Einstein (41 page)

Soon afterward Bohr gave a more satisfactory interpretation of this strange behavior of atomic particles. He pointed out that “position” and “momentum” are two different aspects of a small mass (e.g., an electron) in much the same way that the particle properties and wave properties are two aspects of the photon. To say that a particle is located in a certain limited region of space is exactly analogous to the statement that light-energy is concentrated in a photon, and to define the momentum of a particle is analogous to the emphasis on the wave aspect of light. Both material particles and light have the dual characteristics of particles and waves, but their behavior is neither contradictory nor haphazard. Bohr emphasized again “Mach’s requirement” that we should make only such statements as can be tested by definite physical experiments. According to him, it depends solely on the specific arrangement of apparatus used whether the emission of light and of electrons has to be described as a wave or as a beam of moving particles. According to this view, the two types of properties exhibited are “complementary” features of the same physical object. What we observe depends on what observable reaction of our subatomic phenomena we bring to a test. This conception has been called
Bohr’s theory of complementarity
.

Bohr’s point of view is therefore even more different from Newtonian mechanics than Einstein’s theory of relativity. In Bohr’s conception, we cannot describe what “actually” occurs in space while, say, light is emitted by the sun before it hits the earth. We can describe only what we observe when a measuring apparatus is hit by light. We can, for example, describe whether or not the light from the sun hits a certain spot on a screen. Or, to express it more precisely: We cannot describe “physical reality” by describing the path that a particle traverses
in space, but we can and must describe only the observations made on various physical instruments arranged at different points in space and time. Physical laws link together these observations, but not the positions or paths of the particles or photons. This viewpoint was interpreted as being in agreement with
positivistic
philosophy which asserts that science cannot discover what actually happens in the world, but can only describe and combine the results of different observations.

Since the beginning of the twentieth century more and more emphasis has been placed on the conflict between the above view that science can only describe and systematize the results of observations and the view that it can and must investigate the
real world
. This controversy became particularly acute among the physicists in central Europe. Max Planck was the spokesman for the latter view, which he called the “metaphysical” view, and he directed his sharpest polemics against those who seemed to him to be the most radical representatives of the opposite side. In particular he attacked Mach’s positivistic conception of science, which agrees with Bohr’s view.

About this time a reformulation of positivism started in Vienna and Prague. The new movement was closely related to “Mach’s requirement.” The core of the movement was the
Wiener Kreis
(Vienna Circle), Moritz Schlick, R. Carnap, O. Neurath, and others. In this country it was described as
logical positivism
and established contact with related established tendencies such as
pragmatism
and
operationism
. In England a similar movement is headed by Bertrand Russell.

Since the positivistic conception of physics had been stimulated strongly by Einstein’s pioneer work in the theory of relativity and in atomic physics, many persons regarded Einstein as a kind of patron saint of positivism. To the positivists he seemed to bring the blessing of science, and to their opponents he was the evil spirit. Actually his attitude to positivism and metaphysics was by no means so simple. The contradictions in his personality that we have observed in his conduct as a teacher and in his attitude to political questions also manifested themselves in his philosophy.

Einstein recognized wholeheartedly the great success of
Bohr’s theory in explaining the many phenomena of atomic physics, but from a more philosophical standpoint he was not ready to admit that one must abandon the goal of describing physical reality and remain content only with the combination of observations. He was aware that it was not possible, as Newton had thought, to predict all future motions of all particles from the initial conditions and the laws of motion. But perhaps, thought Einstein, physical events could be described in terms of a new theory as yet unknown. It would consist in a system of field equations so general that they would contain the laws of motion of particles and of photons as special cases.

I must admit that over a long time I myself believed that Einstein was an adherent of the positivistic interpretation of Bohr’s theory. In 1929, at a congress of German physicists in Prague, I delivered an address in which I attacked the metaphysical position of the German physicists and defended the positivistic ideas of Mach. After my address a well-known German physicist with whose philosophical views I was not acquainted rose and said: “I hold to the views of the man who for me is not only the greatest physicist of our time, but also the greatest philosopher: namely, Albert Einstein.” Thereupon I felt a sense of relief and expected the speaker to support me against my opponents, but I was mistaken. The speaker declared that Einstein rejected the positivistic theories of Mach and his supporters and that he regarded physical laws as being more than combinations of observations. He added that Einstein was entirely in accord with Planck’s view that physical laws describe a reality in space and time that is independent of ourselves.

At that time this presentation of Einstein’s views took me very much by surprise. It was oversimplified, indeed, but I soon realized that Einstein’s partly antagonistic attitude toward the positivistic position was connected with his attitude toward Bohr’s conception of atomic physics. Shortly afterward I saw a paper by Lanczos, one of Einstein’s closest collaborators, in which he contrasted the theory of relativity with Bohr’s theory in the following manner: Einstein’s general theory of relativity is the physics corresponding to the metaphysical conception of science; Bohr’s theory, on the other hand, is in accord with the radical positivistic conception. I was quite astonished to find the theory of relativity characterized in this manner, since I had been accustomed to regarding it as a realization of Mach’s program.

Not long afterward — I believe it was in 1932 — I was visiting
Einstein in Berlin. It had been a long time since we had conversed personally, and consequently I knew little of his stand on questions about which he had not published anything. We discussed the new physics of Bohr and his school, and Einstein said, partly as a joke, something like this: “A new fashion has now arisen in physics. By means of ingeniously formulated theoretical experiments it is proved that certain physical magnitudes cannot be measured, or, to put it more precisely, that according to accepted natural laws the investigated bodies behave in such a way as to baffle all attempts at measurement. From this the conclusion is drawn that it is completely meaningless to retain these magnitudes in the language of physics. To speak about them is pure metaphysics.” In this statement, among other things, he apparently referred to magnitudes such as the “position” and “momentum” of an atomic particle.

Hearing Einstein talk in this way reminded me of many other discussions to which his theory of relativity had given rise. Repeatedly the objection had been raised: if magnitudes such as the “absolute temporal interval between two events” cannot be measured, one should not conclude that consequently it is completely meaningless to speak of this interval and that “absolute simultaneity” is simply a meaningless conglomeration of words. Einstein’s reply to this argument had always been that physics can speak only about magnitudes capable of being measured by experimental methods. Furthermore, Professor P. W. Bridgman regarded Einstein’s theory of simultaneity as the best illustration of the fruitfulness of his “positivistic” requirement that only magnitudes having an “operational definition” should be introduced into physics. Consequently I said to Einstein: “But the fashion you speak of was invented by you in 1905?” At first he replied humorously: “A good joke should not be repeated too often.” Then in a more serious vein he explained to me that he did not see any description of a metaphysical reality in the theory of relativity, but that he did regard an electromagnetic or gravitational field as a physical reality, in the same sense that matter had formerly been considered so. The theory of relativity teaches us the connection between different descriptions of one and the same reality.

Actually Einstein has been a positivist and empiricist since he has never been willing to accept any perennial framework for physics. In the name of progress in physics he claims the right to create any system of formulations and laws that would be in agreement with new observations. For the older positivism the
general laws of physics were summaries of individual observations. For Einstein the basic theoretical laws are a free creation of the imagination, the product of the activity of an inventor who is restricted in his speculation by two principles: an empirical one, that the conclusions drawn from the theory must be confirmed by experience, and a half-logical, half æsthetic principle, that the fundamental laws should be, as few in number as possible and logically compatible. This conception hardly differs from that of “logical positivism,” according to which the general laws are statements from which our observations can be logically derived.

In the twentieth century, when Einstein created his special theory of relativity, and even more so when he produced his general theory, it became evident that physical theories were to an ever increasing degree no longer simple summaries of observational results, and that the path between the basic principles of the theory and the observational consequences was more involved than had formerly been thought. The development of physics from the eighteenth century to Einstein was accompanied by a correspondent development of philosophy. The conception of general laws as
summaries
of observations gave way more and more to the conception that laws are creations of the imagination, which are to be
tested
by observation.
Mach’s Positivism
was replaced by
Logical Positivism
.

In the Herbert Spencer Lecture which he gave at Oxford in the summer of 1933 shortly before he left Europe forever, Einstein presented the finest formulation of his views on the nature of a physical theory. He spoke first about the physics of the eighteenth and nineteenth centuries — that is, the period of mechanistic physics:

“The scientists of those times were for the most part convinced that the basic concepts and laws of physics were not in a logical sense free inventions of the human mind, but rather they were derivable by abstraction — that is, by a logical process from experiment. It was the general theory of relativity that showed in a convincing manner the incorrectness of this view.”

After Einstein had emphasized that the fundamental physical concepts were products of invention or fictions, he continued:

“The conception here outlined of the purely fictitious character of the basic principles of physical theory was in the eighteenth and nineteenth centuries far from being the prevailing one. But it continues to gain more and more ground because of the ever widening gap between the basic concepts and laws on the one side and the consequences
to be correlated with our experience on the other — a gap which widens progressively with the developing unification of the logical structure — that is, with the reduction of the number of the logically independent conceptual elements required for the basis of the whole system.”

As in so many aspects of his life and thought, we also note a certain internal conflict in Einstein’s attitude toward the positivistic conception of science. On the one hand, he felt an urge to achieve a logical clarity in physics such as had not previously been attained, an urge to carry through the consequences of an assumption with extreme radicalism, and was unwilling to accept any laws that could not be tested by observation. On the other hand, however, he felt that even
Logical Positivism
did not give sufficient credit to the role of imagination in science and did not account for the feeling that the “definitive theory” was hidden somewhere and that all one had to do was to look for it with sufficient intensity. As a result Einstein’s philosophy of science often made a “metaphysical” impression on persons who are unacquainted with Einstein’s positivistic requirement that the only “confirmation” of a theory is its agreement with observable facts.

In his general theory of relativity Einstein had treated the force of gravity as due to a gravitational field. Matter gave rise to a gravitational field, which in turn acted on other material bodies to cause forces to act. Einstein had taken this force into account by means of curvature in space. A similar situation existed for electrically charged particles. Forces act between them, and they could be taken into account by considering the electric charges to give rise to an electromagnetic field, which in turn produced forces on other charged particles. Thus matter and gravitational field were exactly analogous to electric charge and electromagnetic field. Consequently Einstein sought to build a theory of “unified field” which would be a generalization of his gravitational theory and would include all electromagnetic phenomena. He also thought that in this way he might be able to obtain a more satisfactory theory of light quanta (photons) than Bohr’s, and derive laws about “physical reality” instead of only laws about observational results.

The great success of the geometrical method in the general theory of relativity naturally suggested to him the idea of developing the new theory in the structure of four-dimensional space. In this case it must have still other characteristics besides the curvature which takes care of gravitational effects.