I am convinced that purely mathematical construction enables us to find those concepts and those lawlike connections between them that provide the key to the understanding of natural phenomena. Useful mathematical concepts may well be suggested by experience, but in no way can they be derived from it. Experience naturally remains the sole criterion of the usefulness of a mathematical construction for physics. But the actual creative principle lies in mathematics. Thus, in a certain sense, I take it to be true that pure thought can grasp the real, as the ancients had dreamed.
Einstein , p. Einstein started to routinely claim that this was the lesson he had drawn from the way in which he had found general relativity Norton What warrant is there for thus trusting in simplicity? At best one can do a kind of meta-induction. The success of previous physical theories justifies our trusting that nature is the realization of the simplest that is mathematically conceivable. That is why, in practice, simplicity seems to determine theory choice univocally. An indication that the map of philosophical positions was drawn then in a manner very different from today is to found in the fact that this principle found favor among both anti-metaphysical logical empiricists, such as Carnap, and neo-Kantians, such as Cassirer.
It played a major role in debates over the ontology of general relativity and was an important part of the background to the development of the modern concept of categoricity in formal semantics for more on the history, influence, and demise of the principle of univocalness, see Howard and One can find no more ardent and consistent champion of the principle than Einstein.
The principle of univocalness should not be mistaken for a denial of the underdetermination thesis. The latter asserts that a multiplicity of theories can equally well account for a given body of empirical evidence, perhaps even the infinity of all possible evidence in the extreme, Quinean version of the thesis. The principle of univocalness asserts in a somewhat anachronistic formulation that any one theory, even any one among a set of empirically equivalent theories, should provide a univocal representation of nature by determining for itself an isomorphic set of models.
The unambiguous determination of theory choice by evidence is not the same thing as the univocal determination of a class of models by a theory. When, in , Einstein wrongly rejected a fully generally covariant theory of gravitation, he did so in part because he thought, wrongly, that generally covariant field equations failed the test of univocalness. What Einstein realized in was that, in , he was wrongly assuming that a coordinate chart sufficed to fix the identity of spacetime manifold points. The application of a coordinate chart cannot suffice to individuate manifold points precisely because a coordinate chart is not an invariant labeling scheme, whereas univocalness in the representation of nature requires such invariance see Howard and Norton and Howard for further discussion.
Here is how Einstein explained his change of perspective in a letter to Paul Ehrenfest of 26 December , just a few weeks after the publication of the final, generally covariant formulation of the general theory of relativity:. The physically real in the universe of events in contrast to that which is dependent upon the choice of a reference system consists in spatiotemporal coincidences.
Those statements that refer to the physically real therefore do not founder on any univocal coordinate transformation. For they have all spatiotemporal point coincidences in common, i.
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These reflections show at the same time how natural the demand for general covariance is. CPAE, Vol. Giovanelli or possibly by a conversation with Schlick Engler and Renn, Spacetime coincidences play this privileged ontic role because they are invariant and, thus, univocally determined. Schlick argued that Mach was wrong to regard only the elements of sensation as real. Spacetime events, individuated invariantly as spacetime coincidences, have as much or more right to be taken as real, precisely because of the univocal manner of their determination.
Note, again, that underdetermination is not a failure of univocalness.
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Schlick, of course, went on to become the founder of the Vienna Circle, a leading figure in the development of logical empiricism, a champion of verificationism. The question is this: Do such univocal coincidences play such a privileged role because of their reality or because of their observability. Clearly the former—the reality of that which is univocally determined—is important. But are univocal spacetime coincidences real because, thanks to their invariance, they are observable?
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Or is their observability consequent upon their invariant reality? Einstein, himself, repeatedly stressed the observable character of spacetime coincidences, as in the 26 December letter to Ehrenfest quoted above for additional references and a fuller discussion, see Howard Schlick, still a self-described realist in , was clear about the relationship between observability and reality.
He distinguished macroscopic coincidences in the field of our sense experience, to which he does accord a privileged and foundational epistemic status, from the microscopic point coincidences that define an ontology of spacetime manifold points. Mapping the former onto the latter is, for Schlick, an important part of the business of confirmation, but the reality of the spacetime manifold points is in no way consequent upon their observability. Indeed, how, strictly speaking, can one even talk of the observation of infinitesimal spacetime coincidences of the kind encountered in the intersection of two world lines?
In fact, the order of implication goes the other way: Spacetime events individuated as spacetime coincidences are real because they are invariant, and such observability as they might possess is consequent upon their status as invariant bits of physical reality. For Einstein, and for Schlick in , understanding the latter—physical reality—is the goal of physical theory. Three years earlier, the Bonn mathematician, Eduard Study, had written another well-known, indeed very well-known defense of realism, Die realistische Weltansicht und die Lehre vom Raume Einstein read it in September of Pressed by Study to say more about the points where he disagreed, Einstein replied on 25 September in a rather surprising way:.
I am supposed to explain to you my doubts? By laying stress on these it will appear that I want to pick holes in you everywhere. So, away we go! For me, a hypothesis is a statement, whose truth must be assumed for the moment, but whose meaning must be raised above all ambiguity. This division is, to be sure, not an arbitrary one, but instead …. The answer might be that realism, for Einstein, is not a philosophical doctrine about the interpretation of scientific theories or the semantics of theoretical terms. I just want to explain what I mean when I say that we should try to hold on to physical reality.
We are, to be sure, all of us aware of the situation regarding what will turn out to be the basic foundational concepts in physics: the point-mass or the particle is surely not among them; the field, in the Faraday - Maxwell sense, might be, but not with certainty. If a physical system stretches over the parts of space A and B, then what is present in B should somehow have an existence independent of what is present in A.
What is actually present in B should thus not depend upon the type of measurement carried out in the part of space, A; it should also be independent of whether or not, after all, a measurement is made in A. If one adheres to this program, then one can hardly view the quantum-theoretical description as a complete representation of the physically real. If one attempts, nevertheless, so to view it, then one must assume that the physically real in B undergoes a sudden change because of a measurement in A.
My physical instincts bristle at that suggestion. However, if one renounces the assumption that what is present in different parts of space has an independent, real existence, then I do not at all see what physics is supposed to describe. Realism is thus the thesis of spatial separability, the claim that spatial separation is a sufficient condition for the individuation of physical systems, and its assumption is here made into almost a necessary condition for the possibility of an intelligible science of physics. But the true significance of the separability principle emerged most clearly in , when as hinted in the just-quoted remark Einstein made it one of the central premises of his argument for the incompleteness of quantum mechanics see Howard and It is not so clearly deployed in the published version of the Einstein, Podolsky, Rosen paper , but Einstein did not write that paper and did not like the way the argument appeared there.
In brief, the argument is this. Separability implies that spacelike separated systems have associated with them independent real states of affairs.
A second postulate, locality, implies that the events in one region of spacetime cannot physically influence physical reality in a region of spacetime separated from the first by a spacelike interval. Consider now an experiment in which two systems, A and B, interact and separate, subsequent measurements on each corresponding to spacelike separated events. But quantum mechanics ascribes different theoretical states, different wave functions, to B depending upon that parameter that is measured on A. Therefore, quantum mechanics ascribes different theoretical states to B, when B possesses, in fact, one real physical state.
Hence quantum mechanics is incomplete.
One wants to ask many questions. First, what notion of completeness is being invoked here? It is not deductive completeness. The next question is why separability is viewed by Einstein as virtually an a priori necessary condition for the possibility of a science of physics.
And a field theory like general relativity can do this because the infinitesimal metric interval—the careful way to think about separation in general relativistic spacetime—is invariant hence univocally determined under all continuous coordinate transformations. Another reason why Einstein would be inclined to view separability as an a priori necessity is that, in thus invoking separability to ground individuation, Einstein places himself in a tradition of so viewing spatial separability with very strong Kantian roots and, before Kant, Newtonian roots , a tradition in which spatial separability was known by the name that Arthur Schopenhauer famously gave to it, the principium individuationis for a fuller discussion of this historical context, see Howard Separability together with the invariance of the infinitesimal metric interval implies that, in a general relativistic spacetime, there are joints everywhere, meaning that we can carve up the universe in any way we choose and still have ontically independent parts.
But quantum entanglement can be read as implying that this libertarian scheme of individuation does not work. Can quantum mechanics not be given a realistic interpretation? This idea first found its way into print in a brief article in the Times of London Einstein A constructive theory, as the name implies, provides a constructive model for the phenomena of interest.
An example would be kinetic theory. Examples include the first and second laws of thermodynamics. Ultimate understanding requires a constructive theory, but often, says Einstein, progress in theory is impeded by premature attempts at developing constructive theories in the absence of sufficient constraints by means of which to narrow the range of possible constructive theories.
It is the function of principle theories to provide such constraint, and progress is often best achieved by focusing first on the establishment of such principles. According to Einstein, that is how he achieved his breakthrough with the theory of relativity, which, he says, is a principle theory, its two principles being the relativity principle and the light principle.
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Nor was it only the relativity and light principles that served Einstein as constraints in his theorizing. Einstein is here alluding the famous entropic analogy whereby, in his photon hypothesis paper, he reasoned from the fact that black body radiation in the Wien regime satisfied the Boltzmann principle to the conclusion that, in that regime, radiation behaved as if it consisted of mutually independent, corpuscle-like quanta of electromagnetic energy. The quantum hypothesis is a constructive model of radiation; the Boltzmann principle is the constraint that first suggested that model.
There are anticipations of the principle theories-constructive theories distinction in the nineteenth-century electrodynamics literature, James Clerk Maxwell, in particular, being a source from which Einstein might well have drawn see Harman At the turn of , Hendrik A.
Probably many other examples could be find. Only in recent decades, Einstein constructive-principle distinction has attracted interest in the philosophical literature, originating a still living philosophical debate on the foundation of spacetime theories Brown , Janssen , Lange What made that possible? One explanation looks to the institutional and disciplinary history of theoretical physics and the philosophy of science.
Each was, in its own domain, a new mode of thought in the latter nineteenth century, and each finally began to secure for itself a solid institutional basis in the early twentieth century.