GENERAL INTRODUCTION

The Mechanical Universe is a project that encompasses fifty-two half-hour television programs, two textbooks in four volumes (including this one), teachers' manuals, specially edited videotapes for high school use, and much more. It seems safe to say that nothing quite like it has been attempted in physics (or any other subject) before. A few words about how all this came to be seem to be in order.

Caltech's dedication to the teaching of physics began fifty years ago with a popular introductory textbook written by Robert Millikan, Earnest Watson, and Duanc Roller. Millikan, whose exploits are celebrated in Chapter 12 of this book, was Caltech's founder, president, first Nobel prizewinner, and all-around patron saint. Earnest Watson was dean of the faculty, and both he and Duane Roller were distinguished teachers.

Twenty years ago, the introductory physics courses at Caltech were taught by Richard Feynman, who is not only a scientist of historic proportions, but also a dramatic and highly entertaining lecturer. Feynman's words were lovingly recorded, transcribed, and published in a series of three volumes that have become genuine and indispensable classics of the science literature.

The teaching of physics at Caltcch, like the teaching of science courses everywhere, is constantly undergoing transition. Caltech's latest effort to infuse new life in freshman physics was instituted by Professor David Goodstein and eventually led to the creation of The Mechanical Universe.

THE SEARCH FOR ORDER

In the sixteenth and seventeenth centuries, that brilliant awakening known as the Renaissance was brought to an end as Europe turned its attention to a spirited debate on the finer points of Christian theology. This period, known as the Reformation and Counter-Reformation, culminated in the bloody Thirty Years' War (1618–48).

At that time, three individuals devoted to physics and astronomy laid the foundations for the enormous scientific achievements of Isaac Newton.

THE COPERNICAN REVOLUTION

We find it difficult to imagine the frame of mind of people who once firmly believed the earth to be the immovable center of the universe, with all the heavenly bodies revolving harmoniously around it. It is ironic that this view, inherited from the Middle Ages and handed down by the Greeks, particularly Greek thought frozen in the writings of Plato and Aristotle, was one designed to illustrate our insignificance amid the grand scheme of the universe – even while we resided at its center.

Aristotle's world consisted of four fundamental elements – fire, air, water, and earth – and each element was inclined to seek its own natural place. Flame leapt through air, bubbles rose in water, rain fell from the heavens, and rocks fell to earth: the world was ordered.

TOWARD AN IDEA OF ENERGY

The law of conservation of energy is a fundamental law of physics. No matter what you do, energy is always conserved. The total amount of energy in the universe is, has been, and always will be the same as it is right now. So why do people tell us to conserve energy? Evidently the phrase conserve energy has one meaning to a scientist and quite a different meaning to other peopie, for example, to the president of a utility company or to a politician. What then, exactly, is energy?

THE DISCOVERY OF THE ELECTRON

In the 1890s physicists were puzzled by cathode rays, by glowing gases, and by applegreen fluorescence that appeared in their investigations of light. To study the light emitted by various elements, researchers had developed a specially designed glass apparatus called the cathode-ray tube. To two pieces of metal, called the cathode and anode, which pass through the tube, would be connected a powerful electric voltage as illustrated in Fig. 12.1. When the gas was at atmospheric pressure, nothing happened, but when the pressure was reduced to one-hundredth of atmospheric pressure, the gas began to glow. The end of the tube near the anode glowed, or fluoresced, with an eerie green color, except where a shadow of the anode appeared. Evidently, the cathode emitted mysterious rays – cathode rays – that streamed from the cathode to the anode. Some of these rays hit the glass, creating the green light, but others were blocked by the anode, creating a shadow.

THE END OF THE CONFUSION

In 1543, Copernicus published his book, and a tremor rocked the foundations of the Aristotelian worid. A century later the Aristotelian world lay in ruins, but nothing had arisen to replace it, Galileo and Kepler had made mighty discoveries, but there was no central principle that could organize the world. The unified harmony of the Aristotelian view had been replaced by buzzing confusion.

Galileo was concerned not with the causes of motion, but instead with its description. The branch of mechanics he reared is known as kinematics; it is a mathematically descriptive account of motion without concern for the causes.

NEWTON AND THE SPEED OF SOUND

At any given moment in the development of physics, there are certain experiments or measurements that are just barely possible. These are not the most difficult experiments we can imagine, but they are the most difficult experiments that one can effect with existing equipment. These experiments become a challenge to the artistry and imagination of the most gifted experimenters. A typical example today might be the detection of gravity waves. Toward the end of the seventeenth century, one such state-of-the-art experiment was to measure the speed of sound.

Sound is some sort of disturbance that most often travels to our ears through air. Sound also travels through liquids and solids, but in any case a medium, that is, a substance, is needed in order for the sound waves to travel. In a vacuum, there is no sound.

THE QUEST FOR PRECISION

Not long after Copernicus published his revolutionary book, Tycho Brahe (1546–1601) provided a multitude of new observations that, despite his own intentions, provided crucial support for the Copernican hypothesis.

CELESTIAL OMENS: COMETS

Ancient astronomers earned their keep and kept their heads by making accurate predictions of the arrival of the seasons and of such troubling ceiestiai events as solar and lunar eclipses. As their technical expertise improved, astronomers learned to predict even the wandering motion of the planets. Yet at times, interlopers, such as meteors, appeared in the night sky. Although meteors seemed as unpredictable as the weather (and were thought to be related to it, which is why their name shares the same Greek root with the science of weather – meteorology), even meteor showers were observed to occur regularly. For example, the most spectacular meteor showers occur every year in mid-August.

Nevertheless, objects occasionally and mysteriously appeared in the heavens which were not planets or meteors. Trailing plumes of cold fire, these objects were named comets, from the Greek word meaning thing with hair. Because their appearance was unpredictable, comets were interpreted as omens of impending disaster.

ARISTOTLE'S DESCRIPTION OF MOTION

Before we began to learn to read Galileo's mathematical book of the universe, our descriptions of nature were qualitative and verbal. For centuries, only words were used to describe the motion of objects, words based on the writings of the Greek philosopher Aristotle from the fourth century B.C. Aristotle's descriptions of motion centered on the idea of a “natural place.” Each of the four elements – earth, air, water, fire – that composed all matter on Earth had a natural place. The lowest natural place was assigned to earth, at the center of the cosmos. The natural place for water was just above that of earth. Since air was lighter than earth or water, it was assigned a natural place above water. Fire was given the highest place. When any element or object made of that element was in its natural place, it remained at rest. According to Aristotle, when an object was removed from its natural place, it possessed a potentia, or tendency, to return to its place. If uninhibited, any object would be drawn back to its natural place.

THE PERFECTION OF CIRCULAR MOTION

In the fourth century B.C., Greek philosophers turned to the sky and asked, How can we explain the cycles of change – the motions of the stars, sun, and planets? One such philosopher was Plato (427–347 B.C.), who believed that the senses are not to be trusted, that rather the universe can be understood only by pure thought.

IF THE EARTH MOVES: ARISTOTELIAN OBJECTIONS

In 1543 the revoiutionary book appropriately entitled De Revolutionibus Orbium Coelesiium (On the Revolutions of the Celestial Spheres), was published by Nicolaus Copernicus.

TOWARD AN UNDERSTANDING OF ENTROPY

In this chapter we turn our attention to the entropy principle, a concept which, like Newton's second law, is an organizing principle for understanding the world. The principle is relatively simple to state, but understanding its meaning is more challenging.

Through theoretical studies of Carnot's work in 1865, the German physicist Rudolph, Clausius introduced a new physical quantity closely linked to energy. He called it entropy a word which sounds like energy and comes from the Greek word for transformation. The use of entropy provides a way to analyze the behavior of energy in transformation.

To obtain an intuitive feeling for the concept of entropy, let's start with a familiar mechanical system we've used many times: Galileo's experiment with a ball rolling down and up two inclined planes.

TEMPERATURE AND PRESSURE

Everybody talks about the weather, and that usually means the temperature, an inescapable part of our environment. Yet Newton's laws of mechanics tell us nothing about temperature. Is there any connection between mechanics and temperature?

In Chapter 13 we saw a connection. If you drop a block from above a table, its potential energy first turns into kinetic energy, and then is transformed into thermal energy when the block hits the table. After a while the only evidence that those events occurred is a slight warming of the surroundings, that is, a small increase in temperature.

What really happens is that the kinetic energy of the falling block is turned into the energy of motion of atoms and molecules. The energy is still there, but the motions are in random directions, not the organized motion of a whole block of matter.