Quantum Mechanics: The Physicist turns Philosopher

Some History. All great scientific revolutions have philosophical consequences. Quantum mechanics is no exception. As Max Planck was the first to note, the introduction of the universal constant h posed a serious threat to a deterministic conception of the universe. It became conceivable that nature made leaps after all. Furthermore, these leaps seemed to be of a random character. The indeterministic nature of the quantum world made itself felt in four separate developments: a) in the Planckian explanation of blackbody radiation (1900) and b) in the radioactive decay law (1901). Both involve statistical considerations at a fundamental level. The emission and absorption of discrete quanta by atoms in black bodies is a probabilistic event. So is the escape of -particles or -particles from the nucleus, causing the chemical changes, which Rutherford and Soddy linked to the radioactive activity. Furthermore, c) Einstein (1917) expressed his concern about indeterminism by noting that quantum mechanics failed to predict the direction of outgoing radiation in spontaneous emission. Finally, d) Heisenberg and Bohr used the double-slit Gedankenexperiment in the late 1920s to enthusiastically embrace the new indeterministic view of nature. This was in stark contrast to Planck and Einstein, who adhered to a fundamentally deterministic view of the universe. The double-slit experiment, which was realized experimentally for the first time in 19611, has acquired a proud history in quantum mechanics and has been reproduced in many different versions. Its association with questions of causality in quantum mechanics has lasted to the present day.

Three Responses from Physicists. In the face of the difficulties of interpreting these results in the sense of traditional notions of causality, physicists developed a number of responses, which can be conveniently divided into three groups.

Planck, Einstein and von Laue adopted the Conservative Response. According to this view, quantum mechanics was incomplete in the sense that it failed to specify the spatio-temporal trajectories of the quantum systems. The ability of physicists to predict the precise trajectories of particles from a set of initial conditions and of known laws was seen as the hallmark of (classical) physics. The trouble with this view is that it stands firmly in the shadow of the Laplacean demon. Laplace had identified causality and predictive determinism. Unwittingly, both Planck and Einstein argued for a retention of the Laplacean identification of determinism and causality.

Heisenberg, Bohr and Pauli adopted the Radical Response. This view leads to the rejection of the notion of causality in quantum mechanics. The argument proceeds from the experimental failure of predictive determinism to the adoption of acausality in quantum mechanics. Ironically, this view also operates under the shadow of the Laplacean demon. The reason given for adopting acausality, is the validity of the Heisenberg indeterminacy relations in quantum mechanics. The indeterminacy principle has served generations of physicists (and philosophers) to conclude that quantum mechanistic is acausal. There are two scenarios. The antecedent conditions of the state of a quantum mechanical system cannot be fully known experimentally (either momentum or position, energy levels or time in energy state can be measured accurately). The consequent conditions of the quantum system, after interference with the measurement apparatus, cannot be predicted with precision. Ergo: the law of causality fails to apply in quantum mechanics.

Sommerfeld, de Broglie and the later Born adopted the Philosophical Response. This view leads to a separation of the notions of causality and determinism. It holds that even though determinism fails, causal accounts may still be given in quantum mechanics. This required a notion of probabilistic causality.2 A probabilistic notion of causality no longer satisfies the demand for precise spatio-temporal prediction of trajectories. Individual atoms in an atom beam, split in a Stern-Gerlach apparatus, have a 50% chance of travelling along the upper or the lower trajectory. But it is still possible to give a causal account of the splitting of the atom beam and the chances of individual atoms to travel along the split beams. Similarly many of the famous experiments, which established quantum mechanics, give rise to causal accounts: the Frank-Hertz experiment (1914), the Stern-Gerlach experiments (1920), Compton Scattering (1923) and the Davison-Germer experiment (1927) can all be given a causal interpretation, based on the

2 For the present purposes it suffices to say that according to a model of probabilistic causality, a cause raises the probability of its effect. If we ignore the doubts, expressed by some writers that a cause notion of probabilistic causality. A thought experiment due to de Broglie's perfectly illustrates this new notion of causality.

Consider a phenomenon, A, such as the firing of an electron gun at a crystal, which is always succeeded by one of several phenomena, B1, B2, B3, …Bn. These may be scintillation effects at different points on the surface of a screen erected near the crystal. Furthermore, none of the phenomena, B1, B2, B3, …Bn will be recorded if A is absent (Figure I).

Figure I: De Broglie’s causal thought experiment.3

In this situation, we would consider that A must be the cause of B. It is a case of causality without determinism because quantum mechanics cannot predict which of the phenomena, B1, B2, B3, …Bn will actually occur at which place and time on the surface of the screen. While there are regions of the screen where the probability of impact is greater than in other regions, it is impossible to make any statements about the path and impact area of individual electrons. But there is a causal dependence of the scintillation effects on the firing of the electron gun.

These examples of causal accounts in quantum mechanics may lead to a conditional view of causality, similarly to Mackie's INUS model. There is a tendency in the literature to associate Mackie's model of causality with a deterministic view of the universe. This association is mistaken for two reasons. Mackie himself excluded it.4 And a notion of probabilistic causality can be formulated to express an antecedent set of necessary and sufficient conditions, commonly called the cause, which raises the probability of the appearance of the consequent set of conditions, commonly called the effect.

must always raise the probability of its effect, we arrive at another popular formulation, i.e. probabilistic causation is deterministic causation of probabilities.

3 L. de Broglie, Continu et Discontinu en Physique Moderne. Paris: Albin Michel (1941), 64-66

B1

B2

B3

~A

~B1,2,3

1 C. Jönsson, ‘Electron Diffraction at Multiple Slits’, American Journal of Physics 42/1 (January 1974), 4-11[first published in Zeitschrift für Physik 161 (1961)].