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, 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. The experimental evidence suggested that these two notions must be separated. Causality and indeterminism were compatible with the behaviour 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 affected conceptions at its very core.
Ever since Thomas S. Kuhn pointed out the importance of the history of science for the philosophy of science, it has become customary for philosophers of science to support their philosophical considerations by appeal to real-life science. The philosopher seeks evidence for some general principles about the nature of science often from some historical case studies. If there is a common territory between science and philosophy, as many writers have affirmed, it must also be possible to go from science to philosophy. This is indeed what some of the greatest scientific minds throughout the centuries have attempted to do. Their reflections fall into the oldest branches of philosophical thinking: ontology or the question of what the basic constituents of nature are; epistemology or the question by which tools the human mind can acquire knowledge about the external world; ethics or the question of what moral responsibility scientists have with respect to their discoveries.
In such contributions, scientists, prompted by the most recent discoveries in their respective fields, provide interpretations of science and the natural world and thereby contribute to their understanding. The heartbeat of science is at its most philosophical rhythm when major conceptual revisions or revolutions are afoot and scientists feel the need to go beyond the mathematical expressions of natural processes to reach a level of understanding which assigns some physical meaning to the mathematical comprehension of the natural world or offers a re-interpretation of the nature of the scientific enterprise. What is interesting in this process from a philosophical point of view is that empirical facts filter through to the conceptual level and bring about changes in the way the world is conceptualised. ‘Old notions are discarded by new experiences’, as Max Born once said. The common territory between science and philosophy lies in this interaction between facts and concepts. In re-interpreting the natural world in the light of new experiences the scientist becomes an active participant in the shaping of human views about the surrounding world. Physicists like Max Planck perceived the ‘confusing amount of new evidence’ as a threat to the established mechanistic physical worldview. Such reactions reveal the role of the philosopher-scientist of which scientists are fully aware:
‘History has shown that science has played a leading part in the development of human thought.’
In this paper, I shall briefly consider the role of understanding in the natural sciences and then investigate how old notions have indeed been discarded by new experiences in a specific case in the history of science: the impact of quantum theory on the notions of causality and determinism. The concern of this paper is less with deep-rooted metaphysical assumptions or presuppositions which may have guided or misguided the research of scientists and much more with the impact of scientific discoveries on general conceptions of nature.
Concepts like understanding and meaning are usually associated with particular aspects of the social sciences: Social life produces and reproduces symbolic meaning and the social scientist needs to acquire an understanding of the inherent symbolic meaning invested in social life by adopting the viewpoint of a passive participant. In the natural sciences understanding usually means the assignment of representable physical mechanisms and causal processes to the formal, mathematical aspects of physical theories as they may be expressed in scientific laws. This is achieved via the introduction of various kinds of models. The formalism of statistical mechanics, for instance, is given an interpretation in the kinetic model of gases. After some hesitation Planck interpreted the quantum of action, h, – which he had introduced into his distribution law for blackbody radiation- not as a fictitious entity but as a real physical constant. This interpretation led to a radical rethinking of the physical worldview and, as we shall see, to a re-interpretation of fundamental notions with which scientists described the natural world.
It is sometimes affirmed that although the metaphysical principles of science can change they ‘are assumed to be true independently of any scientific experience.’ The present considerations present a counterexample: Specific scientific discoveries led to a reshaping of some of the fundamental notions with which scientists construct a physical worldview. By implication this means that no scientific experience prior to the discovery had led to a questioning of these fundamental notions. Hence the status of these notions changes as a function of scientific experience. It is the contention of this paper that some fundamental, even metaphysical notions of science have undergone changes as a direct result of scientific discoveries. This claim runs counter to one part of the Duhem-Quine thesis, which holds that empirical results only affect the periphery of the conceptual scheme and do not touch core conceptions at its very centre. More specifically, the aim of this paper is to show how fundamental scientific discoveries had an immediate impact on the way scientists interpreted the structure of the natural world by reference to such fundamental notions as causality and determinism.
These notions came under scrutiny after the first experimental successes of the budding quantum theory. Often these two notions are used interchangeably. This identification of strict causality with predictive determinism is a feature of classical physics as eternalised in Laplace’s spirit. For this superhuman demon the whole past, present and future state of the physical world is stretched out before his very eyes like a filmstrip. There exists no novelty, no genuine becoming. From the present frame, all other frames are predictable or retrodictable, given the knowledge the demon possesses of the laws of physics and the boundary conditions of the physical universe. Furthermore, one frame causes the next and is caused by the previous frame. Hence causality and determinism are identical for the Laplacean demon. However, a central conceptual change which occurred as a result of experimental evidence from quantum mechanics was the separation of these two notions. Central figures like Born, de Broglie and to a certain sense Planck came to hold that causality and indeterminism were compatible.
To understand this separation it is important to recall that three new physical experiences – the phenomenon of radioactivity, the need for quantum discontinuity and the experience of spontaneous emissions and absorptions – jointly led to a reflection on quantum behaviour and the appropriateness of the classical notions of causality and determinism.
(a) The discovery of the phenomenon of radioactivity (Becquerel, Curie) around the turn of the century and the establishment of the decay law – – by Rutherford and Soddy (1900), with its characteristic half-life decay curve, were amongst the first indications that the causality concept of classical physics may have to come under scrutiny. But Rutherford was not prone to philosophical reflections so that it was left to a later generation of physicists to point out that the discovery of the decay law had wide-ranging philosophical consequences. The impossibility to determine which particular atom in N will disintegrate ‘seemed to remove causality from a large part of our picture of the physical world,’ as James Jeans observed. Soon after the discovery of the decay law further events began to cast serious doubts on the adequacy of the classical notions of causality and determinism. The major event of these years was (b) the emergence of the quantum concept in the work of Max Planck. Planck’s introduction of the constant h into physics – which meant the insertion of discreteness and discontinuity – was at first an ad hoc manoeuvre to make his energy distribution law in blackbody radiation compatible with the experimental evidence (avoidance of ultraviolet catastrophe). To drive home the inevitability of this new physical constant Planck uses an analogy: Water whipped up by the wind will slowly be transformed from ordered waves to an unordered calm sea, from ordered molar energy to unordered molecular energy, when the wind dies away. It may be suspected that radiant energy in a cavity will also experience a slow transformation from ordered long infrared waves to unordered short ultraviolet waves. On the analogy with the water waves the infrared waves are expected to disappear and be transformed into ultraviolet waves. This is however not the case: the energy transformation reaches a maximum in the region of visible light (), and then rapidly falls off towards shorter and longer wavelengths. In order to account for this behaviour Planck postulated the hypothesis that energy can only be absorbed and re-emitted in discrete bundles of energy: quanta or multiples of the constant h. Experiments soon showed that h was a new fundamental physical constant and this interpretation provided it with a physical meaning. For a theoretical justification for the distribution law Planck turned to Boltzmann’s connection between entropy and probability, as expressed in the equation:
(where W is the thermodynamic probability and k is the Boltzmann constant; more precisely, W is a measure of the number of micro-conditions, which are compatible with a given macro-condition of a physical system in a certain thermodynamic state). In his search for a justification Planck remained in the realm of statistical mechanics, in which the concept of the statistical law was of paramount importance. He was struck by the existence of two different types of laws in physics: the dynamical laws of classical mechanics which he took to be deterministic causal laws and statistical laws which reflected lawlike tendencies of ensembles of particles (as for instance in the decay law). Planck always lamented the co-existence of these two types of laws in physics because they led to two different kinds of causal connections between physical states: on the one hand the necessary link between cause and effect familiar from classical physics, on the other hand the merely probable link between an ensemble of thermodynamic systems and their evolution towards a maximum state of entropy.
From the introduction of the quantum in blackbody radiation Planck was naturally led, through its connection to statistical laws, to a consideration of causality. As we shall see his solution was to differ from that of Einstein whose work on statistical mechanics and quantum theory provided the third new experience (c), which led to a re-questioning of the concept of causality. Einstein’s deep worry about the fate of causality in quantum mechanics had its root in a process known as spontaneous emission. Both the absorption and emission of photons in atomic processes which govern whether atoms are in a ground state or excited state have a quantum nature. Although Einstein was able to write down equations which govern these processes, he was deeply worried by their philosophical implications. He found it unacceptable, as he wrote to Max Born, to do without ‘complete causality’ even in quantum-like processes like absorption and emission of photons. He found the idea unbearable that an electron which had received a pulse of light energy should freely choose the moment and the direction of its escape from the atom. Admittedly, the knowledge of initial conditions and general laws failed to lead to precise predictions – a fundamental assumption of classical physics. But this pragmatic failure of determinism did not particularly worry Einstein. He held instead that the notion of ‘complete causality’ had a clear sense and he took it to mean ‘predictive determinism’, which is closely associated with the differential equations of physics.
Thus there were at least three new physical experiences which led to a questioning of old notions. Einstein’s refusal to abandon the idea of strict causality, even though in practice it may not be possible to adhere to the classical programme of strict predictability gives a hint of how some of the central figures reacted to this situation. It is at this juncture that the concepts of causality and determinism part and go their separate ways. Einstein himself, as Max Born reminded him, had spoken of the need to constantly check the usefulness and justification of concepts against experience. Despite popular impressions that quantum mechanics has spelt the end of causality, quantum mechanics does not abandon causality. But the classical identification of determinism and causality, so well captured in the Laplacean spirit, comes to an end. Quantum mechanical evidence led to a careful distinction between determinism and causality. It is significant to note that neither Planck, Born, de Broglie nor even Heisenberg or Bohr gave up the notion of causality. But these central notions shifted ground in response to the new experimental evidence.
Determinism is the one concept which has to be abandoned in quantum mechanics due to the statistical nature of quantum mechanical experiments and measurements. Determinism even in classical physics describes an ideal situation: from the knowledge of boundary conditions and the presence of universal laws it should be possible to make precise predictions about the future spatio-temporal location of the system under consideration. Conversely, from the existence of present conditions and the knowledge of general laws it should be possible to retrodict past initial conditions. Heisenberg describes this strict causal determinism as the principle of Newtonian physics. It implies strict predictability and retrodictability:
Astronomy provides the paradigmatic example. But even in this case idealising conditions need to be introduced. The planetary laws, formulated by Kepler and Newton, describe the behaviour of one planet under the gravitational influence of a central sun, and abstract from the existence of other planets.
Indeterminism becomes the important notion for quantum-mechanical systems: it does not stand for a model of random events at the root of all physical processes. Indeterminism refers only to the incomplete determination of boundary conditions, as expressed in Heisenberg’s Indeterminacy Relations for micro-systems. Hence indeterminism does not mean the absence of lawful regularities, even in micro-systems and hence does not mean complete unpredictability. It means that the present initial conditions, either in terms of time and energy or in terms of location and momentum, of individual particles in micro-systems cannot be sufficiently determined to predict their precise space-time trajectories. Note that Heisenberg’s indeterminacy relations express a fundamental indeterminacy at the level of the behaviour of quantum systems and not merely at the level of human knowledge about them.
Causality comes to depict, in the words of Planck, a lawful connection in the temporal succession of events. Born defines causality as ‘the belief in the existence of mutual physical dependence of observable situations.’ However, this general characterisation of causality takes us away from a seemingly sounder classical model of a causal mechanism between two macro-objects: a bullet penetrates a block of wood and causes the block to move forward. If the mass and velocity of the bullet and the mass of the wood block are known, it is perfectly possible to determine the velocity of the wood block after the impact and to calculate the distance covered, taking into account the effects of friction. Given the initial conditions of the material objects and the knowledge of the law of the conservation of momenta, it is possible to tell a causal story. But this model of mechanistic causality is quite different from the model of conditional causality to which quantum mechanics appeals. Quantum mechanics cannot trace the space-time trajectories of atomic particles with complete determination in terms of classical parameters. The model of mechanistic causality may have inspired the classical identification of causality with determinism. But even in the classical world of macro-particles and mechanical forces, mechanistic causality and determinism did not always coincide. For instance, using Kepler’s third law, , it is possible to determine, from the knowledge of the orbital period of a planet, its average distance from the sun. Yet this is a functional law: neither of its terms can serve as an antecedent, contiguous cause on the model of classical causality. What experiments in quantum mechanics typically show is that a quantum mechanical system can be prepared in a desired state; the effect of interference with this state can then be measured; but the measurement only records a statistical result.
Thus we see emerge two conceptions of causality only the latter one of which, in terms of a physical dependence of a set of posterior conditions on a set of antecedent prior conditions, is adequate for quantum-mechanical systems. We have seen that the mechanistic model of causality fails in the world of quantum mechanics because of the ubiquity of the indeterminacy relations. But strictly speaking, inexactness in the measurement of initial conditions and idealisations of laws is also a feature of classical physics. There must therefore be a deeper reason for the inapplicability of the model of mechanistic causality to the quantum world. At this point Max Planck‘s philosophical instinct pointed to the deeper level of ontology to fathom the reason why mechanical causality fails in quantum mechanics: the quantum physicist needs to question the very ontology of the classical world. The blame is to be laid at the door of the model of the classical particle. Planck and other quantum physicists called for the abandonment of the notion of the material point particle. It is this fundamental Newtonian ontology – the corpuscular nature of the world, governed by mechanistic laws of motion in a homogeneous space – which inspired mechanical causality. But when quantum mechanics showed that the tracing of causal mechanisms between events, required by mechanistic causality, failed for quantum events, the classical identification of determinism and causality inspired the erroneous picture of an indeterministic and acausal microworld. In fact it is the very ontology of classical mechanics which fails to be supported by the evidence of quantum mechanics. And for the physicist the nature of reality is determined by the nature of the evidence.
The decisive input to the new world picture consistent with quantum mechanics was provided by Max Born with his probabilistic interpretation of quantum mechanics:
The motion of particles follows the laws of probability, but the probability itself propagates in accordance with the causal law. (…) This means that the knowledge of the state in all respects in a particular moment determines the distribution of the state in all later times.
This is just the notion of conditional causality understood as physical dependence between physical parameters. The dependence sought now is not that between single causes and effects but between a set of antecedent conditions of a whole quantum system and the probability of occurrence of consequent conditions. The probability of the consequent conditions of a quantum system is expressed in the Born rule: . Note that this conditional notion of causality is compatible with the indeterminism of the quantum world. In the famous two-slit experiment it is not possible to determine through which slit an electron passes without disturbing its momentum; but this disturbance destroys its characteristic coherence pattern. Any interference with the electron destroys information needed for the precise determination of its trajectory, and hence destroys its coherence effects. By contrast non-interference with the electron in the two-slit experiment results in its characteristic coherence patterns but at the cost of knowledge about its trajectory. The point is, however, that the very acts of interference and non-interference, which either destroy or create the intensity patterns, are causal processes, which lead to observable effects. Hence in quantum mechanics we still have conditional causality, understood as physical dependence of effect events on cause events, but no longer classical determinism. The task of philosophy will be to overcome these two different conceptions of causality and find a model of causality, which fits both classical and quantum mechanics.
This paper has attempted a brief discussion of the complex connection between science and philosophy. The principal theme has been that fundamental notions like causality and determinism, with which scientists construct their worldviews persist, under the tension of ongoing experimental research and undergo change as a direct result of new empirical discoveries.
Department of Social Sciences and Humanities
University of Bradford
West Yorkshire BD7 1DP UK