Part 1

Einstein's Reply to Criticisms (1949)

January 1, 2020

I reject the basic idea of quantum theory. I do not believe that it will be useful for physics.

Born, Pauli, Heitler, Bohr, and Margenau are all firmly convinced that the riddle of the double nature of all corpuscles (corpuscular and undulatory character) has its final solution in the statistical quantum theory.

They believe that=

  • Heisenberg’s indeterminacy-relation is essentially prejudicial in favour of the character of all thinkable reasonable physical theories in the mentioned sense.

I believe that quantum theory has an incomplete description of physical systems.

Quantum theory is the only one which unites the corpuscular and undulatory dual character of matter in a logically satisfactory fashion.

But I dont like the way that it completely describes any (individual) real situation as it supposedly exists irrespective of any act of observation.

Whenever the positivistically inclined modern physicist hears such a formulation his reaction is that of a pitying smile.

He says to himself= “there we have the naked formulation of a metaphysical prejudice, empty of content, a prejudice, moreover, the conquest of which constitutes the major epistemological achievement of physicists within the last 25 years. Has any man ever perceived a ‘real physical situation’?

How can a reasonable person still believe that he can refute our essential knowledge by drawing up such a bloodless ghost?” Patience!

The above laconic characterisation was not meant to convince anyone; it was merely to indicate the point of view around which the following elementary considerations freely group themselves. In doing this I shall proceed as follows= I shall first of all show in simple special cases what seems essential to me, and then I shall make a few remarks about some more general ideas which are involved.

Let us assume a radioactive atom of definite average decay time at a specific location. It emits a lighter particle. By following Gamow, we replace the rest of the atom by a space of atomic order of magnitude, surrounded by a closed potential energy barrier which, at a time t = 0, encloses the particle to be emitted.

In elementary quantum mechanics this is designated by a Psi-function in 3 dimensions, which at the time t= 0 is different from 0 only inside of the barrier, but which, for positive times, expands into the outer space.

This Psi-function yields the probability that the particle, at some chosen instant, is actually in a chosen part of space (i.e., is actually found there by a measurement of position).

On the other hand, the Psi-function does not imply any assertion concerning the time instant of the disintegration of the radioactive atom.

Is this theoretical description the complete description of the disintegration of an atom? No.

An atom decays at a definite time. However, such a definite time-value is not implied in the description by the Psi-function.

If the individual atom has a definite disintegration time, then as regards the individual atom its description by means of the Psi-function must be interpreted as an incomplete description.

In this case, the Psi-function is to be taken as the description, not of a singular system, but of an ideal ensemble of systems.

In this case one is driven to the conviction that a complete description of a single system should, after all, be possible, but for such complete description there is no room in the conceptual world of statistical quantum theory.

The quantum theorist will reply= This consideration stands and falls with the assertion that there actually is such a thing as a definite time of disintegration of the individual atom (an instant of time existing independently of any observation). But this assertion is, from my point of view, not merely arbitrary but actually meaningless. The assertion of the existence of a definite time-instant for the disintegration makes sense only if I can in principle determine this time-instant empirically.

Such an assertion, however, (which, finally, leads to the attempt to prove the existence of the particle outside of the force barrier), involves a definite disturbance of the system in which we are interested, so that the result of the determination does not permit a conclusion concerning the status of the undisturbed system. The supposition, therefore, that a radioactive atom has a definite disintegration-time is not justified by anything whatsoever; it is, therefore, not demonstrated either that the Psi-function can not be conceived as a complete description of the individual system. The entire alleged difficulty proceeds from the fact that one postulates something not observable as “real.” (This the answer of the quantum theorist.)

I dislike this basic positivistic attitude because I think it is untenable. It is the same thing as Berkeley’s principle, esse est percipi. “Being” is always something which is mentally constructed by us, that is, something which we freely posit.

The justification of such constructs does not lie in their derivation from what is given by the senses. Such a type of derivation (in the sense of logical deducibility) is nowhere to be had, not even in the domain of pre-scientific thinking.

The justification of the constructs, which represent “reality” for us, lies alone in their quality of making intelligible what is sensorily given (the vague character of this expression is here forced upon me by my striving for brevity). Applied to the specifically chosen example this consideration tells us the following=

One may not merely ask= “Does a definite time instant for the transformation of a single atom exist?” but rather= “Is it, within the framework of our theoretical total construction, reasonable to posit the existence of a definite point of time for the transformation of a single atom?” One may not even ask what this assertion means. One can only ask whether such a proposition, within the framework of the chosen conceptual system — with a view to its ability to grasp theoretically what is empirically given — is reasonable or not.

Insofar, then, as a quantum-theoretician takes the position that the description by means of a Psi-function refers only to an ideal systematic totality but in no wise to the individual system, he may calmly assume a definite point of time for the transformation. But, if he represents the assumption that his description by way of the Psi-function is to be taken as the complete description of the individual system, then he must reject the postulation of a specific decay-time. He can justifiably point to the fact that a determination of the instant of disintegration is not possible on an isolated system, but would require disturbances of such a character that they must not be neglected in the critical examination of the situation.

It would, for example, not be possible to conclude from the empirical statement that the transformation has already taken place, that this would have been the case if the disturbances of the system had not taken place.

E. Schrödinger first called attention to a modification of this consideration, which shows an interpretation of this type to be impracticable.

Rather than considering a system which comprises only a radioactive atom (and its process of transformation), one considers a system which includes also the means for ascertaining the radioactive transformation — for example, a Geiger-counter with automatic registration-mechanism. Let this latter include a registration-strip, moved by a clockwork, upon which a mark is made by tripping the counter.

From the point of view of quantum mechanics=

  • this total system is very complex
  • its configuration space is of very high dimension.

But there is in principle no objection to treating this entire system from the standpoint of quantum mechanics.

Here too the theory determines the probability of each configuration of all its co-ordinates for every time instant. If one considers all configurations of the coordinates, for a time large compared with the average decay time of the radioactive atom, there will be (at most) one such registration-mark on the paper strip.

To each coordinate configuration corresponds a definite position of the mark on the paper strip. But, inasmuch as the theory yields only the relative probability of the thinkable co-ordinate-configurations, it also offers only relative probabilities for the positions of the mark on the paper strip, but no definite location for this mark.

In this consideration the location of the mark on the strip plays the role played in the original consideration by the time of the disintegration. The reason for the introduction of the system supplemented by the registration-mechanism lies in the following.

The location of the mark on the registration-strip is a fact which belongs entirely within the sphere of macroscopic concepts, in contradistinction to the instant of disintegration of a single atom. If we attempt [to work with] the interpretation that the quantum-theoretical description is to be understood as a complete description of the individual system, we are forced to the interpretation that the location of the mark on the strip is nothing which belongs to the system per se, but that the existence of that location is essentially dependent upon the carrying out of an observation made on the registration-strip.

Such an interpretation is certainly by no means absurd from a purely logical standpoint, yet there is hardly likely to be anyone who would be inclined to consider it seriously. For, in the macroscopic sphere it simply is considered certain that one must adhere to the program of a realistic description in space and time; whereas in the sphere of microscopic situations one is more readily inclined to give up, or at least to modify, this program.

  • If one maintains that quantum theory can completely describe a physical system, then it leads to very implausible theoretical conceptions.
  • But if one maintains that the quantum-mechanical description is the description of ensembles of systems, then those difficulties of theoretical interpretation disappear

My conclusion is=

  • in quantum theory, there is no such thing as a complete description of the individual system.
  • the attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations. These become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.

In that case the whole “egg-walking” performed in order to avoid the “physically real” becomes superfluous. There exists, however, a simple psychological reason for the fact that this most nearly obvious interpretation is being shunned.

If quantum theory does not pretend to describe the individual system (and its development in time) completely, it appears unavoidable to look elsewhere for a complete description of the individual system. In doing so, it would be clear from the very beginning that the elements of such a description are not contained within the conceptual scheme of the statistical quantum theory.

With this one would admit that, in principle, this scheme could not serve as the basis of theoretical physics.

Assuming the success of efforts to accomplish a complete physical description, the statistical quantum theory would, within the framework of future physics, take an approximately analogous position to the statistical mechanics within the framework of classical mechanics. I am rather firmly convinced that the development of theoretical physics will be of this type; but the path will be lengthy and difficult.

A quantum theoretician might even admit that the quantum-theoretical description refers to ensembles of systems and not to individual systems. But he still clings to the idea that the type of description of the statistical quantum theory will, in its essential features, be retained in the future.

I think that the point of view of quantum theory — taken as theoretical possibility — is incontestable.

For me, however, the expectation that the adequate formulation of the universal laws involves the use of all conceptual elements which are necessary for a complete description, is more natural. It is furthermore not at all surprising that, by using an incomplete description, (in the main) only statistical statements can be obtained out of such description.

If it should be possible to move forward to a complete description, it is likely that the laws would represent relations among all the conceptual elements of this description which, per se, have nothing to do with statistics.

A few more remarks of a general nature concerning concepts and [also] concerning the insinuation that a concept — for example that of the real — is something metaphysical (and therefore to be rejected). A basic conceptual distinction, which is a necessary prerequisite of scientific and pre-scientific thinking, is the distinction between “sense-impressions” (and the recollection of such) on the one hand and mere ideas on the other. There is no such thing as a conceptual definition of this distinction (aside from, circular definitions, i.e., of such as make a hidden use of the object to be defined). Nor can it be maintained that at the base of this distinction there is a type of evidence, such as underlies, for example, the distinction between red and blue. Yet, one needs this distinction in order to be able to overcome solipsism. Solution= we shall make use of this distinction unconcerned with the reproach that, in doing so, we are guilty of the metaphysical “original sin.” We regard the distinction as a category which we use in order that we might the better find our way in the world of immediate sensations. The “sense” and the justification of this distinction lies simply in this achievement. But this is only a first step. We represent the sense-impressions as conditioned by an “objective” and by a “subjective” factor. For this conceptual distinction there also is no logical-philosophical justification. But if we reject it, we cannot escape solipsism. It is also the presupposition of every kind of physical thinking. Here too, the only justification lies in its usefulness. We are here concerned with “categories” or schemes of thought, the selection of which is, in principle, entirely open to us and whose qualification can only be judged by the degree to which its use contributes to making the totality of the contents of consciousness “intelligible.” The above mentioned “objective factor” is the totality of such concepts and conceptual relations as are thought of as independent of experience, viz., of perceptions. So long as we move within the thus programmatically fixed sphere of thought we are thinking physically. Insofar as physical thinking justifies itself, in the more than once indicated sense, by its ability to grasp experiences intellectually, we regard it as “knowledge of the real.”

After what has been said, the “real” in physics is to be taken as a type of program, to which we are, however, not forced to cling a priori. No one is likely to be inclined to attempt to give up this program within the realm of the “macroscopic” (location of the mark on the paper strip “real”). But the “macroscopic” and the “microscopic” are so inter-related that it appears impracticable to give up this program in the “microscopic” alone. Nor can I see any occasion anywhere within the observable facts of the quantum-field for doing so, unless, indeed, one clings a priori to the thesis that the description of nature by the statistical scheme of quantum-mechanics is final.

The theoretical attitude here advocated is distinct from that of Kant only by the fact that we do not conceive of the “categories” as unalterable (conditioned by the nature of the understanding) but as (in the logical sense) free conventions. They appear to be a priori only insofar as thinking without the positing of categories and of concepts in general would be as impossible as is breathing in a vacuum.

From these meagre remarks one will see that to me it must seem a mistake to permit theoretical description to be directly dependent upon acts of empirical assertions, as it seems to me to be intended [for example] in Bohr’s principle of complementarity, the sharp formulation of which, moreover, I have been unable to achieve despite much effort which I have expended on it. From my point of view [such] statements or measurements can occur only as special instances, viz., parts, of physical description, to which I cannot ascribe any exceptional position above the rest.

The above mentioned essays by Bohr and Pauli contain a historical appreciation of my efforts in the area of physical statistics and quanta and, in addition, an accusation which is brought forward in the friendliest of fashion. In briefest formulation this latter runs as follows= “Rigid adherence to classical theory.” This accusation demands either a defence or the confession of guilt. The one or the other is, however, being rendered much more difficult because it is by no means immediately clear what is meant by “classical theory.” Newton’s theory deserves the name of a classical theory. It has nevertheless been abandoned since Maxwell and Hertz have shown that the idea of forces at a distance has to be relinquished and that one cannot manage without the idea of continuous “fields.” The opinion that continuous fields are to be viewed as the only acceptable basic concepts, which must also be assumed to underlie the theory of the material particles, soon won out. Now this conception became, so to speak, “classical;” but a proper, and in principle complete, theory has not grown out of it. Maxwell’s theory of the electric field remained a torso, because it was unable to set up laws for the behaviour of electric density, without which there can, of course, be no such thing as an electro-magnetic field. Analogously the general theory of relativity furnished then a field theory of gravitation, but no theory of the field-creating masses. (These remarks presuppose it as self-evident that a field-theory may not contain any singularities, i.e., any positions or parts in space in which the field laws are not valid.)

Consequently there is, strictly speaking, today no such thing as a classical field-theory; one can, therefore, also not rigidly adhere to it. Nevertheless, field-theory does exist as a program= “Continuous functions in the four-dimensional [continuum] as basic concepts of the theory.” Rigid adherence to this program can rightfully be asserted of me. The deeper ground for this lies in the following= The theory of gravitation showed me that the non-linearity of these equations results in the fact that this theory yields interactions among structures (localised things) at all. But the theoretical search for non-linear equations is hopeless (because of too great variety of possibilities), if one does not use the general principle of relativity (invariance under general continuous co-ordinate-transformations). In the meantime, however, it does not seem possible to formulate this principle, if one seeks to deviate from the above program. Herein lies a coercion which I cannot evade. This for my justification.

Nevertheless I am forced to weaken this justification by a confession.

If one disregards quantum structure, one can justify the introduction of the gik “operationally” by pointing to the fact that one can hardly doubt the physical reality of the elementary light cone which belongs to a point. In doing so one implicitly makes use of the existence of an arbitrarily sharp optical signal. Such a signal, however, as regards the quantum facts, involves infinitely high frequencies and energies, and therefore a complete destruction of the field to be determined. That kind of a physical justification for the introduction of the gik falls by the wayside, unless one limits himself to the “macroscopic.”

The application of the formal basis of the general theory of relativity to the “microscopic” can, therefore, be based only upon the fact that that tensor is the formally simplest covariant structure which can come under consideration. Such argumentation, however, carries no weight with anyone who doubts that we have to adhere to the continuum at all. All honour to his doubt — but where else is there a passable road?


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