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Wanted: Schrödinger’s Cat Dead or Alive!

Joy Christian tells the bizarre tale of quantum reality.

What is all this fuss about Schrödinger’s cat? And who cares whether the cat is alive, dead, or in limbo? Well, you do, if you care at all about the reality of the external world.

The concept of external reality, independent of human observers, has always been one of the trickiest in philosophy. Nevertheless, we all think we have a fairly good idea, albeit different from one another, of ‘what is real’. For example, we rarely ever doubt the ‘reality’ of our physical surroundings; let alone the reality of our inner emotions and feelings. Such a ‘Commonsense Realism’, however, leads us into trouble at a moment’s reflection on questions like: Are the physical objects we see ‘out there’ what they appear to be perceptually, or are they what the mathematical physicists tell us they are? Are the tables and chairs what they seem to be, or are they mostly empty space with relatively few constituents like atoms and molecules? We recognise that there may be more (or less!) to the reality of physical objects than meets the eye. The use here of expressions like ‘atoms’ and ‘molecules’ plunges us into a philosophical tradition called Scientific Realism. This is the view that things may not be what they seem to be perceptually but that nevertheless there is an underlying reality independent of conscious observers, and science is closing-up on this reality bit by bit. Admittedly, expressions like ‘atoms’ and ‘molecules’ are theoretical constructs which change with the development of the sciences; nineteenth century physicists would not have recognised the atom by the description of it so familiar to contemporary scientists. Nevertheless, Scientific Realism is slightly more satisfactory than Commonsense Realism in that it evades the above mentioned troubles of the latter. It replaces naive ideas of reality with refined concepts provided by the prevalent scientific theories. Consequently, we can relax, says the Scientific Realist, and keep on believing in the reality of the external world independent of us.

Unfortunately, our most fundamental contemporary physical theory, Quantum Theory, is intrinsically non-Realistic; that is, at least in its orthodox interpretation, it is by its very nature at odds with Realism. This non-Realistic aspect of Quantum Theory may be succinctly demonstrated by the fact that if the theory is Realistically interpreted, then, as we shall see, it predicts strange behaviour for the objects of our everyday world. For instance, it predicts that a cat, an object of our everyday world, can be objectively in an equivocal state of being neither alive nor dead!

Does this mean that there is something wrong with Quantum Theory? On the face of it, this does not seem likely. It is a very elegant, mathematically highly sophisticated, and experimentally welltested theory of the physical world. Theoretical physicists unanimously esteem its great aesthetic appeal. Its immense accuracy, enormous predictive power, and splendid universality are unprecedented. For instance, one of the tests of its predictions recently established that it is accurate at least to 1 part in a billion billion billion (1027) parts! And its universality is repeatedly exhibited by the fact that the equations of quantum mechanics govern and predict the behaviour of objects from microscopic subnuclear particles like quarks to mind-boggling cosmic entities like black-holes. With such strength, the theory seems to be in no danger whatsoever as far as physics is concerned; it predicts all the right numbers for the behaviour of every conceivable physical object. Even so, some experts argue that we should not accept Quantum Theory as a God-given gospel, and instead should search for a better framework theory because conceptually Quantum Theory makes absolutely no sense!

As alluded to above the conceptual problem at the heart of Quantum Theory is not its failure to explain exotic objects like quarks and black-holes, but its inability to unequivocally explain the objects of our everyday world – objects like chairs, coffee-mugs, and cats. One of the basic tenets of quantum mechanics is the Superposition Principle, which requires the coexistence of every possible alternative state of a given quantal object at a given time. For instance, if an electron, an elementary particle, has two alternatives at its disposal – first of being ‘over here’ and second of being ‘over there’ – then the principle says that the quantum mechanical state of the electron is a superimposed state representing both of these possibilities. In other words, the electron does not have a definite property of being positioned ‘over here’ or being positioned ‘over there’, instead it is at both of these places at the same time! All the quantum effects of the world follow from this principle, and are repeatedly confirmed by experiment. Accordingly, some of the properties of a given quantal system (like the location of the electron in the above example) may not be definite but only ‘potential’, and these ‘potentialities’ coexist. Moreover, according to Quantum Theory, we can make only probabilistic inferences about the ‘actualisations’ of these properties upon appropriate ‘measurements’. What makes the theory so unusual is the fact that the probabilities involved in such inferences are fundamentally different from the ones we encounter in our everyday world. In ideal circumstances we know that we have 50/50 chance of getting heads or tails if we flip a coin, an object of our everyday world. Here also we can make only a probabilistic inference about the outcome, but that is merely due to our lack of knowledge of all the possible influences exerted on the coin while being flipped. Quantummechanical randomness, on the other hand, is not due to anybody’s lack of knowledge; it is ‘objective’. And this fact makes Quantum Theory essentially and intrinsically indeterministic, unlike the Newtonian theory of physics which applies to our everyday world.

Einstein, one of the forefathers of Quantum Theory, did not like this objective randomness in our most fundamental physical theory, hence his famous comment that “God does not play dice!” He devised an ingenious argument with two of his collaborators to show that the theory is at odds with Realism. They showed that if the theory is assumed to describe the external observerindependent reality, then it predicts that moving (say) an electron could instantly have an effect on another electron a long distance away. This ‘action at a distance’ is not only amazingly counterintuitive, it would also imply an influence faster than the speed of light – which is at odds with the spirit of Einstein’s Theory of Relativity. Einstein therefore concluded that Quantum Theory must be flawed. However, when in 1982 a Frenchman called Alain Aspect finally managed to test the argument experimentally, he found that this ‘action at a distance’ really does take place! This triumph of Quantum Theory, nonetheless, probably would not have undermined Einstein’s commitment to Realism. For what motivates most working scientists is a belief in an objective, nonanthropocentric reality.

Schrödinger’s cat
Figure 1: The Schrödinger’s Cat Paradox
A cat is placed in a closed box with a flask full of poisonous gas, some radioactive material, a detector and a hammer. Things are arranged so that in the course of an hour there is a 50/50 (objective) chance of one atom decaying and emitting a particle. If this happens the detector will trigger a relay releasing the hammer which will break the flask, killing the cat. But according to Quantum Theory the state in which a particle has been emitted and that in which it hasn’t are superimposed – it exists in both the states simultaneously. And this means that if Quantum Theory is right the cat is both alive and dead at the same time.
© Matt Gardner 1993

Another pioneer, Schrödinger, was also unhappy with his own theory. He proposed a thought-experiment which dramatises the central conceptual problem of the theory: if the theory is thought to be in harmony with Realism, then certain determinate properties apparent in our everyday world (like the definite space-time position of the pen in my hand) are indeterminable by the theory. He considered a cat in a closed box, as shown in the figure, with a flask full of poisonous gas coupled to a quantal system, which is in a superimposed state of two alternative possibilities of actualisations. Quantum Theory then predicts (due to the linear nature of its dynamical equations) that the cat will not have a definite state of either being alive or being dead; it will instead be (ontologically) in limbo – neither alive nor dead! This is an example of how, when an object of our everyday world is coupled to a quantal system, the theory predicts superposition of the object’s macroscopically distinct states, which we do not find in the everyday Newtonian world; we do not find our coffee-mugs both empty and full at the same time!

Schrödinger used his poor cat in the thought-experiment to dramatise this unacceptable prediction of the Realistically interpreted quantum mechanics. The problem he made conspicuous is essentially that of reconciling the quantum ontology with the familiar ‘classical’ ontology of our everyday world: given the fact that the physical world is made out of quantum constituents with their indeterminate properties, how do we understand – in an objective, nonanthropocentric manner – the emergence of apparent definite properties (like the cat’s property of being definitely alive or being definitely dead) of the objects of our everyday world?

To answer this question, and to evade this dilemma of Schrödinger’s Cat, many attempts have been made in the last six decades to provide alternative interpretations of the theory. They range from the bizarre and problematic ‘allquantum’ Many Worlds Interpretation – which purports continuous branching of one universe into parallel universes each with either a definitely alive or a definitely dead cat – to the rather regressive ‘all-classical’ Causal Interpretation – which refuses to face the radical metaphysical innovations of Quantum Theory. A more conservative intermediate position, doctrinally assumed by most practising physicists with instrumentalistic proclivities, is known as the Copenhagen Interpretation. This orthodox interpretation, however, commits us to a dualism by maintaining that our world is partly classical and partly quantal, without specifying a clear line of demarcation between the two parts. None of the interpretations offered so far have all the desiderata of making the theory both scientifically satisfactory as well as unequivocally compatible with Realism. It is indeed a supreme irony that such a marvelously accurate and beautiful theory defies any attempt to reconcile it with a Realistic interpretation of its mathematical formalism. And for a theory so comprehensive and universal, this surely is an embarrassment. Some philosophers have thus concluded: Well, so much for the Realism! They resort to exploration of possible ‘anti-Realistic’ positions. Others dissent: It is a mistake to take such an extreme philosophical stand based solely on a contingent scientific theory. The debate goes on.

Fortunately, from the point of view of physics, all the doors are not closed yet. There are already preliminary attempts, by a small group of physicists, not just to provide a consistent new philosophical interpretation, but to modify a part of the mathematical formalism of the theory. One also comes across cocktail-party discussions where it is argued that, in order to do away with all the undesired conceptual difficulties, it is better to dream-up a completely new framework theory instead of tinkering with the present one. Needless to say that these are all very high prices to pay just to save the ‘reality out there’, which, alas, is so dear to us! Quantum Theory in its present crystallised form is a scientific jewel, a culmination of the painstaking cerebral labours of innumerable physicists over the past ninety years. These physicists have scrutinised and refined it by passing it through numerous stringent logical and empirical tests. Even a slight tinkering with its formalism is bound to upset the delicate balance achieved by these labours between the theory and the empirical facts. On the other hand, however, we want our cats to be either alive or dead – not in limbo!

Further reading: Quantum Reality by Nick Herbert (Doubleday, New York 1985)

© Dr. J.J. Christian 1993

Dr. Joy Christian trained as a physicist and is now a research fellow in philosophy of science at Wolfson College, Oxford.


A Brief History of Quantum Theory

Along with the Theory of Relativity, Quantum Theory completely revolutionised physics. It happened like this:

1900: Max Planck discovers that you can’t get energy in infinitely small amounts, because fundamentally it comes in little lumps (or ‘quanta’).

1905: Albert Einstein successfully explains the photoelectric effect by assuming that light (a form of energy) comes in little packets (‘photons’). This was startling because light had previously been thought to be a type of wave, but Einstein had shown that light could sometimes behave like a stream of particles.

1913: Niels Bohr shows that electrons in atoms occupy certain orbits (or energy levels) and not others. Later, de Broglie suggests that if light (a wave) can behave like particles, then maybe particles, such as electrons, can behave like waves. In that case Bohr’s ‘stable orbits’ are in fact standing waves, like the modes on a vibrating string. Various experiments then showed conclusively that particles can indeed behave like waves and can exhibit diffraction, interference and all the effects exhibited by, say, ripples on a pond.

1920s: Schrödinger, Heisenberg and others build on this idea of ‘wave/particle duality’ to develop the mathematical rules known as quantum mechanics. They then use this to explain the structure of atoms, radioactivity, chemical bonding and an enormous number of other things too.

However, the new theory had worrying implications. Streams of particles behave like waves, but what about individual particles? In 1926 Max Born reinterpreted the new theory and suggested that the waves describe only the probability of a particle being in a particular state at a particular time. And so objective randomness and indeterminism entered modern physics, like sin entering the Garden of Eden…

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