes useful again. Many now appeal to this theory to explain the cut off point between the quantum level and the macro level, simply based on the amount of information needed to define a quantum state. As we have seen we simply don’t have enough information available to see a macro object as anything other than a classical object, so for us that is exactly what it is. But here we return to the same problem as before, either this means our perception changes the nature of the reality of these objects or our perception is an illusion and the objects really are in superposition all the time. This last point leads many physicists and philosophers to reject the idea of the wave function collapse out of hand, and to claim the function remains true of the world at all times. The strong interpretation of this is that the uncertain properties really do have a definite state it is just that we do not know it. A compelling argument for this is science can not contain something that has not been observed under experimental conditions, and while the wave function has been invariably correlated to observed phenomena no one has ever seen a wave collapse, or even understands what it might mean. The simplest way of denying the wave collapse is through the postulation of the ontological reality of the quantum world. The most conservative form of which is Karl Popper’s Propensity Theory. Popper argues that the quantum world is relatively normal in that there really are definite electrons with real properties which only pass through one slit in a screen etc. The only oddity is the existence of a propensity field which influences the behaviour of the particles, a phenomenon we see as probability. Thus in an interference experiment each photon travels through a different slit to its predecessor in a way that builds up an interference pattern, all under the influence of probability. Most philosophers regarded this idea as crazy, as it incorporates a new force into physics, the force of probability. Something that seems an unnecessary addition to the say the least. But taking the theory at face value we can say it allows us to make sense of QM by postulating an underlying reality not described in the wave function and regarding the quantum equations to be mere probability calculating tools rather than reflections of reality. Thus a weak form of Copenhagenism can be maintained in that the wave function accurately describes an abstract world of possibility. This kind of theory is thus called a ‘hidden variable’ theory. Another kind of hidden variable theory is that of David Bohm. Bohm’s ideas are highly complex and not a little vague, but what he suggests is essentially a reworking of de Broglie’s pilot wave theory into a field theory, for Bohm particles are real entities in an ‘explicate order’, but which emerge from and are guided by a universal field which constitutes an ‘implicate order’. Some properties are explicate in that they belong to distinct particles, other properties are implicate in that they belong to the universal field and not to any isolated particle. Through this shared field every event is thus implicated in every other event and a Realist holism emerges. Details of what this precisely means are sketchy however. All these hidden variable theories however have been disproved by the violation of Bell’s inequalities as predicted by the famous theorem of John Bell. This theorem was literally a test for hidden variables, we do not need to go into the details here other than to say Bell set out a series of conditions, technically referred to as inequalities, that applied if an underlying reality in accord with normal concepts of probability also applied. In other words if a real underlying reality existed and the wave function really was a tool for calculating the probabilities of events within the context of this reality, then the Bell inequalities applied and could not be violated. A few philosophers argue against this view, but the vast majority except it. Unfortunately for Realists Bell’s inequalities were violated in the famous Aspect experiment. This demonstrated the apparent non-locality of QM, in that an entangled pair of particles were shown to influence each other instantaneously regardless of their distance. If one of the pair was measured to have an up spin then though quantum mechanical laws the other particle instantly acquired a down spin even if it was a light year or more away. This for one violated Relativity Theory in demonstrating instantaneous effect or non-local causation something impossible if an underlying physical reality underlay quantum reality. Thus it was concluded that there was no underlying physical reality in this sense. This sunk most hidden variable theories, but Bohm claimed immunity through his claim that while position was a real property, determining a particle as real, other properties such as spin were field properties and not real in the everyday explicate sense. Thus Relativity and Bell’s inequalities were not violated. This is interesting but Bohm’s theory is not explicit enough to elaborate on this point and so smacks of having a cake and eating it for most people. The Realists did not give up here however. They used the results of the Aspect Theory to add to the list of evidence of an incompatibility between State Reduction theories (wave function collapse) and Relativity theory, as the former happily accepted Non-locality as a consequence of their understanding of quantum mechanics, namely that state reduction operated as if space-time did not exist. Rejecting both hidden variable theories and state reduction theories the Realists thus developed the most conservative approach to QM yet devised, Relative State theory. Relative State theory was first set out by Everett in his famous paper on the topic. Unfortunately his definition of it was not precise enough to determine what it actually entailed and there are currently five different interpretations of it, these include two so called many-world interpretations, a many-histories interpretation, a manyminds interpretation and a modal interpretation. The essense of Everett’s thesis is that the wave function accurately describes the state of the world and never collapses, instead what happens is that we only experience one of the possibilities described. Supporters say this is the only interpretation that followers directly from experience and the quantum maths itself and imports no other concepts, therefore by Ockham’s razor must be true. However what this interpretation consists of is problematic. The classic answer is attributed to de Witt, who simply says it means that the universe splits into multiple worlds when a measurement is made and the observer splits with it, so that we are just one observer in one branch of this hypothetical multiverse measuring just one possibility indicated in the wave function. This has inspired many good science fiction stories, but what it entails is too much for most philosophers to take seriously. To start with and ignoring the obvious extravagances of the thesis, what does it mean to say the universe splits? No one has ever observed the universe splitting, so why should we include it as a postulate if we are to exclude other unobserved postulates like wave collapse. Further more it totally disrupts our concept of time, in fact time does not exist under it, either as a continuous dimension in our universe or as a flow, all that remains is continual division and branching of the universe. But most serious of all it infringes the very basis of science, which is supposed to be about producing generalisations that correspond to the observable universe as testable in a laboratory. In contrast the manyworlders are claiming that the wave function applies in the most part to other universes not in contact with ours, that cannot be tested in the laboratory. To counter these claims David Deutsch came up with a version of manyworlds state relativism that allowed interaction between worlds, arguing that this was demonstratable in quantum physics. He thus argued that many-world theory could be experimentally distinguished from state reduction theories through his famous computer-brain idea. What happens here is that a computer-brain observes the quantum world and records a single measurement, if state reduction holds this process is irreversible as one option now no longer exists, but if the universe has split both options are available in that the two universes can be made to interact through interference so that the quantum state is rescrambled and the branches recombine. The computer-brain thus returns to its prior state but has the record of an earlier single state in its memory. The manyworlds theory is thus confirmed. This is clever but is unfortunately totally unsound philosophically. The very idea of two branches of the universe, two essentially separate universes by definition, interacting is a contradiction of terms. Further more in what dimension are these universes supposed to branch given that only three dimensions of space seem possible under existing physics. To solve this problem many philosopher-physicists talk of a manyhistories hypothesis. In this version of state relativism there is not a branching of worlds but a focusing on one history. This is similar to Feynman’s ‘sum of trajectories’ hypothesis, in which a particle takes all possible paths but only one becomes actual, but unfortunately is equally unclear. The argument goes that all the possibilities in the wave function represent equally real histories, but that we only experience one of these. Why varies between versions of this idea, some say that only logically consistent histories are stable enough not to mutual cancel, others that we only have the capacity to perceive one of these histories. The latter version is the most elucidated in that it calls on decoherence theory again to show that it is our lack of information that causes us to perceive the world classically and be ignorant of its quantum nature. But this only recalls all the problems with decoherence theory, either it is saying that the classical world is an illusion, which seems too weak, or that our perceptions effect reality, which is too strong for most and undermines the Realist agenda. Those that think the classical world is an illusion have developed the many-minds interpretation. Here it is not the universe that splits but the mind of the observer. Thus our local conscious mind is only away of one possibility, where as all the others still exist but are only accessible to unconscious parts of our mind to which we do not have access. The problem with this is that it seems to work by shifting all the philosophical problems away from quantum physics and into the still unresolved theory of consciousness. A kind of sweeping under the carpet. What a theory of consciousness would be like if the many-minds interpretation held is incomprehensible to many. For these reasons it seems all the candidates for relative state theory are flawed. However one possibility remains, the modal interpretation of the philosopher Baz van Fraassen. Here it is suggested that the possibilities described by the wave function are modal states rather than real ones, in that they describe possible worlds. What this means is that probability is simply that which governs which world becomes the actual world. This sounds plausible, though invokes worries about a return to Propensity theory. More seriously it is contrary to experimental evidence which demonstrates the various possibilities in the wave function can interact with each other. What this would mean are that the other possible worlds are in someway real. If we want to avoid a collapse of this position into a many-worlds theory, we thus have assume two levels of reality a potential reality, a kind of ghost world, and an actual reality. The actual reality is the one we experience on measurement. The problem with this approach is that it seems to be equivalent to a version of Bohr’s theory. One last attempt to come up with an interpretation of QM is known as Quantum Field theory. The motivation behind Quantum Field theory is the reconciliation of QM with Relativity theory. But it also has other secondary effects that resolve some of the problems of QM. There are many versions of Quantum Field theory one of the most interesting more or less dispossesses of particles altogether, and postulates that each point in spacetime is a quantum system in its own right. Thus the ontology is of a universal field and particles are explained as excitation states in the point systems, that migrate across the system as waves in the greater field. This is not only appealing in that it solves the wave particle duality problem, but in that it reconciles wave mechanics and relativity equations. There is still a problem in that any wave collapse cannot be reconciled with Relativity, but some seek to solve this by again denying the existence of the wave collapse, thus adopting another conservative account of the formalism of QM. This is an attractive option but it has a lot of technical problems. Paramount amongst these is its complexity, many versions of it are too complex to practically compute, and those that are seem to imply the denial of ‘infinities’ through a ‘renormalization’ process and other mathematical ‘fiddle factors’. Even if these technical problems can be overcome Quantum Field theory doesn’t solve all our philosophical problems. While it removes wave particle Complementarity, the dynamical forms of Complementarity involved in the propagation of energy within the field remain. That is the wave trajectories within the field (what ever it turns out to be) are still in superposition, which is a particular problem if we deny wave collapse. Therefore many of the philosophical problems associated with superposition remain. To end this week I want to say a little about the ethical problems effected by our interpretation of QM. I will say more about this next week but I think it will be useful to give a brief precursor tonight. Basically the issue is one of free will. Obviously the exact problem depends on our view on moral philosophy itself, which would demand a separate series of lectures in its own right, however I will simplify things by stating that in my opinion the only issue in ethics that really matters is the exercise of individual free will, without which there can be no morality at all. The problem with some versions of QM (for instance the many-worlds hypothesis and Bohm’s theory) is that they are strictly deterministic. They allow no room for free will, thus if we adopt them we have to give up all conception of morality. While the indeterministic versions do not necessarily help with free will as they are based on random chance rather than choice, some varieties of QM are distinctly helpful. In particularly I am thinking of those interpretations, such as Bohr’s, which involve a Kantian view of the world. It is well known that Kant’s philosophy is amongst the few philosophies that actually allow for free will to exist. Thus if we want to retain our moral view of the world we need to hope that the Kantian versions of QM turn out to be true. I shall elaborate a little more on this next week, and comment on other aspects of morality effected by QM, when I look at the larger picture of Realism and QM and speculate on what actually may be going on in the world. A summary of interpretations (from Wikipedia) Interpretation Deterministic? Waveform One real? Universe? Avoids Avoids hidden Local? collapsing variables? wavefunctions? Copenhagen interpretation (Waveform not real) No No Yes Yes No Yes Copenhagen interpretation (Waveform real) No Yes Yes Yes No No Consistent No No Yes Yes Yes Yes Histories Consciousness causes Collapse No Yes Yes Yes No No Everett manyworlds interpretation Yes Yes No Yes* Yes Yes Bohm interpretation Yes No Yes No No Yes Also in 1927 Bohr stated that space-time coordinates and causality are complementary