QUANTUM PHYSICS AND PHILOSOPHY

QUANTUM PHYSICS AND PHILOSOPHY Special invited article published in the December 2005 issue of Computer Society of India Communications (pp. 22-28)

Ravi V. Gomatam, Ph.D. [email protected]

INTRODUCTION Philosophy used to be the primary mode of inquiry in pre-scientific times. However, since the 17th century, science has replaced philosophy as the primary source of our knowledge about truth and reality, at least as far as the natural world is concerned. But philosophy and science are still intertwined. In this article, we shall examine how modern physics, particularly quantum physics, might yet require solving a major philosophical problem for its own further fundamental progress. Physics, like all fields of science, is tied to observations. These observations, made either in the natural world or in the laboratory, are but our interpretations of sense experiences, and they must necessarily be reported using ordinary language. Physicists, like everyone else, do not question the validity of ordinary language when used to report their observations. The attitude that ordinary language description of experience is in fact a description of the world is called “naïve realism.” There is an entire branch of modern Western philosophy that is devoted to critically examining the assumptions behind the everyday language we use to describe the macroscopic world in which we live and the validity of naïve realism as an adequate description of the world. This branch of philosophy is called “ordinary language philosophy.” Surprisingly, it has something in common with quantum physics: insight into the inadequacy of ordinary language to describe observable reality. It is this connection between ordinary language philosophy and quantum physics that we shall explore in this article. In the process, we shall also offer a basic introduction to both basic philosophy and basic quantum theory.

A Brief Introduction to Contemporary Analytic Philosophy The start of modern Western philosophy is rooted in the powerful realization, due to the sixteenth century French philosopher and mathematician Rene Descartes, that ordinary language can describe only our own private experiences, not directly the underlying external reality. Consider what we could mean when we say “I see a chair in front of me.” While we believe we are seeing a chair, all we can and do see is the light reflected from 1

it, not the purported chair itself. In fact, not even that; we only see the image that the reflected light forms on the retina. Thus, when I report seeing an object, any reference to the external world in this report can go only as far as my eyes' retina. However, further simple reflection shows we do not even perceive the image on the retina directly. The image generates electrical signals that travel to the brain and cause neuronal firings. Yet we do not even perceive these neuronal firings directly. Thus, while when I perceive a chair, what I am actually experiencing, according to these reductionistic models, are the neuronal firings in the brain stimulated by the light reflecting off the chair. Our perceptions, no matter how strong and no matter how well corroborated by the reports of others, must necessarily be individual. Thus, when I report seeing “a meter needle pointing to +1,” I am describing just my own, individual subjective experience. In such reports, there is no reference to anything outside my experience. Therefore, I have nothing in principle to justify concluding that there is an external world and that its structure corresponds to the form of my experience, i.e. that there really is a meter pointing to +1 in the world at the moment of my observation. This conclusion requires extra argumentation, one that realist philosophers have tried hard to provide for 500 years. Surprisingly, however, they have been thus far unable to logically justify the naïve realism that forms the basis of everyday thinking. [For more on contemporary philosophy and its basic issues in this regard, see Russell, Bertrand (1912); Palmer, Donald (1991); Tarnas, Richard (1993)] Of course, in our everyday living, we are not disturbed by such philosophical doubts. We assume that any doubt about everyday experience corresponding to the world is purely of a logical nature, since under most ordinary conditions, there are true experiences that we all routinely have, as borne out by the practical success of the assumption that the objects of our experience directly correspond to objects in the external world. Physicists too invoke this assumption, called naïve realism, routinely by default, when they use ordinary language to report their observation experiences. They believe that an observation experience, say that of “a meter needle pointing to +1” does correspond to the real state of a macroscopic meter needle in the external world. Indeed, in the pre-quantum or classical era of physics, there was no compelling need to examine the validity of this everyday naïve realism within physics at the level of reporting our observations. Quantum theory remarkably tells us that if the quantum state were to be applied to the macroscopic measuring device also, the state of the meter needle in the world at the point of observation, according to quantum theory, is a superposition which (whatever else it is) is not a definite pointer state, even though our experience of the meter needle is that it always points to a definite reading. In other words, pending other assumptions, quantum theory questions some or all parts of our current everyday naïve realism, much as philosophers have questioned it in the past. We shall now provide a very non-technical introduction (with minimal mathematics) to this and related aspects of quantum theory that makes it radically unlike the theories in physics that preceded it. 2

Quantum Theory – A Brief Introduction Just as Newton’s classical mechanics describes the motion of macroscopic or “big-sized” objects—that is, objects whose motion we can see with unaided senses, such as falling apples and revolving planets—quantum theory began by attempting to describe the supposed motion of a different class of objects such as atoms, electrons and photons. The big-sized objects are called classical objects, and the objects described by quantum theory are called quantum objects. The two behave differently, in the following way. Imagine a marble that is internally so constructed that, when thrown, it splits into two identical halves and travels in two different directions. Let us say, we place two identical wooden boards in both their paths at the same distance, so that both halves hit the respective wooden boards with the same energy at the same instant. Suppose we hear a clear sound when one marble piece hits one board, while the other marble makes no sound when it hits the other board. Wouldn’t we be surprised? Indeed, very. Happily, marbles don’t behave so. However, quantum objects — atoms, electrons and photons and the like — do something very much like the above. Given a choice to travel along one of two paths, although the quantum objects do not themselves split, we are led to conclude that they are simultaneously “present” in all paths in some as yet un-understood non-classical sense, and yet get detected only in one path. , we shall now show that this conclusion comes mixing together our analysis of an experiment in part using the quantum formalism (which involves a mathematical language) and our analysis of the same experiment in another part via the observations (which involves ordinary language). Consider the typical two-slit experiment, in which a low-intensity source emits quantum objects, light or material particles, one at a time. We place a two-slit screen (i.e. a screen with two holes in it) on the path of these quantum objects and place some detectors behind it. This effectively gives a chance for the quantum to go from the source through one slit or both slits to reach the detectors. What do they do? The theory tells us the following. Associated with each of the two possible paths are two complex-valued entities, say, a1 and a2, called the “probability amplitude” whose absolute square gives respectively the probability for finding an individual quantum in the respective paths, if only one of the paths is available to it (i.e. the other path is closed off). Thus, the probability for finding the quantum on either path will be, P1 = a12 P2 = a22 Now, with both paths open, the probability for finding the quantum in at least one of the paths must be 100%. Classically, that probability should be given by P1 + P2. However, as per quantum theory, the probability amplitudes must be added and squared, and their absolute value taken in order to get the probability. This 3

means, in the two-slit experiment we are dealing with, probability for the quantum object to take either one of the two paths is P (E1 or E2) = | (a1 + a2)2 | = | (a1)2 + (a2)2 + a1 a2* + a1* a2 | = P1 + P2 + | a1 a2* + a1* a2 | where the * represents the complex-conjugate.

In other words, classically, probabilities add; quantum mechanically, the probability amplitudes add, leading to the presence of the extra product terms in the quantum case. What this means is that in quantum theory, although the quantum object is detected only in one path as a single undivided entity in any actual observation, as if it propagates as a particle along a single determinate path, the non-zero quantum amplitudes for all the possible paths also play a role in determining the overall probabilities for each occurring event. Indeed, the observed quantum interference effects are correctly captured by the quantum statistical description only because of the presence of these product terms. Thus, the quantum amplitudes must be deemed as representing an as yet ununderstood non-classical presence of the electron or photon on both paths at the two-slit screen. This is per the mathematical formalism of the theory. What about the experiments?

Indeed, in actual experiments, the observed behavior of the individual quantum at a location far off from the two-slit screen is different depending on the status of both slits. When only one slit left open, there is an accumulation of spots everywhere, with maximum intensity in line with the slit. When both slits are open, individual spots continue to appear on the screen, but unexpectedly they don’t appear in certain places where they previously appeared when only one slit was open. Eventually an interference pattern accumulates. Conclusion: where the emitted individual quantum will arrive (on a photographic plate placed far-off from the two-slit screen) depends on whether one or both slits are open at the twoslit screen. In other words, the observed behavior of the quantum object at the distant photographic plate is influenced by the physical status of both slits at the two-slit screen. This behavior is the counter-part of the formal feature that the non-zero quantum amplitudes corresponding to both possible paths play a role in determining the overall probabilities for each event.

The idea that the quantum object has a presence at (or, “goes through”) both slits is, nevertheless, still only an inference in the above experiment. To be sure, we need to observe whether the quantum indeed goes through both slits by placing detectors close to the two-slit screen. In that case, however, only one detector always fires. 4

It is physically impossible to simultaneously place the same detectors both near and far away from the two-slit screen. The two experiments are mutually exclusive. If we insist on visualizing the experiment in terms of the quantum objects “going from” the source to the detector, one experiment (placing detectors far away) offers inferential confirmation of the theoretical description that the unobserved quantum object goes through both slits, while another experiment (placing the detectors close to the slit) suggests that the observed quantum object is always in one of the two localized regions. This is the most surprising aspect of quantum theory. Nobel-laureate quantum physicist Richard Feynman went as far as to say “it is the only mystery in quantum mechanics.” It has been mathematically further shown that in some experimental situations (involving two or more correlated quantum particles), any consistently classical realist approach will not correctly predict the observed statistics. This theorem, known as Bell’s theorem, has also been experimentally verified. Thus, it has become clear that no complete return to a classical conception of physical reality is possible with respect to quantum theory. It would be more correct to consistently interpret quantum theory in a non-classical manner in all the experiments. Quantum physicists have therefore been led to the conclusion that although its observed presence is always at only one localized region, the unobserved quantum object is present at both slits in some as yet un-understood “non-classical” way. The unobserved non-classical state of the quantum is called the state of “superposition”. In 1935 Schrödinger, the Austrian physicist who created the quantum mechanical equation of motion that is now named after him, reminded everyone that as per quantum theory, even unobserved macroscopic objects such as detectors must enter superposition. This is shown by applying the quantum mechanical description to the joint system of the microscopic quantum and the macroscopic measuring detector. Then, the two systems, together, enter into a state of superposition. Yet, the macroscopic object too, even more than its microscopic counterpart, upon observation, is always seen, rather directly, to be in a classically determinate state. To dramatize this conclusion, Schrödinger further let this quantum effect to propagate eventually to a “cat” so placed in an experimental situation that its unobserved (i.e. superposed) state would correspond to that of a superposition of the possible states of “being alive” and “being dead.” To show this, Schrödinger imagined a cat trapped in a steel container along with a Geiger counter containing a small amount of radioactive substance. The amount is so small that within the period of one hour, there is equal probability that one atom will decay or not decay. At any given instant, if decay occurs, the Geiger counter discharges and causes the release of a hammer which breaks a small flask of hydrocyanic acid and kills the cat. If not, the cat will not be killed. A quantum theoretic description of the joint system (of a single atom and the cat) should also be in superposition, 5

i.e. not correspond to a definite state for either system. Again, observation will always reveal the cat to be either dead or alive, no other state.

Taking (correctly) the unobserved state of superposition to be a true description of microscopic and macroscopic objects — the empirical success of quantum theory gives every reason to suppose it is — quantum physicists presently assume that the state of superposition transforms, either actually or effectively, into a classical definite state at the point of an observation. But explaining the actual mechanism of this change is known as the “measurement problem”. A number of solutions have been proposed to the measurement problem, but none has won universal acceptance yet. All are compatible with quantum theory, but none is entailed by it since none of these solutions offer any new predictions that offer empirical grounds for preferring one over the rest. In addition, none of them manage to leave us with a feeling that quantum theory is really “understood.” They replace one mystery, wave-particle duality, with another unobservable mystery (a random collapse, or a superposition of “many-worlds”, or a zero-energy pilot wave etc). For a very good, non-technical but accurate description of the classically problematic features of quantum theory, and a review of the various solutions proposed to understand it and their limitations, as well as Bell’s theorem, see Rae (1989). It is clear that our understanding of quantum superposition needs a lot more clarification. Nevertheless, physicists and computer scientists are currently attempting to already put our present understanding of the quantum superposed state to practical use in diverse areas such as quantum computing [See Behera (2005)], and quantum information [See Duwell (2005)]. Quantum Physics and Philosophy — A connection If quantum theory is right, and its empirical adequacy leaves no room for doubt here, the so-called macroscopic "real" world, quantum theory proves, is only an artifact of our own act of observation—exactly what the philosophers were arguing. It has taken centuries for physics to catch up with the insight of the early modern philosophers, but quantum theory takes us significantly past the philosopher’s critical doubt. Whereas, philosophers (realists and anti-realists, alike) have so far assumed that everyday realism means present-day naïve realism (the external reality stands in one-to-one correspondence with our experience of it), under a realistic construal quantum theory tells us that not only there is a macroscopic reality underlying the physicist’s observation experience but that ordinary everyday reality is different from what our simple sense experiences tell us. No wonder realist philosophers have so far failed to prove naive realism through philosophy. This is an exciting and radical implication of quantum theory. Classical mechanics, by and large, leaves our everyday intuitions untouched. Einstein’s 6

relativity theory, however, already leads us in this direction since it revises the physicist’s conception of space and time in an abstract mathematical world (called the Riemanian 4-manifold) and has some profound implications for our ordinary notions of space and time [See Weissmann (2005)]. Quantum theory may cause a further and more radical revolution in our ordinary thinking. For, upon our above analysis, to really understand the nature of the quantum world, common sense thinking in its own domain, i.e. our notion of everyday reality as we think of it independent of science, needs a revision. That is a task certainly requiring input from philosophy. Since, such a revision to common-sense thinking must ultimately make quantum theory itself understandable in terms of these new everyday intuitions, it will be philosophy in interaction with science. However, by and large physicists and philosophers haven’t risen to the occasion. As already mentioned, most quantum physicists assume, in the tradition of Niels Bohr, who famously debated with Einstein over quantum theory, that an observation experience can only “make its appearance within the framework of our customary points of view and forms of perception.” Thus, currently, in using quantum theory physicists assume that that this radical nature of everyday reality either goes away at the point of observation or that a macroscopic object can be approximated to be essentially a classical object at the point of observation for all practical purposes. Although this way of “doing away” with macroscopic superposition has worked tremendously well in practice, it has left the world described by quantum theory impossible to understand in ordinary language, even by quantum physicists. And philosophers, assuming that it is not their task to question physicists’ approach to physics, have simply worked on the measurement problem instead of questioning whether this problem can be avoided in the first place by paying heed to the philosophical lesson that quantum theory imparts. In this regard, the careful reader may have noticed that while we began this article by drawing a difference between the behavior of macroscopic objects and objects treated by quantum theory at present (such as atoms, electrons and photons), we have also discussed the Schrödinger’s paradox which assumes that macroscopic objects too can enter states of superposition. This is not necessarily a contradiction. What Schrödinger’s paradox shows is that macroscopic objects, if conceived quantum mechanically, behave differently than if conceived classically. The paradox arises because, in the case of the macroscopic objects (unlike in the case of the radioactive atom itself) our current naïve realist notions tell us that, given one of many possible macroscopic states, we know that a macroscopic object cannot but be in one of these possible states at all times, even when unobserved. We have seen that philosophers have already shown we do not know this about macroscopic objects any more certainly than we know about the atoms. This suggests that perhaps a proper, realistic interpretation of the quantum state of superposition is possible if we abandon our understanding of superposition at the macroscopic level as a superposition of classically determinate states and strive for a different understanding. The revisions at the level of common-sense thinking that quantum physics calls for will have relevance for the field of artificial intelligence which attempts to 7

capture our common-sense reasoning. At present, quantum computing is based on the correctness of the quantum formalism, and the correctness of Born’s statistical rule, but does not require things beyond that, such as an understanding of the state of superposition. Nevertheless, if we can come up with a revision to our present understanding of superposition, it stands to reason that it will have implications for pursuing quantum computation. It is also further known that “information” in some sense is fundamental to quantum theory. Thus, reconceiving the state of superposition may also require making headway in conceiving information as a feature of the physical world. Philosophers have discussed the nature of information too [See Floridi (2005)] and that is another connection between quantum physics and philosophy. It is time to briefly return to Einstein whose 1905 works are commemorated by this issue of CSI communications. Einstein, more radically than most physicists, saw the essential problem posed by quantum theory to be the need for a revision to our ordinary thinking: “I believe that events in nature are controlled by a much stricter and more closely binding law than we suspect to-day, when we speak of one event being the cause of another. Our concept [at present] is confined to one happening within one time-section. It is dissected from the whole process. Our present rough way of applying the causal principle is quite superficial. We are like a child who judges a poem by rhyme and knows nothing of the rhythmic pattern. Or we are like a juvenile learner at the piano, just relating one note to that which immediately precedes or follows. To an extent this may be very well when one is dealing with very simple and primitive compositions; but it will not do for the interpretation of a Bach Fugue. Quantum physics has presented us with very complex processes and to meet them we must further enlarge and refine our concept of causality.” [Einstein, 1933, p. 203-4] In this article we have proposed that even prior to revising our everyday notion of causality, we may have to recast our very conception of naïve realism in a quantum compatible manner at the level of ordinary language description of our sense experiences. Conclusion Physicists do need common-sense thinking to realistically interpret laboratory observation experiences in a manner compatible with the physical theory at hand. We have shown that quantum theory in this regard eventually requires the solution that is part philosophical and part scientific, namely, how to re-conceive naïve realism in ordinary language in a way compatible with quantum theory, so quantum theory can make further fundamental progress. I myself think such a revision of everyday realism is possible. For some exploratory ideas in revising our common-sense thinking to bring it in line with the demands of quantum 8

formalism, see Gomatam (2010, 2004, 1999a, 1999b).

REFERENCES Behera, L. (2005), “Quantum Computing Communications, 29(6), December, pp. 29-39

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An

Introduction”,

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Duwell, A. (2005) “What is Quantum Information?”, CSI Communications, 29(6), December, pp. 40-42. Einstein, A. (1933) Interview by J. Murphy in Planck, Max Where is Science Going? 1933/1981 by Ox Bow Press: Woodbridge, Connecticut, pp. 201-221 Floridi, L. (2005) “Open Problems in the Philosophy of Information”, CSI Communications, 29(6), December, pp. 43-46. Gomatam, R. (2010) "Popper’s Propensity Interpretation and Heisenberg’s Potentia Interpretation—A comparative assessment", In Sengupta, Pradip K. (editor) History of Science, Philosophy and Culture in Indian Civilization. Volume XIII. History of Science and Philosophy of Science, part 6, New Delhi: CSC, p. 301-312. Available online at http://www.bvinst.edu/gomatam/pub-2005-02.pdf Gomatam, R. (2004) "Physics and Commonsense – Relearning the connections in the light of quantum theory" in Chattopadhyaya, D.P. & Sen Gupta, A.K. (Eds.), Philosophical Consciousness and Scientific Knowledge, New Delhi: CSC, p. 179207. Available online at http://www.bvinst.edu/gomatam/pub-2004-01a.pdf Gomatam, R. (1999a) “Quantum Theory and the Observation Problem”, Journal of Consciousness Studies, 6 (11-12), p. 173-190. Available online at http://www.bvinst.edu/gomatam/pub-1999-01.pdf Gomatam, R. (1999b) “Quantum Theory and Information”, a paper read at “Quantum Approaches to Consciousness” Conference, July 28-August 1, Northern Arizona University, Flagstaff, Arizona Palmer, Donald (1991) Does The Center Hold?, Mountain View, California: Mayfield Publication Co. Rae, Alastair (1989) Quantum Mechanics – illusion or reality? Cambridge: Cambridge University Press Russell, Bertrand (1912) The Problems of Philosophy London: Oxford University Press Tarnas, Richard (1993) The Passion of Western Mind, New York: Harmony Books Weissman, G. (2005) “On Einstein’s Special Relativity and it revolutionary 9

implications for the methodology of physics and our understanding of space and time” CSI Communications, 29(6), December, p. 8-21

Following are some resources from the internet on philosophy, philosophy of Physics and quantum mechanics. Philosophy Encyclopedias of Philosophy http://plato.stanford.edu/ Stanford Encyclopedia of Philosophy http://www.iep.utm.edu/ The Internet Encyclopedia of Philosophy http://www.ditext.com/encyc/frame.html Meta-Encyclopedia of Philosophy Dictionary of Philosophy http://www.philosophypages.com/dy/ A Dictionary of Philosophical Terms and Names http://www.ditext.com/runes/index.html Dictionary of Philosophy (Ancient - Medieval - Modern) edited by Dagobert D. Runes (and 72 other authorities) http://en.wikibooks.org/wiki/Introduction_to_Philosophy A simple introduction to philosophy: http://radicalacademy.com/adiphildirectory.htm This site contains introductory essays on the following topics: Philosophy and History of Philosophy; Philosophy in perspective; Philosophy and Common sense; The roots of philosophy; The emergence of philosophy; Philosophy and science; Philosophy and Life; Metaphysics and its Problems. http://consc.net/chalmers/ Home page to David Chalmers (Professor of Philosophy), which is an excellent resource for philosophy topics. Philosophy of Physics http://www.admissions.ox.ac.uk/courses/phph.shtml This is the home page for a 4 year undergraduate level course in ‘Physics and Philosophy’ conducted by the Oxford University. Following are the links to chapters 2, 3 & 5 of the famous book by Werner Heisenberg titled Physics & Philosophy published in 1958. http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb2. 10

htm Ch. 2 (The History of Quantum Theory) http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb3. htm Ch.3 (The Copenhagen Interpretation of Quantum Theory) http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb5. htm Ch 5. (The Development of Philosophical Ideas Since Descartes in Comparison with the New Situation in Quantum Theory) http://physics.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=physics&zu=http %3A%2F%2Fwww.marxists.org%2Freference%2Fsubject%2Fphilosophy%2Fw orks%2Fus%2Fbridgman.htm Percy Bridgman’s classic book titled The Logic of Modern Physics published in 1927 by MacMillan http://physics.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=physics&zu=http %3A%2F%2Fwww.marxists.org%2Freference%2Fsubject%2Fphilosophy%2Fw orks%2Fus%2Fkuhn.htm Chapter titled ‘The Nature and Necessity of Scientific Revolutions’ from Thomas Kuhn’s book The Structure of Scientific revolution. Quantum Theory http://www-groups.dcs.stand.ac.uk/~history/HistTopics/The_Quantum_age_begins.html An introduction to the history of development of Quantum Mechanics. http://plato.stanford.edu/entries/qm/ This Stanford Encyclopedia entry on quantum mechanics is a good place for starting learning quantum mechanics. http://en.wikipedia.org/wiki/Quantum_mechanics Another resource for starting to learn about quantum mechanics. http://theory.uwinnipeg.ca/mod_tech/node143.html A set of online lecture notes intended as an introduction to quantum mechanics and modern atomic physics. http://www.anselm.edu/homepage/dbanach/ph31c.htm#qm This link contains links to articles to most introductory aspects of quantum mechanics and some nice applet illustrations. http://dmoz.org/Science/Physics/Quantum_Mechanics/Resources/ This page contains links to resources for a range of topics related to quantum mechanics. http://www.upscale.utoronto.ca/GeneralInterest/QM.html This link to the ‘physics - virtual book shelf’ contains a collection of articles explaining basic concepts in quantum mechanics.

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