The primary basis for the present acceptance of the theory of the nuclear atom is the practically universal belief that the existence of an atomic nucleus was definitely proved by the experiments of Rutherford in 1911 and subsequent years. Prior to that time it had been believed that a solid material was just what the name implies in common parlance: a continuous and essentially impenetrable substance. But when Rutherford directed alpha particles against a thin metallic plate, he found, contrary to all expectation, that most of these particles passed directly through the plate just as if there were no obstacle in the way at all, and that the majority of those which were deflected changed their direction by only a relatively small angle. Only a very small proportion experienced major direction changes. By mathematical analysis of his results Rutherford was able to determine the approximate size of the region in which major resistance was encountered, and as a result of his work he arrived at the conclusion that practically all of the mass of the atom is concentrated in an extremely small volume, and that the remainder of the region which the atom occupies in the solid state is mostly open space.
Rutherford's experiments have been repeated with additional precision by other investigators, and it appears safe to say that the experimental facts have been firmly established. It must therefore be conceded that Rutherford's first conclusion, as expressed in the foregoing paragraph, is entirely consistent with the observed facts. But here we encounter an example of a surprisingly prevalent feature of present-day physical science: a curious failure to explore possible alternatives. Time and again in the course of the investigation from which this present discussion originated, critical examination of a commonly accepted idea or conclusion has disclosed that it is only one of the possible explanations of the observed facts, and that there are other, sometimes many other, explanations which have an equally good, if not better, claim to acceptance, but which, so far as the records reveal, have never been explored.
In this case the observed facts are entirely consistent with the hypothesis that most of the mass of the atom is concentrated in a very small region, to be sure, but they are equally consistent with the hypothesis that all of the mass is concentrated in this region; in other words, that this is the atom, not the nucleus of the atom. This alternative conclusion gives us a complete and consistent explanation of the results of Rutherford's experiments in terms of existing knowledge. On this basis there is no need to postulate the existence of an atomic nucleus, and Occam's Principle, one of the sound commonsense rules of science, tells us that we should not make unnecessary hypotheses. All of the facts disclosed by the experiments are entirely in harmony with the conclusion that they merely establish the true size of the atom, indicating that it is very much smaller than was previously believed.
Why, then, we may ask, was the hypothesis of a nucleus accepted so readily and so uncritically? The answer is that this is just one of the many cases in science where an assumption seems so plausible on casual consideration that no one takes the trouble to examine it carefully. In the earliest attempts at explaining the structure of matter, a solid was visualized as a continuous body of material substance. Later, when the atomic theory was proposed, the atomists made the very natural assumption that the apparent continuity of the solid is due to the fact that the atoms of a solid are in contact, whereas the much different properties of a gas result from the presence of empty space between the atoms, which leaves them free to move. There is no physical evidence to support this assumption, but when no one questions it, such an assumption acquires the standing of an axiom. “. . . In a liquid or solid,” says Slater, we may merely assume that the atoms fill up most of the space ,22 and it is indicative of the general relaxation of critical standards in current practice that he characterizes this assumption as “direct evidence” of the atomic dimensions.
In such an atmosphere, where everyone assumes that the dimensions of the atom are known, the discovery that all or nearly all of the mass is concentrated in a very small volume in the center of the region occupied by the atom logically leads to just the kind of a conclusion that Rutherford reached: the conclusion that an atomic nucleus has been located. On the other hand, if the true situation is recognized, and it is realized that the previous ideas as to the size of the atom are pure assumptions and that in reality the atomic dimensions were completely unknown prior to Rutherford's experiments, the only legitimate conclusion that can be drawn from these experiments is that they have determined the size of the atom.
It is frequently stated that the dimensions of the atom can be determined by observations on gases, interpreted with the aid of the kinetic theory, utilizing such phenomena as viscosity which are related to the mean free path of the molecules. But when this “evidence” is examined carefully, it is apparent that all we actually find out from this source is that the minimum inter-atomic distance in the gaseous state is comparable with that which exists in the condensed states. This gets us right back to the question as to the significance of the inter-atomic distance in the solid, a quantity which can be readily measured under a wide range of conditions by the use of modern techniques. In Rutherford's era there seemed to be good reason to believe that this distance was a relatively constant quantity, and even today we find statements in our textbooks such as the following: “Each kind of atom maintains, nevertheless, a rather well-defined volume, which shrinks hardly at all even under the influence of strong pressures.23
If statements of this kind are indicative of the general thinking of the profession, it is no wonder that the physicists have been unable to break away from the pattern of 1911 atomic theory. An enormous amount of experimental work in recent years has established just the opposite; it has been demonstrated beyond question that a specific kind of atom may have a wide range of “atomic volumes,” if we use this term to designate the volume determined by the inter-atomic distance, as in the foregoing quotation. Furthermore, this recent work shows that instead of “shrinking hardly at all” under pressure, all solid substances undergo very substantial decreases in volume under high pressures. Cesium, for example, loses nearly two thirds of its original volume under 100,000 atm., potassium more than half. Most substances are much less compressible than these alkali metals, but if sufficient pressure is applied they behave similarly. Metals such as iron, copper, zinc, silver, cadmium, and tin have been reduced to the neighborhood of half their original volumes by pressures around 3 to 4 million atmospheres, and there is no indication that we are approaching any kind of a limit even at the extreme upper end of the experimental pressure range.
This observed compressibility pattern is very difficult to reconcile with current atomic concepts. It is completely at odds with Bohr's original ideas. The sizes of the orbits in the Bohr atom are fixed by quantum considerations and no intermediate orbits are permitted. But if the atoms are in contact, as assumed, then the contraction under pressure means that the orbits (at least the outer orbits, if there are several) are assuming a continuous succession of values. This is a direct contradiction of the basic postulates of the theory. A few decades ago it probably would have been assumed, in order to save the theory, that this continuous decrease in orbital size is a statistical effect, and that the individual orbits maintain one or another of the permitted values, but the latitude for ad hoc assumptions narrows as experimental knowledge increases, and such an assumption is untenable as matters now stand.
A direct comparison of this kind with the current atomic theories which have been developed through extension and modification of Bohr's original ideas is more difficult, since these theories have been dematerialized to the point where they are essentially nothing but mathematical abstractions and it is hard to come to grips with anything specific. As it happens, however, the one major feature of the original theory that has been maintained intact is the quantizationwhatever other changes may have been made, it has always remained a quantum theory-and it is precisely this point at which the conflict with the experimental compressibility occurs. These compressibilities are therefore incompatible with current atomic concepts as well as with the original Bohr atom.
On the other hand, if we accept the straightforward interpretation of Rutherford's experiments and conclude that they establish the true size of the atom as the size now assigned to the nucleus, it is evident that the inter-atomic distance merely represents a point of equilibrium at which the forces of attraction and forces of repulsion between the atoms are equal. On this basis it follows that application of an external pressure will move this equilibrium point inward, and also that the amount of the displacement will be a function of the applied pressure. Thus we arrive easily and naturally at a theoretical explanation of the exact situation which we observe experimentally.
We also find from compressibility measurements on individual crystals that the compression in any one dimension is largely independent of that in the other two. This information fits in naturally and logically with the concept of an equilibrium distance between atoms, but if this static equilibrium is replaced by a dynamic equilibrium of the type required by the nuclear theories, an explanation of the observed characteristics of the response to linear compression becomes very difficult.
Next we note that the sizes of the various atoms as determined from the inter-atomic distances are entirely inconsistent with the atomic magnitudes. Whatever the structure of the atom may be, there seems to be ample evidence to show that the atomic weight is a measure of the number of units of the primary atomic component (whatever it is), which this particular atom contains. There likewise appears to be adequate evidence to support the conclusion that the units of this primary component are all alike, or at least very nearly alike. Current theory visualizes two kinds of primary units, protons and neutrons, but these entities are very much alike and interchangeable, and each proton is supposed to be accompanied by a single electron. Irrespective of the theoretical viewpoint from which the subject is approached, whether we ascribe the volume to the primary component itself or to some secondary component such as the hypothetical electrons, the true volume of the atom should be at least roughly proportional to the atomic weight, since we can hardly take the stand that units of the same kind come in assorted sizes. But the “atomic volumes” as calculated from the inter-atomic distances or from the observed densities are grotesquely out of harmony with the corresponding atomic weights. For example, the sodium atom, which has less than one-eighth of the atomic weight of the gold atom, occupies more than double the volume of the latter. If the supposed nucleus is actually the atom, everything falls into line, since the experimental information indicates that the volume of the “nucleus” is directly proportional to the atomic weight, as the volume of the atom should be.
Furthermore, if we accept the view that the atoms are in contact in the solid state, as present theories contend, we are forced to the rather bizarre conclusion that the sizes and even the shapes of the atoms are extremely variable. On this basis the carbon atom, for example, is spherical in the diamond crystal, with an inter-atomic distance of 1.54 A, but in a graphite crystal the same atom stretches out to 3.40 A in one dimension, while the inter-atomic distances in the other two dimensions remain approximately the same as in the diamond. A variability of this kind in the atom is not inherently impossible, but it is, to say the least, implausible. The alternative interpretation of Rutherford's findings, which puts the atom where the so-called nucleus is supposed to be, does not require any variability in the atom; in this case the variability is in the nature of the relationship between adjoining atoms.
The important point here is that there is independent evidence of the existence of variability in the inter-atomic relationships. Differences in valence such as those which we encounter in the various oxides of nitrogen, for example, show that there are differences in the manner in which the oxygen and nitrogen forces interact. This alternative interpretation therefore requires no ad hoc assumption; the variability in the inter-atomic distances is readily explained, at least qualitatively, by facts already established. On the other hand, there is no independent evidence of any variability in the atom itself of the kind which is required in order to reconcile the observed facts with the nuclear theory. No matter how many different forms of a solid may exist, when we get the substance into the gaseous state and raise the temperature high enough to dissociate it into atoms, we find the atoms of any particular element all alike (except for isotopic differences, which do not enter into this picture). The nuclear theory therefore has to assume an atomic variability of which we have no observational evidence.
This is the same situation that we find all along the line. If we make no unnecessary assumption to start with, and interpret Rutherford's findings simply as a determination of the size of the atom, then we can explain all of our subsequent observations in terms of facts already known from independent sources. But if we make the wholly unnecessary assumption of the existence of a nuclear structure to explain Rutherford's discoveries, then wherever we go we have to set up additional ad hoc assumptions to reconcile the nuclear theory with the observed facts. This is the old familiar pattern that develops whenever we stray from the truth, whether it be deliberately or unconsciously, and it brands as false the whole theoretical fabric to which it applies. The available factual evidence lends no support to the hypothesis that the inter-atomic distance in the solid state is a reflection of the size of the atom, as the nuclear theory contends; on the contrary, this evidence is wholly in accord with the conclusion that the so-called nucleus is in reality the atom itself: the same conclusion which is the most logical interpretation of the results of Rutherford's experiments.
The importance of this conclusion, so far as the status of the nuclear theory is concerned, can hardly be overemphasized. As will be brought out in the subsequent discussion, the basic elements of the theory, without which it is hopelessly lost, rest entirely on the belief that the existence of a nucleus was definitely proved by Rutherford's experiments. The nature of the assumptions involved in setting up these basic elements of the theory is such that even a reasonable doubt as to the validity of Rutherford's conclusion is sufficient to eliminate all justification for making these assumptions, and by so doing, to destroy the theory completely. But the facts brought out in the foregoing paragraphs show that there is much more than a reasonable doubt. There is actually definite evidence that Rutherford's hypothesis is wrong. His work, with the subsequent corroboration of his experimental findings, provided ample proof of the existence of something massive in the center of the region occupied by the atom, to be sure, but neither his work nor any other has produced any evidence to corroborate the existence of the hypothetical outer parts of the atom. On the contrary, a whole series of facts points to the conclusion that there are no such outer parts, and that the massive “something” which Rutherford called the nucleus is actually the atom.
It seems almost incredible that a basic concept such as that of the atomic nucleus could have slipped into the structure of scientific thought without any critical examination of its claim to validity, but the literature of the Rutherford era shows that this is just what happened. The accuracy of Rutherford's experimental work was checked in the usual manner by repeating his experiments under carefully-controlled conditions, and it also took a little time for the scientific community to become accustomed to the idea that a solid substance is composed mostly of empty space, but so far as the records show, this is as far as the scrutiny ever went. No one, either then or since, seems to have given any consideration to the next point that should have been examined after Rutherford's experimental results were verified: the question as to whether the conclusions which he drew from these experimental results were justified.
The scientific profession is quite willing to concede, in principle, the need for the kind of a periodic reexamination of its basic concepts that was stressed in the preceding chapter. Louis de Broglie, for example, emphasizes the point that the history of science shows that “it is proper to submit periodically to a very searching examination, principles that we have come to assume without any more discussion.24 But there is no indication that he ever applied such a searching examination” to the concept of a nucleus. Similarly we find von Weizsaecker speaking of “making a critical examination of the foundations25 of atomic physics, but he starts this “critical examination” with the assumption that the existence of a nucleus was proved by Rutherford's work. These searching and critical examinations have simply failed to get down to bedrock.
One of the strangest aspects of the whole situation is that practically every elementary chemistry textbook published in the last halfcentury contains a diagram of the sodium chloride crystal in which the sodium and chlorine atoms are pictured as occupying relatively small regions at alternate corners of the unit cube, with nothing but empty space in the remainder of the structure. This is just exactly Ihe picture which emerges when we make a careful and critical examination of all of the available evidence, along the lines discussed in the preceding paragraphs. Here is one of those ironies so often encountered in life. The answer has been right in front of us all the time, but no one has been able to rise far enough out of the traditional channels of thought to be able to see that it is the answer.
There are, of course, many other items of more recent origin which are now regarded as evidence in favor of the nuclear hypothesis, and it will be necessary to consider these items individually and in some detail in the subsequent pages. In analyzing them, however, it should be kept in mind that the accepted ideas in these areas have been formulated in an atmosphere dictated by the prevailing impression that the existence of the nuclear atom was already proved by Rutherford's work, and the general attitude toward other developments has largely been determined by this common understanding as to the firmly established status of the nuclear concept. The physicists have not considered it necessaly for any of these new items to furnish a proof of the validity of the nuclear theory, since this would merely duplicate something which presumably had already been done; all that has been required is that the new information should be consistent with the nuclear hypothesis, and even this rather modest requirement has been waived in important instances, notably in connection with the theories of the structure of the nucleus, which will be the next subject of discussion. Now that it is evident that the existence of an atomic nucleus was not proved by Rutherford's work, and that the massive “something” which he located is actually the atom itself, it is clearly appropriate to examine the subsequent developments in this new setting, to see just what difference this will make in the general picture.
According to currently accepted ideas, the atomic nucleus consists of a number of protons equal to the atomic number of the particular element, and enough neutrons to account for the remainder of the atomic weight. Even without the complication of having to consider details this hypothetical structure immediately encounters two formidable obstacles. First, the protons are, by definition, positively charged hydrogen atoms, and at such short distances they will exert very powerful repulsive forces on each other. Existing knowledge therefore tells us that such a structure is impossible; if it were ever formed it would disintegrate with explosive violence. Second, experimental evidence indicates that the neutron is unstable in the terrestrial environment, with a half-life of only about 13 minutes. On the basis of existing knowledge, therefore, the neutron cannot be a constituent of a stable atom.
Nevertheless, if we have positive knowledge that atomic nuclei do exist and that they are composed of protons and neutrons, it then necessarily follows that existing knowledge of the behavior of these particles is incomplete and that they have some different behavior characteristics under nuclear conditions. In the belief, therefore, that the existence of the nucleus was proved by Rutherford's findings, two ad hoc assumptions have been made to reconcile the contradictory items: (1) that some kind of a “nuclear force” exists in opposition to the force of repulsion that would otherwise destroy the hypothetical structure, and (2) that the normally unstable neutron is stable in the nuclear environment.
Such unsupported assumptions should not be lightly made. Their use is a legitimate scientific device and occasionally one of them serves a very worthwhile purpose. The discovery of the neutrino is a case in point. But the employment of such assumptions is uncomfortably close to the ancient custom of attributing all unexplained events to the actions of spirits and demons, and all too often it simply diverts attention from the real problem and impedes the march of scientific progress, just as any other appeal to the supernatural is likely to do. Certainly the piling of one of these unsupported assumptions on top of another cannot be justified under any circumstances, and this is just exactly the situation that the proton-neutron theory is in, now that it has been shown that Rutherford's experiments did not prove the existence of a nucleus. Without definite and positive proof that a nucleus exists, there is no basis on which we can even talk about nucleons, in the face of the known facts which specifically contradict the nuclear structure.
It will no doubt be contended that an important and generally accepted concept of long standing deserves something more than this summary dismissal, and that some further consideration of the matter is in order. But there is nothing further to be said. Even if the positive evidence that the so-called nucleus is actually the atom did not exist, the mere fact that the assumption of the existence of a nucleus is unnecessary is in itself sufficient to eliminate all justification for the drastic steps that have to be taken to put protons and neutrons into a nucleus. We can justify one unsupported assumption, such as the original postulate of the existence of the neutrino, if the result of this one assumption is to bring everything else into line with observed facts and established physical principles, but even in these days when the latitude for speculation and hypothesis is extremely wide, we cannot justify making assumptions that are completely at odds with established facts and principles merely to enable retaining another unsupported assumption.
The hypothetical nucleus was already in an extremely precarious condition, and physicists have realized that unless an answer could be found soon to the oft repeated question, “What holds the nucleus together?, the proton-neutron concept was likely to collapse of its own weight. The only thing that has kept it alive in the face of the complete lack of progress toward an answer to this crucial question is the hitherto firm conviction that the existence of a nucleus of some kind is a positively established fact. Now that it has been shown that the supposed proof of this point is non-existent, the last vestige of justification for the proton-neutron concept is swept away, and further comment is superfluous.
Let us then turn to a consideration of some of the other aspects of the nuclear theory. In this connection it should be pointed out that the assumptions already discussed are by no means the only ones involved in the foundations of the theory. Actually this theory rests upon a long chain of assumptions: an extraordinary product of scientific imagination which is remarkable not only because of the unprecedented number of assumptions that have been called into service in the construction of this one theory, but also because of the drastic nature of some of the assumptions, which postulate behavior characteristics totally unlike anything ever encountered elsewhere in the physical world. The following list of the major assumptions of the theory, which has been prepared to show the relevance of the various subjects that will be discussed herein, illustrates this point, particularly when it is realized that dozens of additional assumptions have been made in working out the details of the theory. In order to arrive at the currently popular atomic picture it is necessary to assume:
(2) that the parts are known sub-atomic particles.
(3) that these parts are arranged in a nuclear structure.
(4) that the orbital components are electrons.
(5) that the orbital electrons do not follow the usual physical laws.
(6) that the nucleus is composed of protons and neutrons.
(7) that there is an unknown “nuclear force” holding the nucleus together.
(8) that there is an unknown factor which makes the neutron stable inside the nucleus.
Thus far, this discussion has shown that the “proof” of assumption (3) hitherto relied upon is invalid, and that without a definite proof of (3), assumptions (7) and (8) are completely unjustified, which leaves assumption (6) without a leg to stand on. Let us next turn our attention to assumptions (4) and (5), which deal with electrons.