When the Scientist turns
Philosopher.
Friedel Weinert
Abstract
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.
I.
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,[1]
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.’[2]
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.
II.
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.’[3] 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.
III.
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.’[4] 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.[5]
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.
IV.
This paper has attempted a brief discussion of the
complex connection between science and philosophy.[6]
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
Email: f.weinert@brad.ac.uk
[1] This is reflected in many books titles: J. Herschel, On the Study of Natural Philosophy (1830); J.H.Fr. Papillon, Histoire de la Philosophie moderne dans ses rapports avec le développement des sciences de la Nature (1876); H. Dingle, Through Science to Philosophy (1937); A.S. Eddington, The Philosophy of Physical Science (1938); J. Jeans, Physics and Philosophy (1943); M. Born, Natural Philosophy of Cause and Chance (1949); W. Heisenberg, Physik und Philosophie (1959). For the sake of brevity only references to direct quotes are given.
[2] M. Born, Natural Philosophy of Cause and Chance (Oxford 1949), p. 2
[3] C. Dilworth, The Metaphysics of Science (Dordrecht 1996), p. 71; italics in original.
[4] Natural Philosophy, p. 124; cf. p. 9
[5] ‘Quantenmechanik der Stobvorgänge.’ Zeitschrift für Physik 38 (1926), p. 54. This characterization is repeated almost verbatim in his Natural Philosophy of Cause and Chance, p. 103
[6] This essay is a revised and updated version of a paper first presented at the 20th World Congress of Philosophy, Boston 1998, available at http://www.bu.edu/wcp/Papers/Scie/ScieWein.htm. The ideas formulated in this paper are discussed at much greater depth in the author’s full-length study The Scientist as Philosopher (Heidelberg/Berlin/New York: Springer 2004).