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    The Development of Ideas Leading to the Atomic Bomb

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    Where did the atomic bomb come from? In this paper, I will look at the development of the ideas needed to create an atomic bomb. Specifically, what did scientists need to know for them to theorize that a cataclysmic explosion would result when a critical mass of certain elements undergo a chain reaction of nuclear fission. However, I will only look at scientific ideas generally, as they progressed towards fission. This development of ideas was propelled by genius, persistence and tenacity, coupled with flashes of insight into the nature of the universe.

    We see that this development is tied closely to the ability to free the teathers of erroneous paradigms and build better models of the universe in their place. We will be concerned, principally, with the development of physics. Einstein wrote the following on the definition of physics: “What we call physics comprises that group of natural sciences which base their concepts on measurements; and whose concepts and propositions lend themselves to mathematical formulation. ” (Weaver, 78) Although physics today is more focused, this is the basis of all science.

    One of the first groups of people to freely think about the universe and make an attempt to explain their world scientifically were the Greeks. II. The Greek Ideology The Greek’s investigation of science demonstrate that their minds were on par with the best of this era, specifically Aristotle (384 – 322 B. C. ), who formed many brilliant theories. He, along with others, put the theories into sophisticated form that created the basis of scientific thought for close to two millennia. In his universe were four “elements”: Earth, Water, Air, and Fire.

    The Earth was the common center of all the solid materials and had a natural place as the center of the universe. If all the solid material sought a location as close to the center as possible, then the Earth had to be a sphere. He had likewise ordered the other “elements” into spheres. Water had its natural place on the surface of the sphere Earth. Air had its natural place on the surface of the sphere Water. Fire had its natural place outside the sphere of Air. Observations corresponded to this view of the universe. However, he performed no experiments.

    He stated that heavier objects would want to move faster toward their respective spheres than lighter objects. It is regrettable that he did not perform any of a number of simple experiments to prove or disprove his ideas. These Greek philosophers worked to explain the motion of matter. Their ordering of the universe defined what happened when an element found itself outside of its sphere. It simply sought its correct sphere. They also did well with basic types of motion, stating that when one object had contact with another it would create motion in that object.

    There were other types of motion they had trouble with. For instance, why does a ball keep rolling even after your hand no longer has contact with it? Another problem that arises from the Aristotelian classification is how would two objects affect each other in a vacuum? Aristotle had theorized that vacuums would create difficulties, but in his day they were only considered a philosophical abstraction. The problem did not need to be dealt with seriously. Nevertheless, motion in the absence of the element Air was unthinkable. For them, Air had inherent physical properties.

    Also, it encompassed everything that could possibly have motion. The absence of Air meant the absence of motion. Before we can answer these questions, however, we must look at when and how observation combined with experimentation. III. Unifying Observation and Thought with Experimentation The Aristotelian universe was generally accepted for about 1600 years. During the late Middle Ages the view began to change slowly. Scholars began to view experimenting as a method of testing theories.

    The following passage explains the beginning of the change in ideas when scientists used experimentation methodically. Historically we may say the revolution in ideas began with Copernicus and his heliocentric theory of the solar system, but Kepler’s work is much closer to modern science than that of Copernicus, for in formulating his three laws of planetary motion, Kepler proceeded much the way the contemporary physicist does in constructing theoretical models of structures such as atoms, stars, or galaxies. Even so, Galileo and Newton were the initiators of modern science, for whereas Kepler’s work was primarily empirical, the work of Galileo and Newton has all the elements of what we now call physics.

    This work was an enormous step forward in that it revealed the relationship between the motion of a body and the forces acting on it. ” (Weaver, 18) Let’s back track slightly to Galileo Galilei (1564 – 1642). It was not until Galileo that the Aristotelian universe collapsed in a flurry of ingenious and conclusive experiments. Galileo did not invent experimentation, but he forever united it with science. For a brief background of Galileo, we turn to Segre’s “From Falling Bodies to Radio Waves. ” Galileo passed the first ten years of his life in Pisa, went to Florence around 1574, and was back in Pisa in 1581, registering as a student of medicine at the university. When he was nineteen years old he became acquainted with geometry by reading books and meeting the mathematician Ostilio Ricci (1540 – 1603). [It can very well be imagined] what a revelation the discovery of geometry must have been for the young man. He was studying something probably distasteful to him, and all of a sudden he found the intellectual for which he was born and which somehow had escaped him previously.

    Probably only passionate love can equal the strong emotion aroused by such an event. ” (Segre, “From Falling… “, 16) Galileo was the first person to create a shop for the pursuit of scientific study. Some experiments dealt with time-keeping, not an easy task four hundred years ago. He dripped water down inclined planes and achieved useful results. He also experimented with rolling balls of various weights on these inclined planes. It is not difficult to prove that the amount of time for the ball to traverse the plane is independent of the mass of the ball.

    In other words, it requires an equal amount of time for two balls of different weights to roll down an inclined plane. From this, and other experiments, he made the generalization that all bodies fell through equal distances in equal times. There were other significant discoveries made. Aristotelian thought was proved incorrect. Or we may say the generalizations made by Galileo provides a base to explain more phenomena when compared to the Aristotelian universe. After other people performed experiments and formed theories, and a hundred years passed, Sir Issac Newton (1642 – 1727) enters the stage.

    Newton developed mathematical tools to help him solve the problems created by his scientific pursuits. The nature of the phenomena he was pursuing forced him to create calculus. The following passage fills in some of the details. “Using the calculus, Newton deduced Kepler’s three laws of planetary motion. This changed the methodology of scientific research forever, for it showed that a correct physical law (Newton’s law of gravity) combined with logic (mathematics) can reveal new truths with relatively little effort and in a relatively short time.

    Kepler’s empirical formulation of the laws of planetary motion represents some sixty man-years of research (thirty years of Tycho Brahe’s observation and thirty years of Kepler’s arithmetic analysis), were as Newton’s derivation took only an hour or two. ” (Jefferson, 19) The development of the correct mathematical tools was an important event. When mathematics is combined with experimentation and thought, a new method of discovering the laws of nature is possible. The importance of this event can not be understated. Here is another example of the power of Newton’s laws, applying thought and using mathematics.

    At the beginning of the 1800’s , Uranus was found to have perturbations in its orbit. These perturbations were different from the orbit calculated by Newton’s law of gravitation. This fact threatened to dismantle the Newtonian universe. Then in the 1840’s, John Couch Adams (1819 – 1892) and Jean Joseph Leverrier (1811 – 1877), believing Newton’s law to be correct, developed a theory which could account for the differences between the predicted location for Uranus and its actual location. This theory was that another planet’s gravitational influence was perturbing Uranus’s orbit. Subsequently, Neptune was discovered.

    Still the difficulty of how objects affected each other remained. We return now to the different types of motion to appreciate the scientific problem facing people in the 17th century. IV. Action at a Distance Recall that the Greeks had difficulty explaining how a ball, once rolling, keeps rolling, and how objects would affect each other through a vacuum. Newton was able to explain the first problem with his first law: An object in motion tends to stay in motions. This is also know as inertia. The ball that is rolling stays in motion because the only way to change its motion is to subject it to more force.

    The problem of motion in a vacuum was more persistent. It was realized that vacuums do exist. The earth’s atmosphere did not extend indefinately. It was possible to see many objects in space affecting each other. Newton was left with the question: How do two objects affect each other when there is no contact and the objects are in space? This type of force was called action-at-a-distance. Newton was not fond of action-at-a-distance, but used it in his theories of gravitation. Two theories were created to explain action-at-a-distance. First, an idea proposed by Aristotle, a vacuum was not really a vacuum, but was made up of ether.

    Ether was a non-Air substance through which forces could travel. The second idea was that these forces moved across the vacuum like tiny projectiles in the same way a thrown object moves through the Air. (Weaver, 17-20) We see here that although the mathematics are useful and produce great results in explaining behavior, the basis for these explanations is shaky. It took the struggle of several generations of scientists to develop a new theory. The new paradigm that emerged was called field theory. V. Field Theory Michael Faraday (1791 – 1867) looked toward field theory to explain the nature of matter.

    As significant as the atomistic models of matter put forward by Dalton and Boltzmann was the continuous field concept put forward in that same period. Michael Faraday’s main ideas was to say that instead of considering that matter is a collection of ‘things’, each with their derivative physical properties, one of which is their field of continuously distributed potential influence on a ‘test body’, it is rather the continuous field of potential influence that is fundamental to the nature of matter, with the ‘thing’ attributes being derivative to the field.

    Thus, in Faraday’s view, the feature of matter that makes it appear as though it were localized is in fact a derivative property of the underlying field that expresses the essence of this matter. ” (Sachs, 54) Applying field theory to formulae of electrical, magnetic, and gravitational phenomena greatly simplified problems of attraction and repulsion. This new theory is important to the development of nuclear physics and the fission bomb. To see why, we must look at what people have been trying to determine for millenia: how and why objects move from one place to another.

    I will restate the development of the paradigms discussed in this paper. Aristotelian thought declared that each element has a natural sphere of rest. Moving an object out of its elemental sphere required a visible force. Much later, thoughts at the time of Newton could not see a force holding celestial objects in their orbits, but Kepler’s and Newton’s laws explained their motion almost precisely. Field theory supplied the force. A field defines the amount of force a test object feels when it is near the center of the field. For the sun, the gravitational field is very large.

    It significantly alters the course of objects far beyond the Pluto’s orbit. Using field theory is an alternate way of explaining how bodies can attract each other when there is no visible evidence that force is being transferred from one object to another. It moved the transportation device from the medium between objects to being a physical property of the objects themselves. Without this change, the possibility for Rutherford to deduce the structure of the atom is remote. Before Rutherford was able to experiment with the atom, the means to be able to experiment needed to be found. VI. Radioactivity

    In December 1895, Wilhelm Conrad Rontgen (1845 – 1923) discovered what he called a “new kind of ray,” and what we now call x-rays. “On the evening of November 8, 1895, Rontgen was operating a Hittorf tube and had covered it entirely with black cardboard. The room was completely darkened. At some distance from the tube there was a sheet of paper, used as a screen, treated with barium platinum-cyanide. To his surprise Rontgen saw it fluoresce, emitting light. Something must have hit the screen if it reacted by emitting light. Rontgen’s tube, however, was inclosed in black cardboard and no light or cathode rays could come out of it.

    Surprised and puzzled by the unexpected phenomenon, he decided to investigate it further. He turned the screen so that the side without barium platinum-cyanide faced the tube; still the screen fluoresced. He moved the screen further away from the tube, and fluorescence persisted. Then he placed several objects between the tube and the screen, and all appeared to be transparent. When he slipped his had in from of the tube, he saw his bones on the screen… he had found ‘a new kind of rays,’ as he termed them in his first publication on the subject. ” (Segre, “From X-rays… “, 20-57)

    He continued experimenting and found that different objects were transparent to his “new rays” in different degrees. He could not reflect nor refract the rays, nor would the magnetic fields he could produce influence the rays. On January 1, 1896, his first paper on this subject was published. Many found it unbelievable; photographs of bones inside of hands and bullets that were lodged within bodies provided the proof. The implications for medicine intrude the mind almost unbiddingly. Rontgen, receiving the first Nobel Prize for Physics in 1901, was unable to determine what caused this new phenomenon.

    Nature did not want to give up its secret of radiation yet. Hernri Bacquerel (1852 – 1908) came one step closer to understanding this phenomenon. His experiments consisted of taking a photographic plate and wrapping it in thick sheets of black paper. He placed a phosphorescent substance, made form a compound of uranium salt, on the plate and left it in the sun for several hours. When he developed the photograph, it had a the image of the substance that produced the phosphorescence. After performing the experiment several times, he concluded that the phosphorescent substance is able to emit radiation that penetrates the paper.

    The weather became overcast and he stored the materials, including a blank photographic plate, in a drawer. Several days passed and he developed the photograph. He discovered that the same image was on the photograph. He deduced immediately that the process is independent of the substance actually florescing. It was not the fluorescence causing the images on the photographs, but something else entirely. Marie Sklodowska Curie (1867 – 1934), about two years after Becquerel’s discovery, in 1897, was ready to do her doctoral thesis. She sought her husband’s — Pierre — advice.

    He suggested that she undertake a study of the “new phenomenon. ” She developed ways to measure the phenomenon with greater precision. She was the first to use the term radioactive for this phenomenon. They discovered several new radioactive elements. They also developed ways to extract the radioactive substances from the samples she used. She and her husband worked in this field for the rest of their lives. VII. Structure of the Atom Shifting the story again brings us to Ernest Rutherford (1871 – 1937) whose experiments in 1898 lead to the conclusion that there were two types of radiation emissions.

    He called them alpha rays and beta rays. In a few years they discovered that beta rays were electrons moving at high speeds. Also, P. V. Villard, in France, discovered gamma rays, which is a “much further penetrating x-ray. ” Rutherford, in 1903 and 1904, through continued experimentation felt that alpha rays are helium nuclei being expelled from the nucleus of these radioactive materials. His observations lead him to counting individual alpha particles. Hans Geiger (1882 – 1945) worked with Rutherford and together were able to determine several important universal constants.

    The experiments also helped to confirm that matter is discrete, and not continuously distributed. Rutherford studied the passage of alpha particles through other objects. Several students helped Rutherford. Ernest Mardsden (1889 – 1970) around 1904 witnessed that occasionally alpha particles were deflected when traveling through a thin metal foil. When Marsden related this observation to Rutherford, he desired to see the experiment himself. (Segre, “From X-rays… “, 20-57) The following excerpt explains Rutherford’s findings: “The big deflections had greatly amazed Rutherford.

    He later said that it was as if someone had told him that having fired a pistol at a sheet of paper, the bullet had bounced back! “Several weeks passed. Then one day in 1911 Rutherford announced that now he knew why Marsden’s particles were deflected at wide angles. And, moreover, he knew the structure of the atom. ” (Segre, “From X-rays… “, 104) At this time, several models of the atom were already hypothesized. Rutherford’s experiments had provided solid scientific evidence that the idea of the atom being like a small planetary system was essentially correct. Rutherford hypothesized that the nucleus contained the positive charges.

    These charges were concentrated in a comparatively small volume of space. This nucleus was circled by a similar number of negative charges. (He knew there were problems with this theory, but he used this theory in the same way that Newton was willing to use action-at-a-distance. It was close enough to make useful calculations. ) The alpha particles that shot into the foil and bounced back were deflected by the nucleus. This deflection was the result of the mutual repulsion two protons have for each other. It is governed by the mathematical description of Coulomb’s law.

    Without field theory, Rutherford would have had to figure out how two very small protons are able to feel each other’s presence inside an atom. But with field theory, he did not need to concern himself with it too much. Rutherford’s next problem dealt with finding the neutron. The neutron had been hypothesized from the fact that helium has a weight of four protons but an electrical charge of only two. The question of the extra weight was perplexing. The idea of a neutral particle, with the properties that are associated with what is now known as the neutron, was first proposed by Rutherford in 1920.

    James Chadwick (1891 – 1974) and Rutherford performed a search for this theoretical particle, but were unable to prove its existence. Shortly, we will see what had to happen first to make the discovery of the neutron possible. Thus, the atom could be shown to exist. Shortly after Rutherford’s evidence that the atom is like planetary system, but on a very small scale, was made known, many people commenced work in this new field which later became known as nuclear physics. Some, such as Rutherford and the Curies, made this topic their lifes’ work.

    The experiments lead to quantum mechanics, which was also worked on steadily through this time period. It is still pursued today, but unfortunately, we will not look at quantum mechanics in this paper. VIII. Fission Frederic Joliot (1900 – 1958) and Irene Curie (1897 – 1956), his wife, were performing experiments in 1931 with polonium, which had been discovered by her mother, Marie Curie. Their experiments produced very strange results; literal transmutations of elements were occurring at the atomic level for which they could not account. They published these results on January 18, 1932.

    When Chadwick saw the report he repeated the experiments, using additional elements, and proved that the radiation contained a neutral particle whose mass was approximate to that of a proton. He called it a neutron in a report sent to Nature on February 17, 1932. Continuing his work found that slow moving neutrons were more apt at producing these transmutations than protons. When he received the Nobel Prize in 1935, he discoursed on the usefulness of the neutron as a catalyst to fission. A small excerpt from his lecture follows. “The great effectiveness of the neutron in producing nuclear transmutations is not difficult to explain.

    In the collisions of a charged particle with a nucleus, the chance of entry is limited by the Coulomb forces between the particle and the nucleus; these impose a minimum distance of approach which increases with the atomic number of the nucleus and soon becomes so large that the chance of the particle entering the nucleus is very small. In the case of collisions of a neutron with the nucleus there is no limitation of this kind. The force between a neutron and a nucleus is inappreciable except at very small distances, when it increases very rapidly and is attractive.

    Instead of the potential wall in the case of the charged particle, the neutron encounters a potential hole. Thus even neutrons of very small energy can penetrate into the nucleus. Indeed slow neutrons may be enormously more effective than fast neutrons, for they spend a longer time in the nucleus. ” (Weaver, 733) As stated in the quote, slow moving neutrons have a greater incidence of affecting the nuclei of the material than fast moving neutrons. By bombarding of the elements, and determining the reactions that took place, physicists found the neutron to proton ratio of a wide range of these elements.

    They also found that the neutron to proton ratio increased as the number of protons in the nucleus increased. The element with the most protons known at the time was uranium. It has 92 protons and 146 neutrons. (It is usually known as uranium-238. ) Bombarding uranium yielded the most spectacular results yet. The uranium atom was actually split into two atoms of approximately the same size — and fission was accomplished. This released significant amounts of energy. It was found that an isotope of uranium, uranium-235, easily fissioned with slow neutrons to yield krypton and barium. Taylor, 353) Unfortunately, uranium-235 is found in naturally occurring uranium only about 1 part in 137.

    Extracting it is not an easy process. (Segre, “From X-rays… “, 210) This provides the last piece of information needed to deduce the possibility of an atomic bomb. IX. Sustained Reactions – The Atomic Bomb In 1940, Otto Frisch and Rudolph Peierls posed an important question. From “Nuclear Fear”, we may read this question. “Exactly what would happen, they asked themselves, if you could cull from natural uranium a mass composed purely of the rare uranium-235?

    Bohr and others had told the public that there could be enough energy there to blow up a city, but nobody had worked it out as a serious technical possibility. Now Frisch and Peierls realized that with fissionable uranium-235 atoms all crammed together, there would be no need for a moderator to slow the neutrons down, since even the fast neutrons emitted in each fission would have a good chance to provoke another fission. The whole chain reaction would go so swiftly that, before the mass had a chance to blow itself apart, a run away [reaction would allow] many of the uranium-235 atoms [to] split and release energy. ” (Weart, 84)

    This question may have been left academic for years had it not been for World War II. As the awesome power of an atomic bomb was realized by leaders of several countries, a race began to be the first to make a working bomb. As a result, a simpler method was discovered than separating uranium-235 from uranium-238. This simpler method starts when uranium-238 absorbs a single neutron a new element, called neptunium-239, is created. (Neptunium-239 has 93 protons and 146 neutrons. ) This element decays into plutonium-239 (94 protons and 145 neutrons). Plutonium is stable and also has the property of undergoing fission with slow neutrons.

    Hence, the atom bomb was conceivable. Plutonium was produced in a reactor. (Weart, 87) The United States was one of the nations was one of the countries searching for the technology to make the atomic bomb a reality. On July 16, 1945, they succeeded when the first atomic bomb was detonated. “In an isolated spot named Alamogordo, moments before first light… , night exploded noiselessly into day. Searing colors – gold, purple, blue, violet, gray – illuminated everything in sight. From the floor of the desert, a ball of fire rose like the sun (only brighter, one report read, ‘equal to several suns in midday’). …

    Thirty seconds later came a blast of burning air, followed almost instantaneously by an awesome roar. A cloud the shape of an immense mushroom ascended nearly eight miles, was caught by the desert winds, and curled into a giant question mark. ” (Stoff, 1) This was the realization of a long trek through history. Thought and experiment combined with field theory, a knowledge of chemical properties of the elements, and the discovery of radioactivity. This gave people the ability to answer the question: What is the structure of the atom? Not only was the structure determined, but it was found that the number of protons and neutrons could change.

    Protons and neutrons together are known as nucleons — particles that inhabit the nucleus. ) Changing the number of nucleons has several names: radioactivity, fission and fusion according to how the atom is changing and what is causing the change. Generally energy is released as a result of this change. Using Einstein’s bold statement that E=mc^2, the nature of this energy became known. The energy is a direct conversion from part of the mass of the atom. As we saw, it was a short technological step to use the same source of energy for the sun, as a source of energy on the earth.

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