Friedel Weinert
Department of
Social Sciences and Humanities
University of
Bradford
Bradford BD7 1DP
UK
The
path of the philosopher is indicated by that of the scientist. H.Reichenbach
I. On September 26, 1905 Einstein’s paper ‘On the Electrodynamics of Moving Bodies’ appeared in the Annalen der Physik. All agree that it is one of the most important scientific papers ever written. But was it a revolutionary paper? Einstein generalizes the Galilean relativity principle to include electro-magnetic phenomena; he postulates the velocity of light in vacuum as an upper speed limit on all phenomena. He uses the Lorentz transformations for the calculation of spatial and temporal measurements in the transition from one reference frame to another. There is much to be said for the view that Einstein’s Special theory of relativity completes classical physics, especially the work of Maxwell. Einstein himself did not see his theory as a ‘revolutionary act’. But Einstein’s work did introduce a philosophical revolution in our fundamental notions. Einstein was not a professional philosopher. He was, in Reichenbach’s judicious phrase, a philosopher by implication. Still, it would be more judicious to characterize Einstein’s philosophical innovations as consequences of his scientific work. Implications can be hidden in the logic of a situation. But Einstein was fully aware of the philosophical dimensions of his scientific work. I prefer therefore to speak of the philosophical consequences of Einstein’s physics. They extend far beyond the familiar reshaping of the notions of space and time. What made Einstein a great physicist was his ability to question unquestioned assumptions in the tradition of physical theorizing. What made him an even greater physicist was his ability to recognize the limits of his own work. This talent led him from the Special to the General theory of relativity and beyond to a general theory of fields. What made him a decent philosopher was his willingness to pursue the philosophical consequences of his physical discoveries.
Einstein followed the logic of the problem situation, which his physical discoveries had created, into the field of philosophy. Many great scientists of his generation followed suit. Think of men like Eddington, Bohr, Born, Heisenberg, von Laue and Planck. Even today many scientists are ready to contemplate the philosophical consequences of scientific discovery. Science therefore has philosophical consequences. But science also relies on philosophical presuppositions. To regard Einstein as a philosopher is to consider his position on a number of philosophical issues. Einstein philosophizes within the constraints of science, in particular his science. His questions are familiar to every philosopher of science: How do theories relate to the external world? What is the nature of reality? What is the nature of time and space? What is the status of scientific theories? What does quantum mechanics tell us about reality? Given the principle of relativity, what is to be regarded as the real?
II. As Einstein philosophizes within the constraints of the theory of relativity, he sets these philosophical questions within a concrete scientific problem situation. His answers derive their significance from this problem situation. Historically, his first concern was the notion of time. When the Special theory of relativity was generalized to the General theory, his second philosophical worry became the notion of space. But with hindsight we can reorder his philosophical concerns into a logically more coherent picture.
Einstein’s fundamental philosophical position arises from the age-old puzzle of how concepts are related to facts. More generally, how do abstract scientific theories relate to concrete empirical data? We can give this question a slightly more philosophical turn by asking how scientific theories represent empirical reality. Einstein’s philosophical worry derived from his dissatisfaction with Newtonian physics as a fundamental theory. When Einstein first aired his worry, in his Obituary of Ernst Mach (1916), he warned against the tendency to regard concepts as thought necessities. Once certain concepts have been formed, often on the basis of experience, there is a danger that they will quickly take on an independent existence. People are tempted to regard them as necessary. Concepts, however, just like theories, are always subject to revisions. Einstein complained that
Philosophers had a harmful effect upon the progress of scientific thinking in removing certain fundamental concepts from the domain of empiricism, where they are under our control, to the intangible heights of the a priori.[1]
What Einstein had in mind were the notions of time and space. Newton had regarded it as necessary to introduce the notions of absolute space and time into his mechanics in order to make sense of his laws of motion. Few classical physicists had questioned Newton’s reasoning, with the notable exception of Leibniz, Mach and Maxwell. So these notions had become part and parcel of classical physics. They had congealed to philosophical presuppositions, to thought necessities. The Special theory arrived at a different result. Temporal and spatial measurements became relativitized to particular reference frames. This was a necessary consequence of embracing the principle of relativity and taking the velocity of light as a fundamental postulate of the theory. Through his own work Einstein witnessed how such fundamental philosophico-physical notions as time and space required conceptual revision. This made him forever suspicious about the sway that such notions could hold over people’s minds.
III.
It is often claimed that the Special theory of
relativity led Einstein to a static view of time. The argument runs as follows:
the Special theory shows that simultaneity cannot be absolute, as Newton
assumed, since this presupposes a propagation of all causal influence at
infinite speeds. Observers in different reference frames, which travel at
relative constant speed with respect to each other, will not agree on the
simultaneous happening of some event, E. If there is no cosmic notion of
time, to which all observers can appeal, time must pass at different rates for
each observer, depending on the speed of the reference frame. Time cannot be an
objective property of the universe. It depends on the perception of observers.
The universe is static, a block universe. The passage of time is an illusion.
Einstein did at times adopt such a philosophy of being. But there are numerous
passages in Einstein’s work, in which he argues for a more dynamic view of
time. Rather than speaking of space-time, as Minkowski did, Einstein often
prefers the expression, time-space. And he points out that time and space do
not have the same status in Minkowski’s four-dimensional world.
The non-divisibility of the four-dimensional continuum of events does not at all (..) involve the equivalence of the space co-ordinates with the time co-ordinate.[2]
In his theory of space, Einstein aligns his thinking to the relationist position, espoused by Leibniz and Mach. Despite his occasional statements to the contrary his whole theory of time-space points towards a philosophy of becoming.
IV. It has not often been observed that Einstein himself became a victim of the power of philosophical presuppositions. Einstein revolutionized our philosophical notion of time by relativizing both time and simultaneity to particular inertial reference frames. But in his lifelong opposition to the Copenhagen interpretation of quantum mechanics he cheerfully disregarded the lesson about thought necessities, which the theory of relativity had taught him. According to Einstein, quantum mechanics was incomplete because it only permitted statistical statements about ensembles of atoms. Quantum mechanics was unable to make precise spatio-temporal predictions about the trajectories of individual atoms. Heisenberg’s indeterminacy principle, whose validity Einstein fully endorses, prevents deterministic spatio-temporal determinations of atomic trajectories. The ability to make such predictions was for Einstein one of the fundamental requirements of science. Only differential equations, he said, would satisfy the demand of the physicist for causality. This demand for deterministic causality is a reflection of Laplacean determinism, which the quantum theory was hoping to overcome. Although the Schrödinger equation is a differential equation, it only applies to the evolution of quantum systems in an abstract Hilbert space. When Einstein warns that a probabilistic view of quantum mechanics will lead to its incompleteness, on the grounds that it does not allow for precise space-time trajectories of atomic particles, he clings to one of the most venerable presuppositions of classical physics. Philosophically speaking, this is an inconsistent attitude. In his criticism of Newtonian mechanics, Einstein bemoans the inability to jettison fundamental notions like absolute space and time. But in his view of quantum mechanics he himself falls victim to belief in strict determinism. Never underestimate the power of presuppositions!
V. To Einstein scientific theories are free inventions of the human mind. No amount of inductive generalizations can lead from empirical phenomena to the complicated equations of the theory of relativity. But science is not fiction. Science assumes the existence of an external world. Scientific theories are statements about the external world. So how do scientific theories relate to the external world? For Einstein the world of experience was the final arbiter of the validity of scientific theories. In Popperian fashion he regarded all scientific theories as falsifiable. The scientist proposes, nature disposes. This made Einstein a critical realist. Scientific theories present hypothetical pictures of the external world. Pictures need not be mirror images. A scientific theory constructs a coherent and logically rigid account of the available empirical data. Its coherence may always come under threat with new empirical discoveries. There is nothing final about the representation of a scientific theory of the external world. Does this mean that there is always a plethora of rival theoretical accounts, which nevertheless are compatible with the available evidence? Does Einstein submit to the postulate of underdetermination, so cherished by many philosophers? Einstein was not a conventionalist about scientific theories. He did not believe that many alternative representations of the empirical world could be sustained. He grants that logically speaking there are always numerous theoretical accounts, which could in principle account for the available evidence. This is due to the fact that theories are free inventions of the human mind. But Einstein also believes that there is one correct theory. The structure of the external world has the power to eliminate many rival accounts. The surviving theory displays such a degree of rigidity[3] that any modification in it will lead to its falsehood. It is all a question of fit.
Einstein employs various analogies to drive home his point. Consider first the rigidity of scientific theories. The analogy of the crossword puzzle will help to illustrate the point. At first we are fairly free to insert various linguistic combinations into the available columns and rows. But we soon realize that the answers in a few columns and rows impose constraints on the remaining answers. In fact the constraints provide feedback loops. Even though we may have thought at first that one answer was right, we may realize that it does not cohere with a later answer of whose correctness we are fairly sure. As we complete the rows and columns the constraints get ever tighter. Eventually the coherence of the crossword puzzle dictates that only the correct answers will fit. Any change in the completed crossword puzzle will have negative repercussions on the remaining answers. A crossword puzzle displays a great amount of rigidity. There are not normally several alternative solutions to it. A crossword puzzle is not a scientific theory. And the newspaper’s key to the correct answer is not the external world. But the analogies help to understand the point. A newspaper reader’s correct solution of a crossword puzzle will fit the answer the puzzle demands. How would a scientific theory fit the external world? Consider how clothes fit a human body. A tall man can wear the clothes of a small man. But they do not fit him very well. And vice versa. There will be clothes for a tall man that will fit his tall frame much better than others. If your shoe size is 7½, no other shoe will fit as well. Could the same not be true of scientific theories? Einstein thought so. Regard the empirical facts and the mathematical theorems as constraints. If their amount and their interconnections can be increased, then many scientific theories will fail to satisfy the constraints. It will usually leave us with only one plausible survivor. For instance, the General theory of relativity was able to explain the perihelion advance of Mercury, where Newtonian mechanics had failed. It does not follow from this argument that the survivor – let us say the theory of relativity – will be true. But it does follow that the process of elimination will leave us with the most adequate theoretical account presently available. New experimental or observational evidence may force us to abandon this survivor. The desire for unification and logical simplicity may persuade us to develop alternative theoretical accounts. Einstein’s extension of the principle of relativity from its restriction in the Special theory to inertial reference frames to non-inertial reference frame in the General theory is a case in point. Although Einstein does claim that there is one correct theory, he cannot mean this in an absolute sense. His insistence on the eternal revisability of scientific theories speaks against this interpretation. What he must mean is that there is always one theory, which best fits the available evidence. This one theory copes best with all the constraints, which logic and evidence erect.
Einstein is a critical realist. He believes in the existence of an external world, irrespective of human awareness. Theories are free inventions of the human mind. Theories are required to represent reality. They represent reality by fitting the constraints of the external world and the demand for logical simplicity. A theory is not a mirror image of the world. It is a mathematical representation, which provides coherence of the empirical data and shows their interconnections. Theories are hypothetical, approximate constructions, which in a process of fitting and refitting, deliver a coherent picture of the external world. In human efforts to understand the world, empiricism and rationalism go hand in hand.
VI. What is physical reality? There was a time when physicists liked to think of the world as a massive clockwork mechanism. Particles populated the universe. They were in constant regular motion. Einstein suspected that this classical picture was mistaken. It required Newton’s absolute space and time and action at a distance. For Einstein, Hertz, Faraday and Maxwell made significant steps in the revision of the physical worldview when they introduced fields as fundamental physical entities. Einstein regarded the theory of relativity as a field theory, which dispenses with action at a distance. But Einstein was never able to overcome the fundamental dualism in the physical worldview between particles and fields. To overcome this dualism is the job of physicists, not philosophers. Current attempts to construct a theory of quantum gravity may eventually lead to success. In his thinking about the nature of reality, Einstein made another significant contribution to philosophy. Einstein became one of the first physicists to realize the significance of symmetries and invariance in science. In doing so he provided a new criterion of what we should regard as objective and physically real.
The starting point is the principle of relativity. In its general form it states that all co-ordinate systems, which represent physical systems in motion with respect to each other, must be equivalent from the physical point of view. In other words, the laws which govern the changes that happen to physical systems in motion with respect to each other, are independent of the particular reference system, to which these changes are referred. But we have already observed that in the transition from one reference system to another some properties change. The classic examples are temporal and spatial measurements, as well as mass determinations. From the phenomenon of time dilation and the relativity of simultaneity some physicists concluded that time cannot be a physical property of the universe. Some transitions to other reference systems do not, however, affect the physical properties. The classic example is the velocity of light in vacuum. The Special theory of relativity postulates that the value of ‘c’ will be the same in all reference systems. Some physical properties are immune to changes in reference systems, while others are not. The velocity of light is the same in all directions and irrespective of whether it is emitted from a moving or stationary source. But the wavelengths of light depend on the movement of the source, as evidenced in the Doppler effect. Symmetry principles determine the immunity to change. While in classical physics, many properties, like time, mass, space, energy were regarded as ‘absolute’, in the Special theory of relativity, many properties became relational. So the question arises, ‘What is real?’. The answer, which Einstein found embedded in the mathematics of the Special theory of relativity was that the invariant is a candidate for the real. Minkowski’s four-dimensional interpretation of space-time provided Einstein with a criterion. Temporal and spatial measurements varied from reference frame to reference frame. They could not be physically real. But the space-time interval, ds, remained invariant for every observer. It was therefore to be regarded as real. We should of course be careful with such criteria. The philosopher must evaluate whether the philosophical consequences, which the scientist claims for a scientific theory, really do follow. It may rightfully be objected that clocks, not observers, measure time in a particular reference frame. Clocks and their measurements are as real in one reference system as in any other. This is true. From a perspectival point of view, what a clock tells us in each reference frame must be regarded as real. But physics is not interested in perspectival realities. Once the symmetries tell us what remains invariant across reference frames, it is not difficult to derive the perspectival aspects, which attach to different reference frames, as a function of velocity.
VII. Philosophical consequences do not flow from scientific theories with logical compulsion. Nevertheless, certain kinds of philosophical positions are more akin to scientific findings than others. Einstein has therefore provided the philosopher with much food for thought. He accused the philosopher of dragging concepts into the den of the a priori. Sometimes, however, the very foundations of science become shaky. This happened twice in Einstein’s lifetime: relativity and quantum theory. Then the physicist himself is forced to become a philosopher through a ‘critical contemplation of the theoretical foundations’. Every true theorist is a ‘tamed metaphysicist’.[4] For, as Einstein observed, science without philosophy is a muddle. And philosophy without science is an empty scheme
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(1905)
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Relativitätstheorie’, Annalen der Physik 55 (1918)
Relativity: The Special and
the General Theory. London: Methuen (1920)
The Meaning of Relativity. London: Methuen (1922)
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‘Quantenmechanik und Wirklichkeit’, Dialectica 2 (1948)
Essays in Physics. New York (1950)
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Ideas and Opinions. London: Alvin Redman (1954)
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Salle: Open Court. 2 volumes
[1] A. Einstein, The Meaning of Relativity (1922), 2, italics in original
[2] Einstein, The Meaning of Relativity (1922), 30
[3] Steven Weinberg, Dreams of a Final Theory (1993) endorses a similar notion of rigidity.
[4] Einstein, ‘Physics and Reality’ (1936), 17; ‘On the Generalized Theory of Gravitation (1950), 342