Cover

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Title Page, Copyright Page

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pp. i-v

Contents

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pp. vi-ix

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Preface

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pp. x-xiii

Galileo’s claim that mathematics is the language of science applies to no science more than to physics. But mathematical description requires great effort. How is that effort begun? The answer, more often than not, is with a drawing of an as yet wordless, pre-mathematical picture of reality. To draw is to see the world in a particular way and to inform the self with an understanding of the world. Drawing the important elements of physical reality diminishes the psychological difficulty of articulating that reality in language of any kind. Subsequent progress allows one to refine an initially crude drawing....

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Dedications and Acknowledgments

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pp. xiv-xv

My father, the late Reverend Wishard F. Lemons, would have been pleased that, finally, I had written a book he could read. Another dear one who has passed on, Anthony Gythiel, friend, literary scholar, and medievalist, encouraged me to include essays on medieval physics. Memory Eternal, Wishard and Tony! To you and to my young grandsons, Abel and Emil, I dedicate this book....

Antiquity

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p. 1

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1. Triangulation (600 BCE)

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pp. 2-4

When a surveyor cannot measure a certain distance directly, say the width of a river or the height of a tree, either by counting paces or by laying out lengths of a standard measure, he can use the properties of triangles to determine the distance. This idea, which goes back to Thales of Miletus (624–565 BCE), is one of the first in the history of physics and mathematics....

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2. Pythagorean Monochord (500 BCE)

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pp. 5-7

One of the simplest musical instruments imaginable, the Pythagorean monochord, is a single stretched string fixed at each end. When plucked, the string vibrates and produces a tone of a particular pitch. Longer and heavier strings produce lower tones just as longer and larger wind and percussion instruments do. These facts must have been known before the time Pythagoras flourished around 525 BCE. After all, musical instruments with several strings of different lengths, such as the oud and the lyre,...

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3. Phases of the Moon (448 BCE)

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pp. 8-11

The various appearances of the moon—new moon (or no moon), tiny crescent moon, quarter moon, gibbous moon (partway between quarter and full), and full moon—are so familiar we may wonder why they need to be explained at all. Yet a certain kind of mind strives to explain complex phenomena, whether familiar or unfamiliar, in terms of simple concepts. These simple concepts should themselves be plausible and explain other phenomena. If successful, the explanation becomes part of a coherent outlook or theory....

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4. Empedocles Discovers Air (450 BCE)

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pp. 12-14

The air that surrounds us is invisible, odorless, and tasteless. Nor does it usually produce sound or resist our movement through it. Of course, sometimes we feel a breeze at our back or a wind in our face. Less frequently tornadoes obliterate solid buildings and gale force winds raise seas that put neighborhoods under water. No doubt our ancestors had been aware of these phenomena for millennia when Empedocles (490–430 BCE), a native of Acragas in Sicily, sought to explain them. He was one of several physically minded, Greek-speaking philosophers or cosmologists...

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5. Aristotle’s Universe (350 BCE)

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pp. 15-18

Have you ever heard someone say “Eventually scientists will figure out how to do it?” You fill in the reference for “it.” Travel faster than the speed of light? Build a heat engine with 100 percent efficiency? Extract energy from the cosmic microwave background? Indeed, it may be that some things once thought impossible will turn out to be quite possible. But it is not true that everything of which we might dream is possible. After all, we live in a world that has a nature : a characteristic way of being and of becoming, of remaining the same and of changing....

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6. Relative Distance of the Sun and the Moon (280 BCE)

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pp. 19-22

Aristarchus of Samos (310–230 BCE) was the first to determine the relative distance of the sun and the moon from the earth. His method, like that of Thales, depends on the properties of similar or same-shaped triangles. But his application—to the relative distances of heavenly bodies—is much bolder. Aristarchus assumed only that the moon receives its light from the sun....

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7. Archimedes’s Balance (250 BCE)

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pp. 23-26

Around 300 BCE, Euclid organized the mathematical knowledge of his time into definitions , common notions , postulates , and the demonstrations of propositions . Some of the definitions are familiar, for instance, “A line is breadthless length,” and some seem a little mysterious, “A straight line is a line that lies evenly with the points on itself.” The common notions are self-evident statements common to all kinds of reasoning such as “Things which are equal to the same thing are also equal to one other.” The...

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8. Archimedes’s Principle (250 BCE)

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pp. 27-30

The story goes that when the solution to a particularly challenging problem came to Archimedes in his bath, he leapt from the tub shouting “Eureka! Eureka!” (I have found it! I have found it!) But what had Archimedes found? According to Vitruvius (ca. 75–15 BCE), the Roman military engineer who told the story almost two centuries after the event, Archimedes had discovered a method for determining whether a crown that had been made for King Hieron of Syracuse was of pure gold, as per instructions, or mixed with silver. The story is a good one—almost too good to be true—for the method Archimedes discovered concerned...

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9. The Size of the Earth (225 BCE)

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pp. 31-34

Eratosthenes (276–194 BCE) was born in the North African town of Cyrene (in modern Libya) and educated in Athens, but lived the greater part of his life in Alexandria where around 244 BCE he became the head of its great library. The wealth of this library’s holdings can be inferred from the task given Callimachus, a contemporary of Eratosthenes—to catalog the library’s books—and the result of his effort: 120 volumes of bibliography. Thus, we can understand Callimachus’s famous complaint, “A great [or large] book is a great evil.” Nevertheless, the library and its great books made Eratosthenes’s career possible....

Middle Ages

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p. 35

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10. Philoponus on Free Fall (550 CE)

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pp. 36-39

John Philoponus (490–570 CE), whose surname means lover of toil, was a Greek Christian who lived and worked as a philosopher, theologian, and scientist in the century immediately following the invasion of Roman Italy by Germanic tribes in 476 CE. While he flourished more than a century after Theodosius (347–395 CE) had established Catholic Christianity as the official religion of the empire in 380 CE, Philoponus was taught by and worked with pagan philosophers associated with the library in Alexandria. Philoponus...

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11. The Optics of Vision (1020 CE)

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pp. 40-43

It is said that an Egyptian caliph of the Fatimid dynasty convinced the Muslim sage Ibn al-Haytham (ca. 965–ca. 1040), also known by his Latinized name Alhazen, to leave his native Basra in Iraq and come to Egypt in order to design and build a waterway that would regulate the flow of the Nile. Upon close inspection, Alhazen found that the project was not feasible. Then, fearing the wrath of the disappointed caliph, Alhazen feigned insanity. This tactic preserved Alhazen’s life at the cost of his forced confinement. Even so, Alhazen was able to continue his scholarly work, and, when the caliph died, he recovered his freedom....

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12. Oresme’s Triangle (1360)

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pp. 44-48

Oresme’s triangle, embedded in figure 20, relates two quantities, speed (in the vertical direction) and time (in the horizontal direction), graphically rather than pictorially. It is possibly the earliest such graph. As such it illustrates a proof of a theorem sometimes called the mean speed theorem or the Merton ruleaccording to which a uniformly accelerated object starting from rest traverses the same distance in a given time as an object moving uniformly at half the accelerated object’s final speed. In expressing this proof in graphic language, Nicole Oresme (1323–1382), who later became the bishop of Lisieux in northwestern France, built upon ideas first articulated in antiquity....

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13. Leonardo and Earthshine (1510)

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pp. 49-52

If this text had a section called “Renaissance Science,” Leonardo da Vinci (1452–1519) would be its exemplar. Yet, while Leonardo was of the Renaissance, he was not the kind of scholar glorified by the humanists of his time: a scholar nurtured in classical history and literature, having perfect Latin, skilled at rhetoric, and able to speak with confidence at public gatherings. Rather, Leonardo’s education was incomplete, his Latin was poor, and he had little interest in public affairs. But Leonardo was a keen observer of nature, an avid experimentalist, and a man drawn to practical applications. While the classically educated scholars of the Italian Renaissance quoted authors, Leonardo cited experience....

Early Modern Period

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p. 53

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14. The Copernican Cosmos (1543)

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pp. 54-57

The task of ordering the visible universe into an intelligible whole or cosmos has long challenged astronomers. According to Aristotle (384–322 BCE), the heavenly bodies—Moon, Sun, wandering stars or planets, and fixed stars—are embedded in rigidly rotating, Earth-centered, transparent spheres. In this way each “star” moves uniformly in a circle around a stationary earth. Ptolemy (90–168 CE), who was a great observer of the heavens, embellished Aristotle’s basic structure in order to better account for what he actually saw: planets whose brightness varied and planets that sped up, slowed down, and sometimes reversed direction....

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15. The Impossibility of Perpetual Motion (1586)

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pp. 58-61

A chain of fourteen identical, spherical beads is draped over a triangular support whose lower edge is parallel to the ground (figure 25). According to Simon Stevin (1548–1620), its Flemish originator and contemporary of William Shakespeare (1564–1616), this clootcrans or “wreath of spheres,” as it has been variously called, must remain stationary even if one assumes the beads can slip without friction on their supporting surface. Suppose, Stevin reasoned, that the wreath were to slip clockwise. Each sphere would soon take up a position previously held by an adjacent sphere. Then the wreath would recover its original aspect, and then slip again, and so on, ad infinitum. Since perpetual motion is clearly absurd, the wreath of spheres must remain in its original position....

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16. Snell’s Law (1621)

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pp. 62-65

One of the most familiar manifestations of refraction is the broken appearance of a straight object resting in and projecting out of a glass of water. While the geometry of this particular phenomenon (involving as it does the light reflected from the object seen, its propagation through water and air, and its reception at the eye) is quite complicated, the essence of refraction is simple....

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17. The Mountains on the Moon (1610)

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pp. 66-69

By the first decade of the seventeenth century the time had come for the telescope to be invented. In Holland several Dutch lens grinders and spectacle makers hit upon the same idea at the same time: a tube that aligned two lenses, one concave eyepiece and one convex light-gathering objective. In 1608 one of these Dutchmen, Hans Lippershey, attempted to patent a spyglass, as it was then called, that enabled one to see “things far away as if they were nearby.” Lippershey’s spyglass only magnified linear dimensions by a factor of three. Even so, because it had obvious military applications, the news of its discovery spread quickly across Europe....

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18. The Moons of Jupiter (1610)

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pp. 70-73

Among the marvels Galileo saw when he turned his newly constructed telescope toward the heavens in the winter of 1609–1610 were the four brightest moons of Jupiter. He dubbed these the Medicean planets in order to flatter the man whose patronage he sought and to whom he dedicated the seventy-page booklet he published in March 1610 that reported on these telescopic discoveries. This was Sidereus Nuncius, that is, Starry Messenger, dedicated to “The Most Serene Cosimo II de Medici, Fourth Grand Duke of Tuscany.”...

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19. Kepler’s Laws of Planetary Motion (1620)

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pp. 74-77

Johannes Kepler (1571–1630) delighted in uncovering the hidden order of the universe. As a young man, he embraced the order in Copernicus’s heliocentric arrangement of the heavenly bodies. Copernicus’s universe was not significantly more accurate than Ptolemy’s geocentric one, but it was more ordered with each of its features logically entailing others.
Crucial to his search for order was Kepler’s encounter with Tycho Brahe (1546–1601) in February 1600. Tycho’s personal resources and connections had allowed him to construct the best pre-telescopic, astronomical observatories of his time: first the Uraniborg observatory on the island of Hven for the Danish...

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20. Galileo on Free Fall (1638)

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pp. 78-81

Galileo’s fame as a scientist rests on his ability to abstract the essential physics from complicated phenomena, to describe that physics in eloquent words and simple mathematics, and to verify that description with cleverly designed experiments. But Galileo had multiple talents. In fact, Galileo did so many things well his twentieth-century biographer, Stillman Drake, claimed that it is “hard to say whether the qualities of the man of the Renaissance were dominant, or those of our own scientific age.” He was an excellent prose stylist, an accomplished visual artist, an ardent gardener, a proficient lute player, and a vigorous debater....

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21. Galileo on Projectile Motion (1638)

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pp. 82-85

In 1616 the office of Inquisitor General of the Roman Catholic Church warned Galileo to “relinquish altogether the said opinion that the Sun is the center of the world and immovable and that the Earth moves.” Consequently, Galileo promised not “to hold, teach, or defend in any way whatsoever, verbally or in writing” the said opinion—a promise that, by publishing Dialogue Concerning Two Chief World Systems in 1632, he broke in the most dramatic way. The conceit of the dialogue that placed powerful arguments in favor of a heliocentric universe and weak objections to it in the mouths of fictitious interlocutors fooled no one....

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22. Scaling and Similitude (1638)

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pp. 86-89

Figure 34, in Galileo’s own hand, appears in his Two New Sciences(1638). Its exquisite shading tells us that these are not mere shapes for which outlines would have sufficed. Galileo wants us to see these figures as bones—bones with which he can illustrate the concepts of scaling and similitude and take the first steps toward a theory of appropriate size.
Mathematical objects with the same shape, such as two triangles that differ only in scale, are said to be similar. Three-dimensional objects, for example, pyramids, with parts in the same proportions yet of different size, are also similar. However, most natural objects and animals with similar shapes occupy a more or less limited range of sizes within which they can be large or small versions of themselves....

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23. The Weight of Air (1644)

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pp. 90-93

The element mercury is relatively rare, but it does not blend easily with other elements in the earth’s crust, and, for this reason, is often found isolated in mineral deposits. These deposits have been mined for millennia for, in spite of mercury’s toxicity, our ancestors valued it, a shiny liquid metal, as a medicine, as an ornament, and for its high density.
Evangelista Torricelli (1608–1647) put mercury’s density, roughly fourteen times that of water, and its liquidity to good use in devising the first barometer as shown in figure 35. He prepared a narrow glass tube sealed at one end, filled it with mercury, stopped the open end with his finger, inverted it, and...

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24. Boyle’s Law (1662)

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pp. 94-99

Robert Boyle’s first trial of the experiment depicted here ended in disaster. He had a long glass tube bent in the shape of a “U” with its two unequal legs parallel to one another. The longer leg was more than six feet in length and the shorter one was sealed at its end. Then he poured mercury into the open end of the long leg of the tube. His object was to record paired values of the distances marked H and h in the third panel of figure 37, H indicating how much higher the mercury is in the longer leg than in the shorter one and h indicating the length of the column of air trapped in the shorter leg. But before he could gather data, he accidently broke the unwieldy tube and, presumably, spilled the expensive mercury....

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25. Newton’s Theory of Color (1666)

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pp. 100-103

Isaac Newton (1642–1727) entered Trinity College, Cambridge University, in 1661, the year after the Restoration of the English monarchy that, in turn, followed the beheading of Charles I and the decade-long dictatorship of Oliver Cromwell. At that time the university had entered a long period of decline from which it did not emerge until after Newton’s death. Although nominally dedicated to educating young men, chiefly for the clergy, the fellows of Cambridge were not obliged to tutor, lecture, publish, or even remain in residence. Many, in fact, chose to absent themselves for months and years at a time. Even so they drew...

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26. Free-Body Diagrams (1687)

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pp. 104-107

At a certain stage in their study, a physics student is called upon to master an important diagrammatic tool: the free-body diagram. Free-body diagrams help us analyze a situation in terms of forces. A general Newtonian principle is that only bodies exert forces on one another. What are the forces and what are the bodies that exert and that experience these forces? A free-body diagram helps us keep track of our answers....

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27. Newton’s Cradle (1687)

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pp. 108-111

Many of us have played with a “Newton’s cradle,” composed of several steel balls suspended as shown in figure 43. A simplified version of this toy composed of only two steel balls best illustrates our concern (see figure 44). The left panel of the diagram shows the black ball hanging at rest while the elevated white ball is released and allowed to swing down and strike the black ball. The right panel shows the two balls shortly after their collision. The white ball is now at rest, and the black ball has now completely captured the motion originally in the white ball. Eventually the black ball will swing up to a level close to that originally occupied by the white ball. Sometimes we hear this device referred to as a “double pendulum,” so called because two pendula mimic the motion of one....

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28. Newtonian Trajectories (1687)

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pp. 112-115

Galileo’s telescopic observations of the mountains and valleys on the moon suggest that the earthly and heavenly realms are similarly composed. Newton (1643–1727) confirmed this suggestion by generalizing to cosmic scale another of Galileo’s discoveries: the parabolic trajectory. Pitch a baseball horizontally and imagine its trajectory unfolding in space and time. The baseball covers ground in direct proportion to the first power t of the time elapsed and falls downward in direct proportion to the...

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29. Huygens’s Principle (1690)

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pp. 116-120

Constantijn Huygens intended that his son Christiaan (1629–1695) follow him into the Dutch diplomatic service and for this reason gave him a liberal education in languages, music, history, rhetoric, logic, mathematics, and natural philosophy and also training in fencing and riding. But when the House of Orange lost its power, Constantijn lost a patron, and Christiaan lost his opportunity in diplomacy. Fortunately for us, Christiaan’s true interests were in mathematics and natural philosophy. He developed the theory of pendulum motion and of colliding objects, invented an algorithm for computing the digits of the irrational number π, and constructed a telescope with which he identified...

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30. Bernoulli’s Principle (1733)

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pp. 121-124

Daniel Bernoulli’s (1700–1782) good fortune (and also his misfortune) was to have his own father as a tutor. Daniel’s father, Johann Bernoulli (1667–1748), was a professor at the University of Basel in Switzerland and the foremost mathematician in Europe. Johann and his brother Jakob, Daniel’s uncle, were among the first mathematicians to master calculus after its invention by Newton and Gottfried Wilhelm Leibniz (1646–1716) in the second half of the seventeenth century. Johann’s son, Daniel, was probably the ablest of several generations of Bernoulli mathematicians....

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31. Electrostatics (1785)

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pp. 125-130

Figure 51 illustrates a well-known interaction: Like charges repel and unlike charges attract. The two kinds of charges are here indicated by a plus and by a minus sign. Insulating strings suspend the charged balls. (An insulator is made of material in which charges are not free to move, and a conductor is of material in which charges are free to move.)
In the eighteenth century these balls were composed of pith (a spongy organic material) and the strings were of silk—both good insulators. The natural philosophers of that time generated positive charge on a glass rod, for instance, by rubbing the rod with their hands, and transferring the charge to the pith balls by stroking the latter with the glass rod....

Nineteenth Century

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p. 131

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32. Young’s Double Slit (1801)

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pp. 132-136

Sometimes one physical theory completely overthrows and replaces another. Such a revolution occurred in the years following the period 1801–1804 when the English polymath Thomas Young (1773–1829) marshaled compelling arguments in favor of the wave theory of light. Young’s arguments were, in part, reinterpretations of data gathered a hundred years earlier by Isaac Newton and, in part, based upon his own simple experiments....

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33. Oersted’s Demonstration (1820)

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pp. 137-140

Once household implements began to be made of iron, people began noticing that nearby lightning strikes sometimes magnetized these implements. But what is lightning? And how does it magnetize iron? In a letter written in 1752, Benjamin Franklin described an experiment whose purpose was to answer the first of these questions. His idea was to fly a kite into a storm cloud so that any electrical charge present would be conducted down its wet string and stored in a glass container, lined inside and out with metal foil, called a Leyden jar. After explaining to his correspondent how to make a kite out of a silk handkerchief, Franklin went on to say, “And when the rain has wet the kite and twine,...

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34. Carnot’s Simplest Heat Engine (1836)

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pp. 141-144

Steam-driven heat engines turned wheels that, in the early nineteenth century, ground corn, wove cloth, moved goods, and lifted water out of English coal mines. By the late nineteenth century, heat engines were powering dynamos that produced electricity—that highly transportable potential to perform work. Remarkably, already in 1824 with the publication of Reflections on the Motive Power of Fire and on the Machines Fitted to Develop that Power, Sadi Carnot (1796–1832) had outlined the general possibilities and absolute limitations of heat engines....

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35. Joule’s Apparatus (1847)

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pp. 145-149

What causes things to heat up and to cool down? Thanks to the widespread use of reliable thermometers in the eighteenth century, one scientist came up with an explanation. According to Antoine Lavoisier (1743–1794), heating was caused by the flow of caloric (a “subtle fluid”) that, as it penetrated the pores of an object, raised its temperature. Caloric was thought to be ingenerate and indestructible, that is, conserved, as it flowed from one object to another. Furthermore, caloric was thought to be weightless and composed...

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36. Faraday’s Lines of Force (1852)

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pp. 150-154

One of Albert Einstein’s earliest memories was of a compass his father had given him. Apparently, the earth, itself a large magnet, cast its influence across empty space and caused the compass needle to point north. Years later Einstein said that on holding the compass he “trembled and grew cold. ... There had to be something behind objects that lay deeply hidden.”
But is the space around a magnet really empty? Michael Faraday (1791–1867) was the first to gather evidence suggesting that what surrounds a magnet is as real as the magnet itself. He referred to this something as the “atmosphere” of a magnet or, alternatively, as its “lines of force.” Today we speak of its magnetic field....

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37. Maxwell’s Electromagnetic Waves (1865)

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pp. 155-158

James Clerk Maxwell (1831–1875) so greatly admired Michael Faraday that he advised readers of his own work first to carefully study Faraday’s 1,100-page Experimental Researches in Electricity (1855). Certainly he had done so and even corresponded with its author, forty years his senior. Eventually, Maxwell paid Faraday the high compliment of constructing a mathematical model of the pictorial concept of which Faraday was most proud: his electric and magnetic lines of force....

Twentieth Century and Beyond

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p. 159

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38. Photoelectric Effect (1905)

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pp. 160-163

Figure 62 depicts the photoelectric effect. Light (upper left) strikes a surface (bottom), breaks loose some of its electrons, and ejects them (upper right) from the surface. Soon after its discovery in 1887, scientists began exploring the curious properties of the photoelectric effect. Chief among them is that only sufficiently high frequency light can eject electrons—so-called photoelectrons—from the surface. How high a frequency is required depends on the composition of the surface. Most metals, for instance, require frequencies at least as high as that of ultraviolet light. If the frequency is too low, for instance, if its color is too red, no photoelectrons are produced no matter how...

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39. Brownian Motion (1905)

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pp. 164-167

Jean Perrin’s (1870–1942) experimental work ended the long debate over whether matter was continuously divisible or not, that is, whether or not atoms exist. Perrin received the 1926 Nobel Prize in Physics for deciding the question in favor of atoms. Among his crucial experiments are those that confirmed Albert Einstein’s theory of Brownian motion—a theory that makes use of atoms and molecules.
Brownian motion—that irregular, back-and-forth, wandering motion of microscopic particles immersed in a liquid—was first observed in 1827 in grains of pollen in water. After showing that neither currents in the water nor the water’s evaporation caused...

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40. Rutherford’s Gold Foil Experiment (1910)

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pp. 168-172

The concept of an atom as a tiny, indivisible building block of the material world is at least as old as the fifth century BCE. Imagine dividing a chunk of matter into smaller and smaller pieces until eventually producing an object that could no longer be divided. This is appropriately an atom, since the very word means uncuttable. Lucretius, a first-century BCE Roman poet, took comfort in the idea that human affairs were mere surface phenomena. Ultimately, all was “atoms and the void.”
Atoms also comforted philosophers because the existence of atoms solved, in part, a philosophical problem. One observes that all things seem to change. But since change is a relative concept, we are moved to ask: “Change with respect to what?” “How can we evaluate change except relative to some...

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41. X-rays and Crystals (1912)

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pp. 173-176

On November 8, 1895, while experimenting with a beam of electrons created within an evacuated glass tube, Wilhelm Conrad Röntgen (1845–1923) accidentally discovered certain “rays” that propagated beyond the end of his tube. These rays seemed to travel in straight lines, made florescent materials glow, and exposed photographic plates. Because the rays traveled through flesh but not through bone, Röntgen used them to photograph the bones in his wife’s hand. He called them X-rays....

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42. Bohr’s Hydrogen Atom (1913)

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pp. 177-181

The Danish public knew Niels Bohr (1885–1962) as a soccer player before it knew him as a physicist. Crucial to the evolution of Bohr’s persona from sports hero to Nobel laureate was the postdoctoral year (1911–1912) he spent in England studying atomic physics, first with John Joseph Thomson at the University of Cambridge and then with Ernest Rutherford at the University of Manchester. Recall that the earlier work of Max Planck (1900) and Albert Einstein (1905) had suggested that the concepts of classical physics were not sufficient for understanding the atom....

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43. General Relativity (1915)

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pp. 182-185

Each side in the Great War of 1914–1918 tried to bleed the other to death. Because newly developed machine guns and heavy artillery made traditional offensive tactics obsolete, furious battles achieved little or nothing of military value—only massive death. The Battle of Verdun, for instance, lasted nine months, created a million casualties, and left two depleted armies occupying much the same ground as before. By the time Germany sued for peace in the fall of 1918, a whole generation of “doomed youth,” in Wilfred Owen’s haunting words, had died “as cattle.”...

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44. Compton Scattering (1923)

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pp. 186-189

Einstein invented the concept of light quanta in 1905 in order to account for the photoelectric effect—that is, for the capacity of ultraviolet light to eject electrons from metallic surfaces. According to his hypothesis, the energy in light is concentrated in bundles or quanta that are, on occasion, entirely and instantaneously transferred to a single electron. The energy E(=hv) in each quantum of light is determined by the frequency v of the light wave with which it is associated where the proportionality constant h is Planck’s constant....

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45. Matter Waves (1924)

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pp. 190-193

Louis de Broglie (1892–1987) (pronounced Louie de Broy) was born into an illustrious family that had since the seventeenth century produced prominent soldiers, politicians, and diplomats for France. Louis’s father was the fifth duc de Broglie. Louis would eventually become the seventh. Educated as a child by private tutors, he completed high school and matriculated at the University of Paris, first studying history, then law, and finally physics—especially theoretical physics. Although World War I interrupted his studies, his older brother, a prominent experimental physicist, arranged for Louis to be posted, for much of the war, to the safety of a telegraph station at the foot of the Eiffel Tower. Demobilized in 1919 Louis returned to the university to finish his doctoral dissertation....

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46. The Expanding Universe (1927–1929)

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pp. 194-197

The daily appearances of the sun and the moon, the moving planets, the starry sky, and the encircling band of hazy or “milky” light called the Milky Way have, over the centuries, invited men and women to consider the universe as a whole. Of what is it composed? Does it move? What is its shape?” are questions they asked and sometimes answered. Observations made with the naked eye or with the aid of a telescope only slightly constrained their speculations. Experiments were, of course, out of the question....

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47. The Neutrino and Conservation of Energy (1930)

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pp. 198-201

When first discovered in the last few years of the nineteenth century, radioactivity troubled physicists. After all, atoms were supposed to be indestructible. How then could they eject their parts? And why did they even have parts? Furthermore, radioactivity seemed to be an inexhaustible source of energy. From whence came this energy? Was it from the atom itself or from the region surrounding the atom, or was it created ex nihilo at the moment of radioactive decay?...

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48. Discovering the Neutron (1932)

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pp. 202-205

James Chadwick (1891–1974) gets credit for “discovering the neutron,” but his actual contribution is not so simply described. He was neither the first to predict the existence of the neutron nor the first to find evidence for its existence. Nor was he the first to realize that neutrons must be elementary rather than composite particles.
Of course, sometimes the word discover is quite appropriate. Ernest Rutherford, for instance, certainly discovered the atomic nucleus. Its existence was unexpected in 1910 when Rutherford, his associate Hans Geiger, and their undergraduate student Ernest Marsden performed an experiment whose inescapable interpretation was that most of the mass of the atom and all of its positive...

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49. Nuclear Fission and Nuclear Fusion (1942)

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pp. 206-210

We are used to things happening in a certain way. Massive objects fall down—not up. Certain materials, like paper, burn easily. Others do not. We may be less familiar with nuclear fission and fusion, but the same general principle applies: A system (massive object, paper, nucleus) changes in a certain way (falls, burns, transforms) only when that system can lose energy in that change. This principle helps us understand nuclear fission and nuclear fusion. The first releases energy slowly in nuclear power plants and quickly in fission bombs. The second produces the energy beaming from our sun and released explosively in a hydrogen bomb....

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50. Global Greenhouse Effect (1988)

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pp. 211-215

The earth absorbs radiant energy from the sun, transforms that energy into longer wavelength, infrared, thermal energy, and reradiates this thermal energy skyward. By intercepting part of this reradiated, thermal energy and directing it back toward the earth’s surface, our atmosphere boosts the temperature of the earth’s surface above what it would be in its absence. Although such heating is commonly referred to as the greenhouse effect, actual greenhouses warm their contents in a different way—in particular, by inhibiting the circulation of air....

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51. Higgs Boson (2012)

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pp. 216-220

As we grow older, some of us put on weight. We slowly begin to feel more massive, or, at least, heavier. And while feeling our weight or our mass is an everyday experience, we have probably never been moved to ask: “Why does anything have mass?” “Where does mass come from?”
Einstein’s E=mc2 or, equivalently, m=E/c2 provides one kind of answer. Evidently, anything with energy E has mass m in the amount E/c2. But consider an elementary particle, for instance, an electron, isolated and at rest, that is, a particle with no parts, no apparent spatial extent, no obvious energy, and no motion. Yet an electron acts as if it has a tiny rest mass of 9﹒10-28 grams. Our question then becomes “Why do elementary particles have rest mass?” Or, if you prefer: “Why do isolated elementary particles at rest have energy?”...

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Afterword

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pp. 221-222

The principle with which I have selected concepts to explore in these essays, those that can be drawn and seen, favors older explanations such as those of lunar phases and of submerged bodies over more recent, less visualizable ones such as those of the global greenhouse effect and of the Higgs boson. Other important contributions to twentieth- and twenty-first-century physics, probability distributions, black holes, chaos, entanglement, and gravity waves did not make the cut because I could not easily represent them in a drawing. No doubt, our drive toward inventing new ways to see physics will, in time, allow these topics to be grasped visually....

Notes

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pp. 223-230

Bibliography

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pp. 231-236

Index

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pp. 237-246