What Science Is and How It Works

Gregory N. Derry

• Publisher: Princeton University Press
• Year: 2002
• Print ISBN: 9780691095509, 9780691058771
• eBook ISBN: 9781400823116
• Topic: History
• Subtopic: Science History
• Index: History

How does a scientist go about solving problems? How do scientific discoveries happen? Why are cold fusion and parapsychology different from mainstream science? What is a scientific worldview? In this lively and wide-ranging book, Gregory Derry talks about these and other questions as he introduces the reader to the process of scientific thinking. From the discovery of X rays and semiconductors to the argument for continental drift to the invention of the smallpox vaccine, scientific work has proceeded through honest observation, critical reasoning, and sometimes just plain luck. Derry starts out with historical examples, leading readers through the events, experiments, blind alleys, and thoughts of scientists in the midst of discovery and invention. Readers at all levels will come away with an enriched appreciation of how science operates and how it connects with our daily lives.


An especially valuable feature of this book is the actual demonstration of scientific reasoning. Derry shows how scientists use a small number of powerful yet simple methods--symmetry, scaling, linearity, and feedback, for example--to construct realistic models that describe a number of diverse real-life problems, such as drug uptake in the body, the inner workings of atoms, and the laws of heredity.


Science involves a particular way of thinking about the world, and Derry shows the reader that a scientific viewpoint can benefit most personal philosophies and fields of study. With an eye to both the power and limits of science, he explores the relationships between science and topics such as religion, ethics, and philosophy. By tackling the subject of science from all angles, including the nuts and bolts of the trade as well as its place in the overall scheme of life, the book provides a perfect place to start thinking like a scientist.

PART I

EXPLORING THE FRONTIERS OF SCIENCE: HOW NEW DISCOVERIES ARE MADE IN THE SCIENCES

Chapter 1

A BIRD’S EYE VIEW: THE MANY ROUTES TO SCIENTIFIC DISCOVERY

Now, I am not suggesting that it is impossible to find natural laws; but only that this is not done, and cannot be done, by applying some explicitly known operation.…

(Michael Polanyi)

HOW DOES A SCIENTIST go about making a discovery? The idea that there’s a single answer to this question (the “scientific method”) persists in some quarters. But many thoughtful people, scientists and science critics alike, would now agree that science is too wide-ranging, multifaceted, and far too interesting for any single answer to suffice. No simple methodology of discovery is available for looking up in a recipe book. To illustrate some of the rich variety in the ways scientists have discovered new knowledge, I have chosen five cases to recount in this chapter: the accidental discovery of x-rays; the flash of intuition leading to the structure of benzene; the calculations through which band structure in solids was discovered; the voyages of exploration inspiring the invention of biogeography; and the observations and experiments resulting in smallpox vaccine.

§1. SERENDIPITY AND METHODICAL WORK: ROENTGEN’S DISCOVERY OF X-RAYS

Working late in his laboratory one evening in 1895, a competent (but not very famous) scientist named Wilhelm Roentgen made a sensational discovery. His experiments revealed the existence of a new kind of ray that had exotic and interesting properties. Because these mysterious rays were then unknown, Roentgen called them x-rays (x standing for the unknown), a name that we still use to this day. After he reported his new discovery, Roentgen immediately became a highly celebrated figure and won the first Nobel Prize in physics just a few years later.

Of course, we now know what x-rays are. X-rays are similar to light, radio waves, infrared and ultraviolet rays, and a variety of other such radiations. All of these things are particular kinds of electromagnetic waves, so called because they are wavelike transmissions in electric and magnetic fields. The major difference between light and x-rays (and all the other types) is the wavelength of the radiation (this is the distance over which the wave repeats itself; different colors of light also differ in wavelength). The energy of the radiation also changes with the wavelength. X-rays have hundreds of times more energy than light, which accounts for both their usefulness and also their potential danger. This high energy also played an important role in Roentgen’s discovery.

The experiments that Roentgen had in mind built on the work of many other nineteenth-century scientists (Thomson, Crookes, Lenard, and others). This work consisted of experiments with something called a cathode ray tube. These devices are not as unfamiliar as you may think; the picture tube in your television is a cathode ray tube. Basically, a cathode ray tube is just an airtight glass container with all the air pumped out to create a vacuum inside, and pieces of metal sealed into the glass wall so that electrical connections outside the tube can produce voltages on the metal inside the tube. If the voltage is high enough, a beam of electrons leaving the metal can be produced. A substance that glows when high-energy rays strike it, called a phosphor, can also be placed inside the tube. When the beam of electrons strikes the phosphor, we can see the presence of the beam by the telltale glow emitted. In essence, this is how your television creates the picture you see on the screen.

In 1895, the existence of electrons was not known (Thomson was soon to discover the electron in 1897). The cathode rays, which we now call electron beams, were at that time simply another mysterious radiation that scientists were still investigating. One important property known to be true of the cathode rays is that they are not very penetrating, that is, do not go through matter easily. For example, cathode rays couldn’t escape through the glass walls of the tube. Lenard had discovered that a thin aluminum sheet covering a hole in the glass allows the cathode rays through, but the rays can then only make it through about an inch of air. All these observations were made using the glow of phosphors to detect the presence of the beam. Roentgen wondered whether some tiny portion of the cathode rays might after all be escaping through the glass walls undetected. The glass itself is weakly luminescent when struck by cathode rays, so the whole tube produces a kind of background glow. If an escaping beam were very weak, the slight glow it caused on a detecting phosphor might be washed out by this background glow of the tube. So Roentgen designed an experiment to test this hypothesis. He covered the tube with black cardboard to screen out the background glow, and his plan was to look for a weak glow on the phosphor he used as a detector when he brought it close to the covered tube wall.

As a first step, Roentgen needed to check his cardboard covering to make sure that no stray light escaped. As he turned on the high voltage, he noticed a slight glimmering, out of the corner of his eye, coming from the other side of his workbench (several feet away from the tube). At first, he thought that this must be a reflection from some stray light that he had not managed to block successfully. But when he examined the source of the glimmer more carefully, he was shocked to discover that it was coming from a faint glow of the phosphor he planned to use later as a detector. Something coming from the tube was causing a slight glow from a phosphor located over thirty times as far away as cathode rays can travel through air. Roentgen immediately realized that he had discovered some fundamentally new kind of ray, and he excitedly embarked upon the task of studying its properties. He found that these rays had extremely high penetrating powers. His phosphor continued to glow when a thousand page book or a thick wooden board was placed between the tube and the phosphor. Even thick plates of metals such as aluminum and copper failed to stop the rays completely (although heavy metals such as lead and platinum did block them). In addition to their penetrating power, Roentgen found that his new rays were not affected by magnetic and electric fields (in contrast to cathode rays, which are deflected by such fields).

In the course of his investigations, Roentgen made another accidental discovery that insured his fame in the history both of physics and of medicine. While holding a small lead disk between the phosphor screen and cathode ray tube, Roentgen observed on the screen not only the shadow of the disk but also the shadow of the bones within his hand! Perhaps to convince himself that the eerie image was truly there, Roentgen used photographic film to make a permanent record. After he completed his systematic and methodical investigations of the properties of x-rays, Roentgen published a report of his findings. The experiments were quickly replicated and justly celebrated. In physics, the discovery of x-rays opened up whole new avenues in the investigations of atoms and turned out to be the first of several revolutionary discoveries (followed quickly by radioactivity, the electron, the nucleus, etc.). In medicine, practitioners quickly realized the diagnostic value of x-rays as a way to look inside the body without cutting it open. The use of x-rays in medicine is one of the fastest practical applications of a new scientific discovery on record.

Roentgen’s discovery of x-rays was a marvelous combination of luck and skill. Discovering something you aren’t looking for, a process often referred to as serendipity, is not uncommon in the sciences. But as Pasteur’s famous maxim says, “chance favors only the prepared mind.” Roentgen’s mind was extremely well prepared to make this discovery, both by his skill in experimental techniques and by his thorough knowledge of the previous work on cathode ray phenomena. Also, Roentgen’s painstaking detailed investigation of the x-rays, following his initial lucky break, was crucial to the discovery process. He recognized the importance of the faint glimmer he did not expect to see.

§2. DETAILED BACKGROUND AND DREAMLIKE VISION: KEKULÉ’S DISCOVERY OF THE STRUCTURE OF BENZENE

The carbon atom has chemical properties that set it apart from all other elements. Carbon is able to form a wide variety of chemical bonds with other elements, particularly with hydrogen, oxygen, nitrogen, and with other carbon atoms. The tendency to form various kinds of carbon-carbon bonds, in addition to the C-H, C-O, and C-N bonds, fosters the creation of complicated chainlike structures in such carbon-based molecules. For these reasons, many thousands of these carbon compounds exist, so many in fact that the study of them is a separate branch of chemistry. This branch is called organic chemistry, because it was once thought that only living organisms could produce these compounds. It’s true that the molecules of living organisms (carbohydrates, fats, proteins) are all in this category, but “organic” is a misnomer in the sense that many organic chemistry compounds have nothing at all to do with life.

We might say that organic chemistry started with the synthesis of urea in 1828 by F. Wöhler. For many years thereafter, organic chemistry proceeded by trial and error, with chemists using their experience and various rules of thumb to synthesize new compounds. Organic chemists had no theory underlying their work and didn’t know the structures of the compounds they created. Around the middle of the nineteenth century, the work of many chemists contributed to a growing understanding of the science underlying organic reactions and syntheses. Prominent among these chemists was August Kekulé. Kekulé’s major contribution to organic chemistry was the idea that a molecule’s three-dimensional structure was a key ingredient in determining that molecule’s properties. The number of atoms of each element making up the molecule is obviously important, but how they are connected to each other in space is equally important. Kekulé’s theories concerning molecular structure in general, along with his determinations of the structures of many specific compounds, advanced the field considerably.

By 1865, Kekulé had worked out the structures of many compounds, but the structure of benzene had proven to be intractable. Benzene is a volatile liquid that can be obtained from coal tar. Benzene is sometimes used as an industrial solvent, but the major importance of benzene is its role as the structural basis for many dyes, drugs, and other important chemicals. Michael Faraday had already determined the atomic composition of benzene in 1825. Benzene consists simply of six carbon atoms and six hydrogen atoms. But forming these six C and six H atoms into a structure that makes sense had defied the efforts of organic chemists, including Kekulé. One major problem with devising a reasonable benzene structure is the 1:1 ratio of C atoms to H atoms. Kekulé had already previously concluded that C atoms make four bonds to other atoms and that H atoms make one such bond, a system that works well for methane (see chapter 18) and similar compounds. But it’s hard to reconcile this idea with the 1:1 ratio of C atoms to H atoms in benzene. Another big problem was the chemical behavior of benzene, especially compared to other compounds in which hydrogen atoms don’t use up all of the available carbon bonds. These other compounds, such as acetylene (the gas used in welding torches), can be chemically reacted with hydrogen to produce new compounds that have more H atoms. Benzene, however, wouldn’t accept any new H atoms in such a reaction.

Kekulé had pondered these problems for a long time. He combed his knowledge of organic chemistry in general, reviewed everything that was known about the reactions of benzene with other chemicals, and expended great effort in order to devise a suitable structure that made sense. Then, Kekulé hit upon the answer in a flash of inspiration. As Kekulé recounts the episode:

I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes.… My mental eye, rendered more acute by repeated visions of this kind, could now distinguish larger structures of manifold conformation: long rows sometimes more closely fitted together all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis.

Kekulé’s vision had suggested to him the ring structure of benzene shown in Figure 1. By having the chain of carbon atoms close on itself, he was able to satisfy the bonding numbers for C and H while leaving no room for additional H atoms. The question then became purely empirical. Does this benzene structure explain all of the known reactions and syntheses involving benzene? Does it predict new reactions and syntheses accurately? To make a long story short, the answer to these questions turned out to be, basically, yes.

Other structures were also proposed for benzene, and a vigorous debate went on for some years. In the end, Kekulé’s ring structure had the most success in explaining the data and became accepted as the correct structure. Some inconsistencies remained; calculated energies for the molecule were higher than the measured energies, and the placement of the three double bonds was distressingly arbitrary. These problems were finally cleared up many decades later when the modern quantum theory of chemical bonding was applied to the benzene ring, showing that all six bonds are really identical (circulating clouds of electrons bonding the carbons might be a more appropriate image than alternating double and single bonds). Meanwhile, Kekulé’s proposed benzene ring was extremely successful in suggesting reaction pathways for commercially important organic compounds. The German chemical industry soon became the envy of the world, producing dyes, drugs, perfumes, fuels, and so on. The solution of the benzene structure problem was a key to much of this activity, which was an important segment of the German economy prior to World War I. Kekulé himself, however, had little interest in commercial ventures and confined his attention largely to scientific understanding.

Figure 1. The structural model of the benzene molecule worked out by Kekulé, often referred to as a benzene ring. The ring structure was inspired by Kekulé’s vision of a snakelike chain of atoms closing on itself.

A number of scientists have reported experiences similar to that of Kekulé. After a prolonged period of apparently fruitless concentration on a problem, the solution seems to arrive all at once during a brief period of relaxation. It’s crucial to immerse oneself completely in the details of the problem before the flash of inspiration can come. An unusual aspect of Kekulé’s experience is the highly visual character of his insight. His earlier development of the structural theory of organic chemistry had also been informed by such visions of dancing atoms, so this seems to have been a general part of his thinking process. Kekulé’s early training had been in architecture, and it’s possible that this training influenced his rather visual approach to chemistry and his tendency to think in terms of the spatial “architecture” of molecules.

§3. IDEALIZED MODELS AND MATHEMATICAL CALCULATIONS: THE DISCOVERY OF BAND STRUCTURE IN SOLIDS

Semiconductors are now an essential part of modern life, forming the heart of integrated circuits and diode lasers. Computers, compact discs, telecommunications, audio amplifiers, television, and many other devices would not exist if we didn’t understand the behavior of semiconductors. The essential concept needed to understand semiconductor behavior is the concept of energy bands separated by band gaps, although few people have ever heard these terms. The existence of energy bands in solid materials was discovered by several people during the years from 1928 to 1931, at a time when semiconductors were merely a laboratory curiosity of little or no interest to anyone. The motivation for the work that led to this discovery was a desire to understand how electrons can even move through metals at all. If you imagine the negative electrons in a metal as moving through the array of fixed positive ions (which are much more massive than the electrons), the problem becomes apparent. The electrons and ions exert strong forces on each other. As the electrons try to move, they soon collide with an ion and are scattered into a different direction. This kind of scattering, in fact, is what causes electrical resistance in the first place. However, all the calculations done before 1928 indicated that the electrons shouldn’t get much farther than one or two ions; experimental resistance measurements required electrons to get past hundreds of ions before colliding. This was a mystery.

In an effort to solve this mystery, Felix Bloch applied the newly invented theory called quantum mechanics to the problem. In the strange world of quantum mechanics, the electrons may be pictured as waves rather than as particles. Bloch also used another recently discovered fact: the ions in a metal are arranged in an orderly periodic fashion (a crystal lattice; see chapter 18). So Bloch’s model (see chapter 6) of a metal consisted of quantum mechanical electron waves traveling through a periodic lattice of positive ions. Bloch succeeded; he was able to calculate the motion of the electrons in such a system, and the results were remarkable. It turned out that the electrons could sail effortlessly through the lattice without hitting ions. Resistance was due to vibrations of the ions and imperfections in the crystal. The results agreed well with experiments.

Another important step was taken by Rudolf Peierls, building on the foundation of Bloch’s work. Peierls kept the same basic model that Bloch used, but now he varied the strength of the forces between the electrons and the ions. In his previous work, Peierls had already shown that a more detailed examination of Bloch’s calculations reveals a “flattening” of the energy curve for the electrons. (This energy curve tells us how the electron’s energy changes as its momentum increases.) His experience with this previous work enabled Peierls to recognize the significance of his new calculations. He discovered that where the flattening of the energy curve ends, the...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Dedication Page
• Contents
• Preface
• Prologue: What Is Science?
• Part I. Exploring the Frontiers of Science: How New Discoveries are Made in the Sciences
• Part II. Mental Tactics: Some Distinctively Scientific Approaches to the World
• Part III. Larger Questions: The Context of Science
• Part IV. Common Ground: Some Unifying Concepts in the Sciences
• Epilogue: So, What Is Science?
• Index