Thematic Essay: Physics, from Leonardo to Hertz

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Introduction

Thematic Essay: Physics, from Leonardo to Hertz

Thematic Essays combine a broad survey of a particular topic with key supplementary readings to create a comprehensive learning experience. In this essay, Nobel Prize-winning physicist Leon Lederman traces the development of classical physics. Accompanying the essay are Sidebars consisting of excerpts from the works of some of the world’s most influential scientific thinkers.

By Leon M. Lederman

The great English physicist Sir Isaac Newton once remarked, “If I have seen further it is by standing on the shoulders of giants.” Indeed, much of what physicists know today can be traced to the giants who lived during a period of scientific discovery often referred to as “classical physics”—a period that runs roughly from the time of Leonardo da Vinci in the 15th century to the time of Heinrich Hertz in the late 19th century. During this nearly 400-year period, many of the key concepts in physics, such as the notion of inertia and the ideas of gravity, electricity, magnetism, and light, were formulated and explained in a few simple equations. Looking back on this period in history and reviewing its accomplishments enable us to understand not only the physics of today but also the modern world we live in.

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The period before the beginning of the Renaissance in the 15th century is often referred to as Western Europe's Dark Ages. The principal intellectual accomplishment during the Dark Ages was the preservation of the writings of the ancient Greeks, many of whom made important contributions to the study of science. This preservation was aided by the Egyptians in Alexandria and by the Arabs, who had translated some of the Greek scientists and philosophers, particularly Aristotle and Plato. With the blessing of the Roman Catholic Church, friars translated the Arabic versions of Greek science into Latin, the language of all educated people in Western Europe.

Largely through the work of the friars, the church accepted the teachings of Aristotle, the greatest of ancient Greek scholars. Unfortunately, Aristotle’s writings on astronomy and physics were misguided, and the church blindly adopted these errors. This uncritical approach made things difficult for Italian astronomer Galileo Galilei and others who created modern physics. By opposing Aristotle, they were opposing the Catholic Church, which was then all-powerful in Europe.

The spirit of intellectual curiosity during the Renaissance era, however, prompted a renewed study of nature without preconceived ideas. One of the most important figures in the Renaissance was Italian artist and scientist Leonardo da Vinci.

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Leonardo da Vinci

Leonardo is regarded as the consummate Renaissance man not primarily for his scientific accomplishments but because of his extraordinary range of achievements. Leonardo was a painter, sculptor, engineer, architect, physicist, biologist, and philosopher—and he was outstanding in each of these fields. Science influenced all of Leonardo’s activities; he believed in observing nature and carrying out experiments. As a painter, Leonardo studied the laws of optics and the optical structure of the eye; he studied human anatomy with the thoroughness of a medical surgeon to understand how to paint the torso. Leonardo’s ability to solve physics problems in the fields of static (stationary) and dynamic (moving) mechanics fortified his civil and military engineering work.

Leonardo excelled at physics. He anticipated the principle of inertia, which would be demonstrated by Galileo nearly a century later. The inertia principle is the idea that an isolated body, not in contact with anything, will continue moving at a constant velocity forever. Understanding this concept required deep insight since it is not at all easy to imagine an abstract idea such as an “isolated body.” Leonardo knew that the speed of a falling body increases over time. He knew that perpetual motion was impossible as a source of power, and he invented a mathematical scheme for proving the law of the lever. Leonardo worked on the flow of water through pipes, designing an irrigation system and channels. He studied wave motion on water and extended this study to waves in the air and the laws of sound.

Leonardo dismissed with contempt all the work of alchemists, astrologers, and magicians. To him, nature was orderly and subject to logical laws. In most of his scientific work, Leonardo was 100 years ahead of his time.

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Copernicus

The first major scientific breakthrough in the mid-1500s was called the Copernican revolution, and it revolutionized science. Nicolaus Copernicus, a Polish mathematician and astronomer, was dissatisfied with the accepted picture of our solar system as developed by the ancient Egyptian astronomer Ptolemy, who did his work around 150 ad. Ptolemy had carefully studied the motion of the planets, but he did so under the assumption that Earth is at rest while the Sun and all the planets somehow move around it. Other early astronomers had noticed that the planets sometimes moved across the sky ahead of the stars, but that they also sometimes reversed themselves. Ptolemy explained this motion as the result of a set of small circles, called epicycles, on which the planets moved. He hypothesized that the epicycles moved on larger circles called deferents, which were centered on Earth, and that the combination of these motions caused the planets’ forward and reverse movements. Ptolemy did not have instruments to make precise observations of planetary motions, and his data were skewed by his failure to realize that Earth also moves.

Copernicus made the bold, courageous assumptions that Earth is just another planet and that it revolves around the Sun. Why bold? Because it seemed absurd to assume that our solid, stable Earth could actually be speeding through space. Why courageous? Because religious authorities had adopted the satisfying idea that the entire universe was centered on Earth, and to refute that idea was to go against the church.

Copernicus’ description of his theory of the universe, written near the time of his death in 1540, is typical of the way modern physicists think and how they strive for simplicity in the description of nature. Copernicus describes his theory of the universe thusly:

As the modern physicist-historian Thomas Kuhn points out, Copernicus’s finding was “an ‘epochal’ turning point in the intellectual development of Western man.” Unlike more modern scientific revolutions, Copernicus’s system of a central Sun orbited by the seven known planets did not immediately affect science, but its social, cultural, moral, and political influence was rapid and profound. Copernicus’s paper was published in 1543, but its influence on physics had to wait for German astronomer Johannes Kepler, Italian astronomer Galileo Galilei, and English physicist and mathematician Sir Isaac Newton. The conceptual challenge centered on the need to understand the new idea that Earth actually moved.

By centering motions around the Sun, the Copernican system made the orbits of planets simpler, whereas the Earth-centric Ptolemaic system created the need for complex epicycles. Still, the overall accuracy of the two systems turned out to be about the same given the relatively low accuracy of astronomical measurements at the time. Consequently, observational precision played a key role in the advance of physics and astronomy. The champion of accuracy in the late 16th century was the Danish astronomer Tycho Brahe, who had valuable help from Kepler.

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Tycho Brahe and Johannes Kepler

Outfitted by the king of Denmark on an island totally dedicated to precise astronomical measurements, Brahe outfitted his laboratory with ingeniously fabricated, beautifully constructed astronomical instruments designed on a large scale. He supplemented the new equipment with a new ethic of continuous and repeated measurement, and he eventually obtained an unprecedented level of precision.

Kepler was Brahe’s assistant. Whereas Brahe was fascinated by the art of making careful measurements of the motion of the planets, Kepler was more interested in trying to arrive at a mathematical picture of the solar system. In the beginning, he followed Copernicus and assumed that the planets performed circular orbits around the Sun, but he soon realized that Brahe’s measurements indicated that the geometric shape of the planets’ orbits could not be a circle.

Kepler converted the calculation of the orbits, described in Brahe’s notebooks as a long string of carefully transcribed numbers, into a figure studied in all geometry classes: the ellipse. Kepler also wrote down several mathematical relations that all planet motions would obey. Kepler’s three laws of planetary motion became famous by the early 1600s and proved to be crucial to the great work of Newton. Kepler’s laws of planetary motion were the direct predecessors of Newton’s theory of gravitation. However, between Kepler’s Sun-centered ellipses and Newton’s revolution, the physics of Galileo and French philosopher René Descartes played crucial roles.