Physics Scratches a Philosopher's Itch Philosopher Elise Crull muses over interpretations of quantum physics, focusing on experiments that cast doubt on our classical notions of causality.By Michael Schirber W hich came first, the chicken or the egg?Both, says Elise Crull, who bases her answer on recent experiments that show that causal sequences can get mixed up in quantum circuits.A philosopher of physics at The City College of New York, Crull studies the results of experiments like these to try and tease out their meaning.Can causality coexist with quantum uncertainty?Why does the arrow of time point in one direction?Crull says that she doesn't yet know the answers to these questions, but she thinks scientists could benefit from pondering such riddles.
Physics Scratches a Philosopher's Itch Philosopher Elise Crull muses over interpretations of quantum physics, focusing on experiments that cast doubt on our classical notions of causality.By Michael Schirber W hich came first, the chicken or the egg?Both, says Elise Crull, who bases her answer on recent experiments that show that causal sequences can get mixed up in quantum circuits.A philosopher of physics at The City College of New York, Crull studies the results of experiments like these to try and tease out their meaning.Can causality coexist with quantum uncertainty?Why does the arrow of time point in one direction?Crull says that she doesn't yet know the answers to these questions, but she thinks scientists could benefit from pondering such riddles.
[I] think I can safely say that nobody understands quantum mechanics. Richard Feynman, The Character of Physical Law Quantum mechanics [is] that mysterious, confusing discipline, which none of us really understands but which we know how to use. It works perfectly, as far as we can tell, in describing physical reality, but it is a ‘counter-intuitive discipline’… Murray Gell-Mann, Questions for the Future [The quantum] postulate implies a renunciation as regards the causal space-time co-ordination of atomic processes. Niels Bohr, Atomic Theory and the Description of Nature It should be emphasized, however, that the probability function does not in itself represent a course of events in the course of time. It represents a tendency for events and our knowledge of events. The probability function can be connected with reality only if one essential condition is fulfilled: if a new measurement is made to determine a certain property of the system. [T]he idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them…is impossible … Werner Heisenberg, Physics and Philosophy [In] quantum theory it is the principle of causality , or more accurately that of determinism , which must be dropped and replaced by something else.… We now have a new form of the law of causality.… It is as follows: if in a certain process the initial conditions are determined as accurately as the uncertainty relations permit, then the probabilities of all possible subsequent states are governed by exact laws. Max Born, The Restless Universe
Motivated by some recent news, a journalist asks a group of physicists: "What's the meaning of the violation of Bell's inequality?" One physicist answers: "It means that non-locality is an established fact". Another says: "There is no non-locality; the message is that measurement outcomes are irreducibly random". A third one says: "It cannot be answered simply on purely physical grounds, the answer requires an act of metaphysical judgement". Puzzled by the answers, the journalist keeps asking questions about quantum theory: "What is teleported in quantum teleportation?" "How does a quantum computer really work?" Shockingly, for each of these questions, the journalist obtains a variety of answers which, in many cases, are mutually exclusive. At the end of the day, the journalist asks: "How do you plan to make progress if, after 90 years of quantum theory, you still don't know what it means? How can you possibly identify the physical principles of quantum theory or expand quantum theory into gravity if you don't agree on what quantum theory is about?" Here we argue that it is becoming urgent to solve this too long lasting problem. For that, we point out that the interpretations of quantum theory are, essentially, of two types and that these two types are so radically different that there must be experiments that, when analyzed outside the framework of quantum theory, lead to different empirically testable predictions. Arguably, even if these experiments do not end the discussion, they will add new elements to the list of strange properties that some interpretations must have, therefore they will indirectly support those interpretations that do not need to have all these strange properties.
This paper examines how such fundamental notions as causality and determinism have undergone changes as a direct result of empirical discoveries. Although such notions are often regarded as metaphysical or a priori concepts, experimental discoveries at the beginning of this century—radioactive decay, blackbody radiation and spontaneous emission—led to a direct questioning of the notions of causality and determinism. Experimental evidence suggests that these two notions must be separated. Causality and indeterminism are compatible with the behavior of quantum-mechanical systems. The argument also sheds some light on the Duhem-Quine thesis, since experimental results at the periphery of the conceptual scheme directly affect conceptions at the very core.
In classical physics, the width, or uncertainty, in a set of experimental results is generally attributable to experimental variations, due to small differences in the objects being tested, or variations in the apparatus used, instrumental factors, environmental factors etc. Experiments in quantum physics also suffer from all these sorts of variations. Consequently, even if we have a collection of systems, all carefully prepared in the same eigenstate of the physical variable in question, we will not always get exactly the same measured value. The spread of this distribution is down to experimental uncertainty. People have not been afraid of drawing philosophical conclusions from the uncertainty principle, almost from the moment the result was first published. Heisenberg himself launched one of the earliest sallies, his target being the principle of causality. Causality, the notion that everything that happens has been caused by something happening previously, has been central to physics for hundreds of years.
Quantum Physics is usually defined as a theory that affirms a primary role of randomness and probability. Eleven well-known quantum experiments are examined and the result is the coexistence of both random and causal type behaviour. We need both to describe experiments. Quantum Mechanics states the general overcoming of causality and this statement constitutes an unlimited generalization, not supported by experience. Determinism and indeterminism are philosophical systems, that universalize causality or chance. The crucial point is the difference between epistemic and intrinsic randomness. In the first aspect, randomness does not have a fundamental meaning, in the second, randomness of the individual event is explained, but the detection of high stable regularities remains to be explained. The article addresses the question of entanglement and various aspects of probability theory, including the law of large numbers, arriving at the thesis that many relevant questions are unresolved. A causal description is in quantum mechanics impossible in principle, given its assumptions. In order not to contradict experience, both points of view are necessary, causal and random. This is the state of the research, to return to and start again from.