by I. I. RABI
NEARLY five years ago, on November 6, 1940, I left my laboratory and classroom at Columbia University to work with a number of other physicists from different parts of the country on the secret development of new weapons of war. Before this paper appears in print, thanks largely to the “atomic” bomb, I hope and expect to be back in my laboratory and, together again with students and colleagues, to resume a life project which was interrupted by “a call to armaments.”
Before the war these physicists almost never had occupied themselves with problems and questions which in any direct way could be called immediately practical. They directed their whole attention to discovering and understanding the laws of the physical universe in a clear, consistent, logical, and often mathematical scheme. They made it their code to communicate these results to others in the most frank, direct, and expeditious manner. Inseparably connected with their scientific work — and no small part of it — was the upbringing of future scientists.
Yet these very men were largely responsible for the discovery and practical development of at least two of the most remarkable and terrible weapons of this war: radar and the so-called atomic bomb. To apply the adjective “terrible” to radar may occasion some surprise to those who only have read or thought about electronic devices in terms of their beneficial peacetime applications. The crews of Japanese ships, however, who found themselves being shelled with devastating accuracy during the darkest hours of night by our warships, and the Germans who experienced saturation bombing from airplanes which were invisible above a dense cover of clouds, will readily testify to the weird and uncanny terror which an unseen but deadly enemy can inspire. The potentialities of this extraordinarily facile and protean instrument of war are disquieting to anyone who appreciates the degree to which radar will heighten the surprise value and accuracy of any weapon.
Speaking for the group of men who created these weapons, I would say that we are frankly pleased, terrified, and to an even greater degree embarrassed when we contemplate the results of our wartime efforts. Our terror comes from the realization — which is nowhere more strongly felt than among us - of the tremendous forces of destruction now existing in an all too practical form. By this, I do not mean to suggest that we who helped to create the new weapons are now overcome with a sense of guilt or regret. These instrumentalities were natural consequences of the scientific knowledge at our disposal, and as such were inevitable. They did help us to win a bitter war in which we were attacked in a most cowardly fashion.
Returning from wartime occupations to his laboratory and classroom, the physicist looks forward to an era of peace and regards anew the future of his science. From the lessons of this war, we know that his science, as he understands it, is possible only in an environment undisturbed by war or even by the threat of war. The physicist has become a military asset of such value that only with the assurance of peace will society permit him to pursue in his own quiet way the scientific knowledge which inspires, elevates, and entertains his fellow men.
Thus by the very success of his efforts in this war, the physicist has been placed in an embarrassing position. The inheritor of the tradition of Galileo, Newton, Faraday, Maxwell, Gibbs, Rutherford, Michelson, and Einstein now is hailed as the messiah who will bring us a new world with push-button facilities, new industries, an expanding economy, and jobs for all. He is assailed with equal fervor by a thoughtful group of citizens who condemn him as the Frankenstein of our time and who hope that he will be placed in protective custody until we have solemnly taken thought of how one should live in an atomic age.
Industry, with considerable success, is trying to lure the physicist from his academic hide-out with glittering pieces of silver and with the promise of unlimited scientific equipment and corps of assistants. Meanwhile our rejuvenated military forces are building giant laboratories (any one of which can use up all of our currently available and really welltrained physicists), and hope to stock them with men who can continue their scientific research and still adhere to the well-meaning but completely impossible regulations of the Civil Service Commission.
The New York Times proposes to alleviate man’s lot by corralling the scientist in large research institutions, which it fondly imagines are of an industrial nature, where he would have as overseers and public guardians a group of wise men who know the important problems better than the scientist himself. Out of this pleasant hell there presumably would emerge cures for cancer and the common cold, rocket devices which would make a trip to the moon a week-end possibility for desk-weary stenographers, and so on. With childish faith in the capabilities of science and a complete lack of any understanding of the nature of scientific creation, the erudite news writer apparently believes that the theory of relativity or quantum mechanics could have been produced on order from wise men in Washington who by some sublime divination realized the necessity for these theories and were able to convince the Director of the Bureau of the Budget that the results would justify the expenditure of the taxpayers’ money.
The universities hope the physicist will return to satisfy the needs of students. It has become obvious to the heads of institutes of learning that the future generation of scientists will be a sorry lot if the best teachers leave the academic circles for more lucrative positions in military or industrial laboratories.
The embarrassment of the physicists stems not only from the fact that they are unaccustomed to being courted with such ardor, but also from their realization, admitted readily by four out of every five, that in the past five years, apart from the development of certain techniques which may be useful in later research, the progress of the science of physics has been less than moderate. The same profound questions which furrowed the brows of physicists before the war and forced them to spend long days and nights in their laboratories are still with us. The physicist returning from the war has no vast amount of literature to digest before he can bring himself up to date in his field, because his own dusty files contain virtually the last words written upon the subject.
With atomic bombs and radar in mind, the skeptic may well ask what the physicist thinks he has been doing these past five years, if not physics. In a more heated vein, he may inquire just what are these problems which the physicist considers to be so important and yet which are so remote from practical possibilities that the intense research work of the war years has not touched upon them perceptibly.
These are probing questions. To answer them the physicist must attempt to explain the two aspects of his science. There is, first, the creative intellectual activity which constantly pushes back the boundaries of our understanding of natural phenomena; second, the industrial activity which applies the results of scientific knowledge and understanding to satisfy material human needs and whimsies. The first is the science of physics proper, and the second is the side of physics which has been called the inheritance of technology. If the science of physics lags, the inheritance of technology is soon spent. In these war years, the inheritance of technology has been exploited to the point where further substantial progress can come only from an advance in the science of physics.
In the past, the science of physics was fifty years ahead of important technological application. For example, Faraday’s experiments on the fundamentals of electromagnetic induction preceded the rise of the electrical industry by half a century. The growth in numbers, size, and quality of our modern industrial research laboratories and the great improvement of our schools of technology are bringing technological application very close to scientific discovery, as we can see from the fact that the infant science of nuclear physics has already resulted in the atomic bomb.
The essential unpredictability of the laws of nature beyond our experience, as exemplified in the great discoveries of the past, makes scientific research a venture, literally, into the unknown. To set out a detailed program with practical goals for truly scientific research is like trying to make a map of a country no one has ever seen and the very existence of which is in grave doubt. Pure science cannot have any goal other than the appeasement of the human spirit of intellectual adventure.
RADAR and the atomic bomb are two results of a planned program of research which made use of known facts and principles. The atomic bomb is an offspring of twentieth-century physics, while radar in principle is the child of nineteenth-century physics wedded to twentieth-century technology. Radar is the easier of the two devices to understand, since everyone is familiar with its stepfather, the radio.
In the latter part of the last century Heinrich Hertz, in Germany, succeeded in demonstrating experimentally the existence of what now are popularly known as radio waves. The existence of such radio waves had been predicted by Maxwell, who, on the basis of Faraday’s electrical experiments, had written a set of mathematical relations defining their properties, which actually were the same as those of visible light, except for differences in wave length. Maxwell had predicted the existence of these waves, but had given no clue as to how to generate them. Hertz’s tremendous discovery, however, showed that visible light was just a special wave-length region of an infinite spectrum of radio waves and that these waves all originated from the motions of electrical charges.
Hertz’s contributions to scientific knowledge resulted in a spectacular unification of a very large variety of isolated phenomena, including light, radio waves, and the motion of electricity — the intellectual tool which has made the whole art of radio possible. The further development of the art and science of radio was concerned with the generation, control, and detection of radio waves. A series of successively brilliant inventions gave us wireless telegraphy, then the radio telephone and radio broadcasting, and finally the “soap opera.” One invention, the threeelement vacuum tube of De Forest, was so outstanding in its consequences that it almost ranks with the greatest inventions of all time. Very few of our modern developments would have been possible without it.
Radar was implicit in Hertz’s original experiments, but it had no practical development until the need arose for a new warning device to forestall surprise attacks by aircraft. Assuming that it took approximately fifteen minutes for a defending force to get its fighter planes off the ground and organized in a position where they could intercept attacking enemy bombers, it was obvious at the outbreak of the war that a warring nation must be warned of the approach of enemy planes within a minimum safety zone of seventy-five miles (at this distance a plane traveling three hundred miles an hour would be only fifteen minutes away). Such an aircraft-detection device obviously would have to work day or night, and in all types of weather. Many persons working in different countries, isolated from one another by walls of secrecy, arrived at practically the same solution at almost the same time. From this, one may be inclined to believe that the problem was not exceptionally profound.
The principle of radar was known throughout the scientific world before the war. The concentration of scientific talent on the development of the technological tools of radar resulted in a tremendous amount of progress in a very few years.
This development was concerned chiefly with the production and utilization of shorter and shorter radio waves, which could be directed as more concentrated beams, in order to obtain increasingly fine details of the objects under radar observation. In other words, the Army wanted to know not only that aircraft were approaching, but the number, type, and disposition of the planes, while the Navy wanted to know whether the invisible and unidentified object caught in the radar beam was a sampan, a battlewagon, or just a rock jutting out of the water. The aiming of anti-aircraft guns by radar was the next (and still secret) step.
It is a fundamental law of physics that in order to produce narrow beams of radio waves the antenna on the transmitter must be many times larger than the wave length of the radio beam. Therefore, to avoid immense antenna structures, such as those which can be seen around any commercial radio station here in the United States, it was necessary to utilize shorter and still shorter wave lengths. The need for less antenna was urgent, because radar sets were installed aboard ships as small as destroyers and in the limited interiors of fighter planes. From the conventional idea of antenna strung between two poles there eventually evolved antennas which were more like searchlight mirrors and were no larger than an oversized salad bowl.
BORROWED from television and adapted to radar was the cathode-ray tube in which the echoes were displayed on the screen of the tube in a form instantly recognizable by the operator. In its final wartime form, radar could take a picture of groups of planes seventy-five miles away or draw a map of a city through thirty thousand feet of cloud layers.
All in all, we now have in radar something which resembles television, except that the picture on the screen is an object as seen through the medium of radio waves, rather than through light waves. Unlike a beam of light, the radio waves are invisible and can penetrate great layers of clouds, smoke, and haze. The future uses of the art of radar lie in two directions: from the ground, and from the air. In the first instance, radar enables us to see aircraft in the sky, regardless of darkness, fog, or the fact that the plane may be many miles away. We can look forward with confidence to the day when there will be no more “lost” planes circling in vain for a place to land.
The control towers on commercial airfields of the future will be able to tell a fog-enshrouded pilot where he is and guide him to a safe landing place. More than that, the radar-equipped man on the ground will be able to direct the course of the plane without the assistance of the pilot. It is a short step from having a man on the ground tell an invisible pilot what to do to having apparatus which controls the movements of the plane without human intervention. From the standpoint of commercial aviation, radar will be a lifesaving device, but the reader also can imagine the deadly possibilities latent in man’s ability to build pilotless aircraft, buzz bombs, rockets, and jet-propelled missiles, each loaded with atomic bombs and able to follow an invisible beam to a predetermined target. He has only to envisage himself on the receiving end of this delivery line to get the feeling that this is a small world and the hiding places are very few.
Even more eerie than the possibility that invisibleeyed groundlings will be kings of all they survey in the air is the second prospect of future radar, that men in the sky will spy upon us from afar and know our every movement. Few objects of any size can escape the radar eye. Ships in even the loneliest waters cannot escape detection by high-flying observation planes, nor can trucks move at night without registering a change on a distant radar screen. Nations in a radar world will have little privacy, and the gap between the very advanced nations, technologically speaking, and the more backward ones is becoming so great that the former, with very slight inconvenience, can wipe out the latter.
As a peacetime instrument, however, flying radar will have multiple uses. Airmen rapidly and accurately can map vast uncharted regions of the world. Clouds will not deter the bird’s-eye view which man will have of waterways, mountains, and impassable jungles. Wherever pilots fly, they will have before them a visual image of the terrain that lies unseen below them. There will be no such thing as “visibility zero”; the radar screen will become an exact aerial road map.
It has been said that every weapon of war brings its own countermeasures, and this is true of radar. However, one should not permit himself to be lulled into a sense of security because of this fact. Only after the impact of a new weapon has been felt can work on the development of countermeasures begin. The rapid rate in which weapons recently have been developed has left the invention of effective defensive devices far behind. It is safer to be on the offensive than on the defensive, and the past few years have proved that a small margin of technical superiority often wins the battle.
THE story of the atomic bomb must be told in a very different manner from that of radar. In the first place, the principles involved are still new to even the most technically-minded persons; and secondly, they are intimately associated with the very structure of matter itself. One must start with the physicists’ picture of the structure of matter as it was in 1919, the year that Rutherford in England effected the first artificial transmutation of nitrogen into oxygen.
On the basis of numerous experiments and close mathematical reasoning, it was believed, and nothing discovered since has changed this view, that all matter is made of unit structures, or atoms. Each chemical clement has its kind of atom. The atoms themselves have structures of increasing complexity as one goes up the scale of atomic weight from hydrogen to uranium, but the architectural scheme is similar for all elements. Each atom has a central massive core, or nucleus, which contains almost all of the mass (weight) of the atom. The nucleus carries a charge of positive electricity and is very small in size — nuclei come about a million million to the inch. Surrounding the nucleus and moving under the intense electrical attraction of its positive charge are the electrons, which are very light, all identical in charge and mass, and negatively charged. Viewed as a whole, the atom is regarded as remotely similar to our planetary system with a massive central sun, the nucleus, surrounded by its electrons, like planets. Here the similarity ends, because the “planets” are not at all identical, and gravitational attraction plays little or no part in atomic structure. The electrical forces in the atom are vastly greater.
The number of electrons which surround the nucleus depends only on the amount of electrical charge on the nucleus. This charge is a definite number of times greater than the charge on the electron, and it is positive instead of negative. The number of electrons is equal to the number of units of positive charge which the nucleus carries. The structure as a whole is therefore electrically neutral, or uncharged, because the positive charges on the nucleus and the negative charges on the elect runs balance out. It was hard then, and is now, to define the size of the atom exactly, but in general there are approximately one hundred million atoms to the inch, so that each is ten thousand times larger than the nucleus alone.
The difference between chemical elements is only in the amount of electrical charge which they carry on the nucleus, and consequently in the number of electrons which surround it. Most chemical elements have more than one variety of nucleus. These varieties of the same chemical element have different nuclear masses (atomic weight), but they all have the same positive charge. These varieties were given the name of isotopes.
The various elements and their isotopes have masses which are approximately an integral number of times the mass of the hydrogen nucleus. Hydrogen was discovered to be the simplest element of all, with a nucleus carrying only one unit of positive charge and consequently surrounded by only one electron. This nucleus was found to be of such importance that it was given a special name — proton. The ratio between the proton and electron mass was 1840 to 1. The electrical charges were equal in amount, but the proton was positive and the electron was negative.
As far as can be observed, chemical elements are usually stable over periods of billions of years in the sense that iron remains iron, and oxygen remains oxygen, without changing to something else. A few exceptions were noted, however, of which radium is still the most famous. Without any external intervention, radium, a metal, spontaneously transmutes itself into another element known as radium emanation, a gas. In the process of transmutation, the radium emits a helium nucleus known as an alpha particle. This splitting of radium into radium emanation and helium (alpha particles) occurs within the nucleus itself.
The alpha particle given off during the self-transmutation of radium was identified as the nucleus of helium, the second element in the table of elements, and was found to have two positive charges but a mass of approximately four in proton units. Since it comes out of the radium nucleus and is positively charged, the alpha particle gathers high speed from the intense electrical repulsion of the highly charged nucleus of radium emanation and therefore comes out with a great deal of kinetic energy — the energy of motion.
Since radium and some other elements were known to disintegrate naturally, it was concluded that the nuclei were complicated structures of unknown units, and there were great hopes of inducing transmutations artificially. When physicists tried to induce artificial transmutations, however, even the intense heat of the electric spark produced no change in any of the nuclei. But in 1910, using alpha particles as fast bullets which could overcome the repulsion of the positive charge on the nucleus, Rutherford fired alpha particles directly into nitrogen nuclei and caused them to change into oxygen nuclei.
The reaction which Rutherford brought about artificially is worth studying very carefully, because a more complicated element, oxygen, was built up from two simpler elements, helium (alpha particles) and nitrogen. From this experiment, physicists learned that not only alpha particles but also protons could emerge from a nucleus.
ATOMIC nuclei are most extraordinary and fascinating objects. Contained in a very small space are a number of unit positive charges which exert great forces of intense mutual repulsion. Yet nuclei are found in general to be extremely stable structures. Under the intense mutual repulsion of the positive charges alone they would blow up instantaneously. It was therefore concluded that there must be some unknown intense forces of attraction which hold these antipathetic components together. What these forces are and how they arise was, and still is, one of the great mysteries of the science of physics. Since scientists could not understand the forces which held nuclei together, they could not understand how much energy was released in a nuclear reaction.
The way to calculate the amount of energy released in a nuclear reaction was discovered not through nuclear experiments but in a manner which is an interesting illustration of how different developments within a science dovetail to form the whole structure. In 1905, Einstein enunciated the theory of Special Relativity from a general consideration of the nature of clocks, the measurement of time, and the remarkable consistency of the velocity of light as measured on different systems moving relatively to one another. As a straightforward deduction from this theory, he enunciated the equivalence of mass and energy.
For our purposes, it can be stated from Einstein’s theory that if there is a change in the energy of some system, such as a nucleus or a collection of nuclei, there will be a perfectly definite equivalent change in mass. This statement gives us one of the most powerful tools in nuclear physics, because it enables us to find the energy released during a nuclear reaction by measuring the change in mass after the reaction. If one can measure accurately the original mass of a nucleus and the masses of the products of a reaction, the difference in the mass will immediately give the amount of energy which has been released for use.
From 1919, the year of the Versailles Treaty and Rutherford’s experiments with artificial transmutation, our story of the development of nuclear physics jumps to 1939, the year of the outbreak of World War II and of the discovery of the nuclear fission of uranium. Those twenty years between two wars were among the most revolutionary in the history of physics. They marked the experimental verification of Einstein’s Theory of General Relativity and the complete revision of our concepts of space, time, and gravitation.
During this period there arose the wondrous intellectual structure which is known as quantum mechanics, which gave us complete and quantitative insight into atoms and molecules and finally wedded phvsics and chemistry into one science. The scientific, philosophical, and moral implications of quantum mechanics, with its rejection of the classical doctrine of causality, have not yet been exhausted by our generation and are hardly known to the educated public.
The greatest experimental development during the brief period of peaceful scientific progress was in the field of nuclear studies. The outstanding event was the discovery of the neutron by the English physicist, Chadwick, in 1932. The neutron is what really makes the atomic bomb tick. It was a brand-new particle previously unknown to physics. The neutron is just perceptibly greater than the proton mass. It is just a bare nucleus without a positive charge, and consequently has no negatively charged electron surrounding it. Since it is neutral, the neutron is not affected by the electrons which surround the nucleus of an atom, and when it is employed as an atom-splitting bullet it can only be stopped or deflected by the nucleus itself. Hence the neutron can readily penetrate inches of lead or other dense material.
This great discovery made it possible to begin to understand the structure of atomic nuclei. They are now assumed to be composed of neutrons and protons, usually of more neutrons than protons. Two chemical elements, such as oxygen and nitrogen, differ from one another by the number of protons in the nucleus, which determines the total nuclear charge. Atoms with the same number of protons in their nuclei, but with different numbers of neutrons, have the same chemical properties and, as explained earlier, are known as isotopes. Some elements have as many as twelve different isotopes. Uranium, which until recently was considered the heaviest of all elements and the last in the periodic table of elements, has been found to have three important isotopes, which have mass numbers 238, 235, and 234. The charge on each isotope corresponds to 92 protons, and the rest of the mass is furnished by neutrons.
Fermi and his school of physicists in Italy were among the first to realize the power of the neutron as an experimental tool for the study of nuclei. Since the neutron carries no charge, there is no strong electrical repulsion to prevent its entry into nuclei. In fact, the forces of attraction which hold nuclei together may pull the neutron into a nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth. The nucleus is violently shaken up by the blow, especially if the collision results in the capture of the neutron. A large increase in energy occurs and must be dissipated, and this may happen in a variety of ways, all of them interesting.
Following Fermi’s lead, physicists all over the world took up with vigor the sport of bombarding nuclei with neutrons. Neutrons were easily obtained. For a few thousand dollars, furnished by a benevolent foundation, one could buy or rent a quantity of radium salt, which gave a very handy neutron source in compact and portable form when it was mixed with powdered beryllium. The neutrons came from the disintegration of beryllium by the fast radium alpha particles. Later on, the cyclotron became a more powerful and more controllable source of neutrons.
The discovery of neutrons began a dramatic sequence of events which led to the atomic bomb. Fermi and his associates commenced around 1934 to study the effects on uranium of neutron bombardment and capture. The results of their experiments were most puzzling. It was assumed that the elements which were produced by the nuclear reaction were in general of greater atomic weight and charge than uranium, but no logically consistent and clear account of the phenomena could be made. The problem continued to baffle scientists until 1939, when Hahn and Strassmann in Germany announced early in that year that barium was one of the products of the bombardment of uranium with neutrons.
The gold strike on the Klondike was as nothing compared with the effect of the Hahn-Strassmann announcement on the tight little world-of physicists. All over the globe, physicists unleashed their cyclotrons, their Geiger counters, and their ionization chambers, and by the end of the year nearly one hundred articles had appeared on the consequences of this discovery by Hahn and Strassmann. One hundred articles is a large number when one considers that usually some sort of experimentation has to be done or some calculations made before a scientific article can be written.
Why all this pother? The answer is simply this: while all other nuclear disintegration previously observed had resulted in the release of an alpha particle, a proton, or a neutron, the emergence of barium from a nuclear reaction was a vastly different matter. Barium has only a little more than half the mass of uranium. The immediate conclusion, soon justified by experimentation, was that the uranium had split into two nuclei of almost equal mass as a result of the neutron capture. Each nucleus had a large positive charge of approximately 40. The two halves therefore flew apart with an enormous release of kinetic energy, and the process was aptly named fission in analogy to the biological splitting of cells. The amount of energy released during the fission process was easily determined by Einstein’s statement of the relation between changes of mass and energy. The sum of the masses of the two fragments was less than the original mass of uranium, which means energy release.
The amount of energy released by the nuclear reaction was found to be two hundred million electron volts, as compared with that released by chemical reactions, which ordinarily is less than five electron volts. The nuclear reaction therefore released more than forty million times as much energy as a chemical reaction between atoms. It did not take the physicists more than a few minutes to realize the implications of these experiments. As an instance, I was residing in Princeton, on sabbatical leave from Columbia, when Professor Bohr, the great Danish theoretical physicist, arrived with advance news of the Hahn-Strassmann experiments. The next morning I visited Columbia and told the news to my colleague, Fermi, who had by that time left Italy to join our faculty. By nightfall he was already speculating on the size of the crater which would be produced if one kilogram of uranium were to disintegrate by fission. Similar scenes were occurring all over the world. The race for the atomic bomb was on.
THE two-billion-dollar questions which had to be answered before the atomic bomb could be realized were: (1) Did all three uranium isotopes undergo fission, and if not, which of the three was the important one? (2) Were any neutrons released during the violent fission process, and if so, how many on the average? (3) Did the remaining non-fissionable isotopes absorb neutrons to any great degree?
As was stated in the official report on the development of the atomic bomb, it was known by 1940 that only U235 (Uranium 235) was important for fission by neutrons of all speeds and that neutrons of certain speeds were captured by U238 to produce U239. It also was known by this time that the average number of neutrons emitted per fission was somewhere between one and three, and that these neutrons were mostly of high speed. These facts were very encouraging and fortunate for our side in this war, because they showed that an atomic bomb was possible and also so expensive that the enemy could not produce it. The reasoning runs like this:-
If more than one neutron is released during the fission process, the fission of one uranium nucleus will produce enough neutrons to set off more than one other uranium nucleus, and the whole process will multiply rapidly with explosive effect, producing what is called a chain reaction. The chain reaction will die out, however, like a fire in wet wood, if less than one neutron is produced per fission or if the neutrons, while passing through the uranium, are absorbed to a sufficient degree by some process which does not produce fission. It is clear that the chunk of uranium has to be large enough for the neutron to do its work by colliding with a fissionable nucleus before it can escape through the surface of the uranium. Also, another important point, the neutrons must be fast to give the chain reaction time enough to consume an appreciable portion of the uranium by disintegration before the gigantic energy release blows the entire bomb apart.
Fortunately for our side, the atomic bomb was bound to be extremely expensive to produce. U235 is only one part in 140 of the mixture of isotopes which ordinarily is bulk uranium. U238, which is 99.3 per cent of bulk uranium, absorbs neutrons and thus would stop the reaction. To make a bomb, pure U235 was needed, and relatively lots of it. The separation of U235 from U238 in bulk was never attempted before and turned out to be a peculiarly difficult and costly process. To ordinary peacetime thinking, it would have been termed impossible because of the expense. Here lay our good fortune, because, unlike any other nation, we had the manpower, the money, and the time to do the task. If the abundant U238 had been the important agent in atomic bombs, our cities would have been obliterated before we entered the war, because our enemies, although short on resources, were fully aware of all the possibilities.
Another side to the development of the atomic bomb is still more eerie. The capture of a neutron by U238 yields U239, which has a property, in common with some other nuclei, of spontaneously increasing its positive charge. The increase occurs in two successive steps by a process which essentially entails the creation of electrons. The electrons are ejected from the nucleus, which becomes an entirely new element, plutonium, of mass 239 and charge 94, instead of the 92 charge of uranium. This element is found nowhere on the face of the earth and represents an entirely new creation. It was suspected and later proved that plutonium also possesses the requisite fission properties to be the new material for a bomb. Plutonium had the advantage over U235, because it was an entirely different element from U238 and consequently, once made, could be separated from U238 by cheap chemical methods.
As a nation we can congratulate ourselves on having leaders in this country who were bold enough to appropriate the vast sums necessary to make this new element, atom by atom, through the bombardment of U238 with neutrons, when no certainty existed that the process would prove successful or that plutonium would be useful in an atomic bomb. Although most people feel, now that success has been achieved, that the effort was justified, one can imagine the fury of the defenders of the treasury if the gamble had turned out otherwise.
I HAVE said that, as a result of the war, science has advanced only moderately, despite these great technical developments. It is not my purpose, nor would it be right, to minimize the vast industry, the keen insight, the resourcefulness, and the imagination of the scientists and engineers who performed these gigantic deeds of scientific valor. Extensive areas of scientific knowledge were consolidated by their efforts, and new scientific tools of a power previously unknown were forged in their laboratories. Our advance in pure science, when we get back to it, may be greatly accelerated by the use of the new techniques developed during the war, if those whose business it is to supply the funds will stand the expense and not insist upon calling the tune.
It might be well at this point to recall some of the still unanswered fundamental scientific questions which physicists were asking themselves in 1940. More than a quarter of a century has passed, for example, since Onnes in Holland discovered the phenomenon of superconductivity. Briefly stated, he found that some metals, such as lead, when cooled to temperatures of a few degrees above absolute zero, suddenly lost all trace of electrical resistance. Once a current was started on a loop of wire at this temperature, the current continued indefinitely. Why? The question is all the more tantalizing since we understand quite well the factors which cause ordinary electrical resistance. One wonders whether the Onnes discovery is an accidental phenomenon or a profound one. All our ideas concerning the conductivity of electricity in metals remain in doubt until this problem is solved. If one were able to produce a resistanceless wire, its effect on the electrical industry would be revolutionary.
The great scientific objective of nuclear physics has been the elucidation of the forces which hold the aggregate of neutrons and protons together within their nucleus despite the strong electrical repulsions of the constituent protons. This primordial force which makes matter as we know it exist at all is unlike gravity or electrical forces, which fall off inversely as the square of the distance between force centers. It is a very short-range force which acts only over distances of about the size of the nucleus and then decreases very abruptly. Yukawa, a Japanese scientist, has suggested that the unknown force may have a connection with a new particle of mass intermediate between the electron and proton. Such a particle, the mesotron, has indeed been found since in cosmic rays and has become a fascinating field of study in itself.
Mesotrons seem to appear in a manner which would delight the professional magician. Apparently very rapidly moving protons such as are to be found in cosmic rays produce mesotrons when they collide with the nuclei of oxygen and nitrogen, the chief components of the earth’s atmosphere. It cannot be said that the mesotrons are ejected from these nuclei, or from the cosmic-ray protons; they simply appear as if by an act of creation during the violent collision. The phenomenon is very new; and for all we know, there may even be a wide variety of mesotrons. The energy which is represented by the mass of the newly created mesotron comes at the expense of the kinetic energy of the fast proton.
The mesotrons themselves seem to be real enough. They have an electrical charge of the same amount as the electron and a mass about two hundred times as great. They produce good, healthy, visible tracks in a Wilson cloud chamber and give every evidence of definite, real, concrete existence. Yet after a brief period of about one millionth of a second, they disappear into limbo, and all that is left is a very ordinary electron and some short-wave light energy of the X-ray variety.
These discoveries and unanswered questions pertaining to the nature of mesotrons and nuclear forces represent the first isolated tentacles which will encompass an interesting field of the physics of the future. Very few advances along these lines of research were recorded during the war years.
To probe still deeper, it is an experimental fact that matter is made up of small units like electrons, neutrons, and perhaps other particles still unknown. One asks oneself why electrons should be all alike. Why should electricity come out in certain definite units like the electron, no more, no less? There exists a positive electron, called the positron, which was discovered by Anderson in California about the same time that the neutron was discovered. The particle is in all respects just like the electron, except that its charge is positive. A positron and an electron can unite in mutual annihilation. All that comes off is some short-wave radiation like X-rays of energy corresponding to the Einstein relation bet ween mass and energy. Conversely, radiation can be destroyed to produce a positron-electron pair. Why does it turn out that these two have exactly the same charge and mass, no matter where or how produced? This is a property of light or electronic radiation which radar research does not touch.
Looking back now to the period before 1932, we seem to have been living in a simple, innocent world. We had the electron, the proton, and light, and all the observable properties of matter were to be worked out in terms of the interplay of these factors. Then in rapid succession there were discovered the positron, the neutron, and the possible varieties of mesotrons, which had hardly entered anyone’s thought before. These particles are all real in the sense that we can obtain direct experimental effects from any of these single isolated particles. But there is another particle, which, if it did not exist, would have to be invented. This one is called the neutrino and, because of its postulated nature, no one has yet devised an experiment, by means of which it might be observed.
The need for the neutrino arises from the method which physicists employ to balance their books. In the physicist’s notebooks there are at least four entries in which the credit and debit sides of the ledger must balance; otherwise the life of the physicist would hardly be worth living, so lawless would natural phenomena appear. The entries come from the socalled conservation theorems. The first of these is the conservation of charge, which states that the total net amount of charge remains constant. If a new positive charge appears somewhere, an equal amount of negative charge will also appear to balance it. The mutual annihilation of an electron-positron pair does not change the total charge.
The second conservation theorem is the law of the conservation of energy, now well known to all. This law states that if energy or mass disappears in one way it must reappear in an equal amount in another. The two other conservation laws - the conservation of momentum and the conservation of spin — are not so familiar but are just as important.
It has been known for a long time that in certain radioactive processes, such as the one in which U239 changes into plutonium by emitting two electrons in successive steps, the last three conservation laws are not fulfilled. The sums of the energy, momentum, and spins of the end products (that is, of the transformed nucleus, the ejected electron, and the radiation) do not balance with what was on the nucleus in the first place. Rather than give up these cherished conservation theorems, we assume that another particle, happily named the neutrino, emerges at the same time as the electron and shares the energy, momentum, and spin with it in such a way as to balance the books. The mass which the neutrino has to have in order to do the job for which it was designed is practically zero.
Admittedly all this may be rather fancy scientific figure-skating, but such speculations have the habit of turning out to be right. Only further research will reveal whether the neutrino must remain a ghost or whether it will take on the flesh and blood of direct experimental confirmation.
We do not know the answers to these questions or to other questions equally searching and fundamental. The development of radar or the atomic bomb was almost irrelevant to them. The answers will surely come if the science of physics continues, and probably from the most unexpected sources. The process of fission was found through chemical analysis; the positron was discovered, of all places, in the study of cosmic rays. After the discoveries are made, it is hard to see how they could have been missed. In the study of natural phenomena, man is a very nearsighted creature, and even the most profound and original man can see but a very short distance. It is a great adventure where close study, patience, intuition, and luck each play a part. It is the last frontier left to the free spirit of man in a crowded world.
The physicist returns from the war to cultivate his science. The answers to his questions will not be the end of all wisdom and knowledge. When scientific enigmas die, they give birth to twins. We are the inheritors of a great scientific tradition and of a beautiful structure of knowledge. It is the duty of our generation to add to the perfection of this structure and to pass on to the next generation the best traditions of our science for the edification and entertainment of all mankind.