The Structure of the Universe

by Paul Halpern, Bruce Gregory (Foreword by)

Overview

The Structure of the Universe by Paul Halpern, Ph.D., originally published in 1996, is a tour of the knowledge of the deep reaches of space and predictions for its future.

Technological marvels such as the Hubble Space Telescope are revealing a wealth of information about the deepest reaches of space. After decades of research, scientists now believe they are closer to discovering the 'missing matter,' the invisible stuff left over from the Big Bang that will determine the ultimate fate of the universe. With each discovery new light is shed on scores of old questions, and at the same time new questions arise.

Product Details

ISBN-13:	9781466881686
Publisher:	Holt, Henry & Company, Inc.
Publication date:	09/16/2014
Sold by:	Barnes & Noble
Format:	eBook
Pages:	131
File size:	6 MB

About the Author

Paul Halpern, Ph.D., is a theoretical physicist and an associate professor at the Philadelphia College of Pharmacy and Science. He is the author of numerous books and articles on astronomy and science. He lives in Philadelphia, Pennsylvania.

Bruce Gregory is an associate director of the Harvard-Smithsonian Center for Astrophysics and a specialist in issue of science education. He is the author of Inventing Reality: Physics as Language.

Read an Excerpt

The Structure of the Universe

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By Paul Halpern

Henry Holt and Company

Copyright © 1997 Paul Halpern
All rights reserved.
ISBN: 978-1-4668-8168-6

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CHAPTER 1

The Parade of Planets

We do not know who among the ancients was the first to discover the planets. (The word planet itself comes from the ancient Greek planetes and means "wanderer.") We can only assume that at some point in history, observers distinguished the behavior of the planets from that of the "fixed stars." It was noted that while ordinary stars seem to move in unison from night to night, the planets appear to track out independent paths. One might say, then, that the unknown people who first noticed this fact were the true discoverers of the planets.

This distinction can be seen by noting that while star patterns, such as the Big Dipper, are essentially stable (in the course of hundreds of years at least) with respect to the stellar background, planets, such as Mars and Jupiter, tend to shift around in the sky. If you stand at a given site in the Northern Hemisphere and look up at the sky, the Big Dipper always appears to be surrounded by the same set of stars. On the other hand, Mars and Jupiter continuously move around the heavens, both with respect to the stars and with respect to each other. These planets might appear close together on one date and far apart months later.

Ancient civilizations believed that the relative positions of the planets influenced events on Earth. Disasters, such as wars, fires, floods, and famines, could be predicted, they thought, by carefully analyzing and interpreting planetary motions. Therefore, early astrologers kept careful records of these movements and periodically issued dire warnings about imminent catastrophes.

The arrival of a conjunction, when two or more planets appear close to each other, was seen as an especially powerful omen. The more planets that were lined up for a particular conjunction, the rarer it was, and the more significance it was assigned by the ancients.

The most detailed early scientific portrait of planetary behavior was developed by the ancient Greeks during the sixth through fourth century B.C. At that time, classical Greek philosophers such as Pythagoras, Plato, and Aristotle imagined that the planets (along with the Sun, Moon, and stars) circle the Earth periodically. Pythagoras and Plato attributed this rhythmic behavior to the mathematical regularity of nature, the same sort of harmony represented by the tonal scales of music and the patterns of planar geometry. Aristotle viewed the movements of the celestial bodies as part of a clockwork physical order, set into motion by a Creator. While the cosmologies of Pythagoras and Plato included metaphysical components, and the universe of Aristotle basically did not, each shared a vision of a concentric series of perfect circular orbits, with the Earth as center.

These early geocentric (Earth-centered) models of planetary motion were far from accurate. They share the assumption that each planet completes a uniform, circular orbit around the Earth. However, in reality, planets do not always appear to trace out one-way paths in the heavens. Rather, they often seem to reverse their directions of motion — moving forward, then backward, then forward again. These reversals take place periodically, and cannot be accounted for by simple circular orbit theories.

To explain this phenomenon, called retrograde motion, the Greek scholar Apollonius of Perga developed, in the third century B.C., a more intricate model of the planets. According to this theory, planets execute smaller circles in space, called epicycles, while following large circular paths around the Earth. The combination of these circuits accounts for periodic reversals of direction.

Ancient Greek notions of astronomy remained prevalent throughout Europe and North Africa for centuries, up until the end of the Middle Ages. These ideas were summarized in the book Almagest (meaning "The Great System" in Arabic), written in the second century A.D. by Ptolemy of Alexandria.

Ptolemy's work, a synthesis of classical knowledge along with some of his own ideas, was seen by medieval scholars as the authoritative account of celestial behavior. It advocated a geocentric model of the planets, including the epicycles of Apollonius, as well as additional geometric elements, called eccentrics (deviations of orbital centers from Earth) and equants (off-center points about which the epicycles move). These complex schemes were included to reproduce observed planetary behavior. In contrast to the intricate orbits of the planets, however, the Sun, Moon, and stars were seen as revolving around the Earth along simple circular paths.

Because of the influence of the Ptolemaic approach, for hundreds of years the Earth was considered to be the physical center of the cosmos, surrounded by a complex array of orbiting bodies. This was the state of affairs until the Renaissance, when a supreme revolution in cosmological thought took place. In this monumental change of perspective, the Earth was dethroned and replaced on its pedestal by the Sun. From that time on, no longer was our planet considered the crux of the universe.

The first important advocate of a heliocentric (Sun-centered) cosmology was the sixteenth-century Polish astronomer Nicholas Copernicus. Copernicus was puzzled by the conflict between the elaborate Ptolemaic system and the Platonic ideal of simple circles. To resolve this contradiction, he revived the ideas of another, less well known, Greek philosopher, Aristarchus, who had written in the third century B.C. that the Earth revolves around the Sun.

After years of careful calculations and ruminations about the implications of a heliocentric universe, Copernicus released in 1543 his most important work, Revolutions of the Celestial Spheres. In this treatise, printed shortly before his death, he advanced the view that Earth and the five then-known planets follow simple circular paths around the Sun. To explain the appearance of the stars as "fixed," he stated that they occupy a spherical shell, centered on the Sun, well beyond the domain of the planets. Finally, he wrote that the Moon alone orbits the Earth.

The Copernican system was considered blasphemous by the Church, which had cast its long shadow over science for quite some time. Because of Christian teachings that the Earth is the sole realm of the physical, and the heavens, that of the spiritual, the Church had firmly aligned itself with the geocentric viewpoint, particularly with the cosmology of Aristotle. By placing the Earth on a par with the other planets, Copernicus implied that these other bodies were physical as well. The Earth ceased to occupy a special place in cosmology. Naturally, this was too much for the Church to accept, and the writings of Copernicus were condemned.

Challenging the doctrine of the Church, Italian philosopher Giordano Bruno published in 1584 the book Of Infinity, the Universe and the World, advocating a Copernican view of the cosmos. Bruno took Copernicus one step further, arguing that not only is there a planetary system around the Sun but that there is one around each of the stars. Moreover, he wrote that the number of stars and planets in the universe is infinite. He did not provide tangible evidence for his hypothesis, but rather employed spiritual arguments to make his case. The Church was even more hostile to Bruno's ideas than it was to those of Copernicus. In 1600, for his heretical beliefs, Bruno was burned at the stake in Rome.

The ideas of Copernicus and Bruno were speculative rather than empirical. Neither philosopher proved definitively that the Earth and planets orbit the Sun; observational evidence was needed to establish their case. Moreover, to make detailed predictions of orbital behavior, a mathematical description of the movements of planets was required. Finally, to link the rules governing planetary motions with those concerning terrestrial interactions, a new set of physical laws needed to be stated. These requirements were adroitly satisfied in the seventeenth century by Galileo, Kepler, and Newton, respectively.

Galileo Galilei was born in Pisa in 1564. In his youth, his scientific productivity was remarkable. While a student at the University of Pisa, he made a number of important discoveries, including the fact that a pendulum of a given length swings at a constant rate, regardless of how far it is initially displaced from its balance point. (According to legend, it was also at Pisa where he performed a simple gravity experiment — dropping two objects, each of different mass, off the Leaning Tower to show that they fall at the same rate.)

In 1592, Galileo left the University of Pisa and was appointed Chair of Mathematics at the University of Padua. The next eighteen years represented the most productive period of his life. His work in dynamics, the science of moving bodies, brought him great renown. But it was his astronomical findings that would make the greatest impact upon science.

With a primitive telescope of his own construction, Galileo mapped out the appearance and behavior of celestial bodies to an unprecedented extent. He discovered mountains on the Moon and satellites orbiting Jupiter. He resolved hundreds of stars, recorded the phases of Venus, and plotted out the motions of sunspots. He published his work in 1610 in a book called The Starry Messenger.

Galileo's extensive research led him to realize that the observed planets and the Moon shared with Earth a number of similar features. He was thereby persuaded to place these celestial bodies on equal footing with each other (more or less) — considering each to be a physical world in its own right. He concluded that the Copernican heliocentric model, rather than the Ptolemaic or Aristotlean approach, would best describe this "democratic" state of affairs. He announced his support for the views of Copernicus in the book Dialogue on the Two Great World Systems, published in 1632. When he died in 1642, the power of his written arguments was already beginning to persuade the bulk of the European astronomical community that the Earth revolves around the Sun.

One of Galileo's trusted correspondents was the German scientist Johannes Kepler. Kepler shared with Galileo a strong belief in Copernican cosmology. Unlike Galileo, however, Kepler espoused his beliefs openly and never wavered in his expressed convictions.

Kepler was born in 1571 in the town of Weil der Stadt, then part of the Holy Roman Empire. Trained in theology and mathematics, in 1601 he was fortunate enough to succeed Tycho Brahe as imperial mathematician in Prague. When Kepler assumed this position, he gained access to several decades worth of naked-eye astronomical measurements of the planets taken by Brahe.

Over the years, Kepler compiled and analyzed this wealth of data, seeking to prove the Copernican description of planetary motion. In particular, he tried to show that the orbit of Mars was a circle around the Sun. But as much as he tried, he couldn't find a circular path that would match up with Brahe's values. He discovered, however, that the orbit of Mars was well matched by a geometric figure called an ellipse. An ellipse is an oval-shaped object with precise mathematical characteristics. Specifically, it is defined in terms of two internal points, called foci, such that the sum of the distance from any given point of the ellipse to one focus plus the distance from that point to the other focus is a constant. After further study, Kepler found that the orbits of each of the then-known planets could be described as ellipses in which the Sun was one of the foci. Thus, he showed that, rather than resembling a series of concentric circles, the orbits of the bodies in the solar system were all ellipses, and all shared one of their two foci, the Sun.

Although Kepler provided science with a mathematically perfect description of how planets move, he failed to explain why they move that way. This latter task was carried out by a man considered by many to be one of the greatest scientists of all time, Isaac Newton.

Newton was born in Woolsthorpe, England, in the year of Galileo's death. During his long scientific career, mainly as a professor of mathematics at Cambridge University, he made substantial, and often pivotal, contributions to the fields of mathematics, physics, and astronomy.

Perhaps Newton is best known for his theory of universal gravitation. Legend has it that his interest in the phenomenon of gravity began when he noticed an apple falling from a tree. Supposedly, he then began to wonder about the nature of the force attracting bodies to one another. Soon he realized that the same pull that draws an apple down to Earth also draws the Moon toward Earth, and Earth toward the Sun.

Newton encapsulated his gravitational theories, as well as other principles of dynamics, in his treatise Principia. In this book, he proved that the elliptical orbits of the planets around the Sun could be predicted by a simple mathematical equation. This law states that the gravitational force between bodies depends inversely on the square of the distance between them. In other words, the closer two objects are, the greater the gravitational attraction, by a factor of the square of the amount of separation. By combining this law of universal gravitation with his laws of motion (also stated in Principia), and applying these principles to the case of the interaction of any given planet with the Sun, Newton showed that the planet would travel in a simple ellipse with the Sun as one focus.

Newton's laws forever changed the science of cosmology. Before Newton, the study of the universe was considered a metaphysical venture. Unable to make predictions based on firm mathematical principles, early scientists needed to use faith and intuition to explain the motions of the planets and stars. Newtonian cosmology, on the other hand, requires no recourse to theology. It embodies a clockwork vision of the universe in which each component is related to every other component through precise equations.

Since the time of Newton, astronomers have discovered three more planets in the solar system: Uranus, Neptune, and Pluto. Like the inner six, these outer planets engage in roughly elliptical orbits. However, scientists now know that the paths traced by planets are not exact ellipses. Following the law of gravity, the planets are not just attracted by the Sun; they are also drawn by each other. Therefore, each planet's trajectory is affected by the gravitational influences of all the others. These mutual attractions are strongest when planets are closest together, and show up as "wobbles" in planetary orbits.

The presence of these wobbles has been used in modern times to predict the existence of unseen planets. After Uranus was discovered by William Herschel in 1781, two scientists — Urbain J.J. Leverrier in France and John Couch Adams in England — independently noticed peculiarities in its orbit. These irregularities turned out to be caused by the planet Neptune — first sighted in 1846 by Johann Gottfried Galle. Each time Neptune is close to Uranus it exerts a stronger pull on that planet. When Neptune moves farther away, its attraction to Uranus is weaker. Thus, because of the influence of Neptune, the path that Uranus takes around the Sun is slightly perturbed.

Pluto, discovered in 1930 by Clyde William Tombaugh, was similarly anticipated because of seeming irregularities in the orbit of Neptune. Though these detected variations turned out to be nothing but observational errors, their apparent existence inspired astronomers to hunt for a ninth planet. With this motivation, Tombaugh searched the sky meticulously and finally found Pluto (which has properties much different than the planet that was predicted from the erroneous data).

There is far more to the Solar System than just the nine planets. Scores of moons (over 60 and still counting) circle these bodies, ranging in diameter from less than ten miles to thousands of miles across. The bulk of these satellites orbit the giant planets: Jupiter, Saturn, Uranus, and Neptune. Earth and Pluto have one moon each, Mars has two, and the rest, none.

The four largest planets also harbor structures known as rings: intricate bands encircling them, composed of billions of chunks of rock and ice. Saturn's ring system has been known since the time of Galileo. The other giant planets' fainter rings were first detected in the late twentieth century.

Between Mars and Jupiter lie the remnants of a planet that never was. Thousands of small stony objects, called asteroids, orbit the Sun within this wide belt. These bodies are the remains of a bath of planetesimals (the precursors of planets, composed of rock and ice) that once filled the inner Solar System within what is now the orbit of Pluto. Except in the asteroid belt region, most of these planetesimals collided with each other numerous times and eventually coalesced into planets. Therefore, except for a few strays, most of the inner Solar System is free of these bodies. However, close to Jupiter mighty gravitational forces prevented the occurrence of large-scale amalgamation. Instead of merging, the primitive rocks remained as asteroids. The asteroid belt remains today as a fossil of what the entire Solar System once looked like before there were planets.

(Continues...)

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Excerpted from The Structure of the Universe by Paul Halpern. Copyright © 1997 Paul Halpern. Excerpted by permission of Henry Holt and Company.
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Table of Contents

Contents

Title Page,
Copyright Notice,
Dedication,
Acknowledgments,
Foreword Bruce Gregory Harvard–Smithsonian Center for Astrophysics,
Introduction Mysteries of Space,
Chapter One The Parade of Planets,
Chapter Two The Spectacle of Stars,
Chapter Three Puzzling Pulsars,
Chapter Four The Lair of Black Holes,
Chapter Five Galactic Rhythms,
Chapter Six Time's Trigger,
Chapter Seven The Age Question,
Chapter Eight The Transgalactic Bubble Bath,
Chapter Nine Ghosts in the Void,
Chapter Ten Light Show,
Conclusion The Fate of the Universe,
Timeline of the Universe,
Further Reading,
Index,
Photo Credits,
Other Scientific American Focus Books,
Copyright,