Galileo

I

INTRODUCTION

Galileo (1564-1642), Italian physicist and astronomer who, with German astronomer Johannes Kepler, initiated the scientific revolution that flowered in the work of English physicist Sir Isaac Newton. Galileo’s main contributions were, in astronomy, the use of the telescope in observation and the discovery of sunspots, mountains and valleys on the Moon, the four largest satellites of Jupiter, and the phases of Venus. In physics, he discovered the laws of falling bodies and the motions of projectiles. In the history of culture, Galileo stands as a symbol of the battle against authority for freedom of inquiry.

II

EARLY YEARS

Galileo, whose full name was Galileo Galilei, was born near Pisa, Italy, on February 15, 1564. His father, Vincenzo Galilei, played an important role in the musical revolution from medieval polyphony to harmonic modulation. Just as Vincenzo saw that rigid theory stifled new forms in music, so his eldest son came to see both the then-dominant physics of Greek philosopher Aristotle and the Roman Catholic theology influenced by it as limiting scientific inquiry. Galileo was taught by monks at Vallombrosa and then entered the University of Pisa in 1581 to study medicine. He soon turned to philosophy and mathematics, and although he left the university in 1585 without a degree, he did receive a useful introduction to the versions of Aristotelian physics current at the time.

III

ARISTOTELIAN PHYSICS OF GALILEO’S TIME

Aristotelians made a sharp division between Earth and the heavens. In the heavens there could be no change except the recurring patterns produced by the circular motions of the perfectly spherical heavenly bodies. The sublunar world (the universe below the Moon) was the region of the four elements—earth, water, air, and fire—and was subject to its own distinct laws of natural motion. Fire, for instance, had lightness, which made it rise vertically, away from the center of Earth. Earthy objects fell naturally downward toward the center of Earth: the heavier the object, the faster its fall. “Natural” motions of the elements took them to their natural place, where they rested. Rest was the natural state of an element; it was motion that needed explaining, since every motion must have a cause. This common-sense physics held sway until Galileo began to undermine it.

IV

GALILEO’S WORK IN PHYSICS

The key to Galileo’s new physics lay in mathematics. While registered as a medical student at the University of Pisa, he increasingly devoted his time to the study of mathematics, with the encouragement of court mathematician Ostilio Ricci. After leaving the university he tutored privately for a time and wrote on hydrostatics, but he did not publish anything. In 1589 he became professor of mathematics at Pisa.

A

Falling Bodies

Before assembled university professors, Galileo reportedly refuted Aristotle’s belief that speed of fall is proportional to weight by simultaneously dropping two objects of the same material but different weights from the Leaning Tower of Pisa. This celebrated story of Galileo’s demonstration that Aristotle was fundamentally mistaken about motion comes from his last pupil and first biographer, Vincenzo Viviani. Though Viviani’s account is sometimes dismissed as legend, it is more probably an exaggerated version of an actual event. Galileo’s manuscript shows that he was still unclear about acceleration in free fall and that he thought more in terms of the characteristic speed of a body of a given material in a given medium.

Yet Galileo could already improve on Aristotle. Galileo considered himself a follower of the ancient Greek scientist Archimedes and abandoned Aristotelian notions of heaviness and lightness in favor of the more useful notion of density. Galileo made his first attempts at producing simple mathematical comparisons of how bodies of varying densities fall in various substances, and he was willing to ignore minor discrepancies, leaving them to be explained by further investigation. He even toyed with the idea of a body resting on a perfectly smooth surface being movable by the slightest of forces—a hint of his later work on inertial motion and a measure of how he was distancing himself from Aristotelian ideas of natural and forced motions.

Galileo’s contract at the University of Pisa was not renewed in 1592, probably because he contradicted Aristotelian professors. The same year, he was appointed to the chair of mathematics at the University of Padua, where he remained until 1610. Galileo’s mathematical work depended on his ability to discern simple mathematical patterns underlying familiar occurrences, such as the free fall of objects to the ground. He combined this with a knack for devising controlled observations in which the looked-for mathematical relationships presented themselves as obvious and as measurable with precision. His fundamental conviction was that the universe is an open book but, as he wrote later in The Assayer (1623), “One cannot understand it unless one first learns to understand the language and recognize the characters in which it is written. It is written in mathematical language.…” 

Galileo’s conviction led to important discoveries in the first decade of the 17th century. He not only recognized that the acceleration of any body in free fall was uniform but he expressed this in a simple law: The distance traveled in free fall is proportional to the square of the time elapsed; that is, in two seconds a body will fall four times as far as it will in one second; in three seconds it will fall nine times as far; and so on.

B

Projectiles and Pendulums

Galileo’s law of falling bodies led to an understanding of the motion of projectiles. Galileo could look at the fall of an arrow or cannonball and see it as made up of two independent motions: The vertical component was uniformly accelerated and conformed to his law of falling bodies; the horizontal motion imparted to the body by the bowman or gunner was at constant speed. When the horizontal and vertical components were combined, the resultant path was a parabola. This seemingly abstract geometrical account had practical consequences for efficient gunnery.

In a similar vein Galileo investigated mechanics and the strength of materials. In his studies of pendulums he discovered that the swing of a given pendulum takes the same time no matter how large its arc. Others soon pointed out that this was true only if the swing did not become too large.

C

Inertia

One of the greatest contrasts between Galileo’s ideas and Aristotle’s ideas is in their underlying models of motion. Galileo believed that an object moving uniformly on Earth’s surface without meeting any resistance would continue to move at the same speed without needing any force to keep it going. Aristotelians, on the other hand, would look for a force to cause the continuing motion. Galileo’s idea approximates Isaac Newton’s first law of motion, according to which a body will continue in its state of rest or uniform motion in a straight line unless interfered with. Although Galileo failed to define uniform motion as a straight line, he made the advance of not treating rest as a state more natural than motion. 

V

WORK IN ASTRONOMY

During most of his time in Padua, Galileo showed little interest in astronomy, although in 1595 he declared in a letter that he preferred the Copernican theory that Earth revolves around the Sun to the assumptions of Aristotle and Ptolemy that planets circle a fixed Earth (see Astronomy: The Copernican Theory; Ptolemaic System).

A

Observations with the Telescope

In 1609 Galileo heard that a telescope had been invented in Holland. In August of that year he constructed a telescope that magnified about ten times and presented it to the doge of Venice. Its value for naval and maritime operations resulted in the doubling of his salary and his assurance of lifelong tenure as a professor.

By December 1609 Galileo had built a telescope of 20 times magnification, with which he discovered mountains and craters on the Moon. He also saw that the Milky Way was composed of stars, and he discovered four satellites circling Jupiter. It was therefore undeniable that at least some heavenly bodies move around a center other than Earth, a finding that did not prove that Copernicus had been right but did fit in well with the Copernican system of the universe. Galileo published these findings in March 1610 in a book called The Starry Messenger.

Galileo astutely used his new fame to secure an appointment for which he had been angling for some time, that of court mathematician at Florence, Italy. He was thereby freed from teaching duties and had time for research and writing. By December 1610 he had observed the phases of Venus and found that variations in the planet’s brightness were much greater than previously thought. These could be explained as a natural consequence of the Copernican system but not by the Ptolemaic system. Galileo naturally took the discovery of Jupiter’s moons and his observations of Venus as confirmation of the Copernican system.

Traditionalist professors of philosophy scorned Galileo’s discoveries because Aristotle had held that only perfectly spherical bodies could exist in the heavens and that their movement was eternal and circular. This view could not be maintained if Venus, for example, was sometimes nearer Earth and sometimes farther away. Nor could Aristotelian theory explain why Venus sometimes appears crescent-shaped, like the Moon. Galileo also disputed with professors at Florence and Pisa over hydrostatics, and he published a book on floating bodies in 1612. Four printed attacks on this book followed, rejecting Galileo’s physics. Aristotelians took shape to be the key to explaining why bodies float, whereas Galileo relied on the relative densities of the floating object and the medium in which it floated. In 1613 Galileo published a work on sunspots and predicted victory for the Copernican theory.

B

Theory of Tides

Only the Copernican model supported Galileo’s ingenious but mistaken theory of tides. According to Galileo’s theory, the motion of Earth’s rotation is alternately added to Earth’s orbital motion and subtracted from it, with the effect that the seas are set sloshing backward and forward. To this simple mechanism, which provided one tide every 24 hours, Galileo had to add further factors, such as the orientation and configuration of seabeds and shores, to make a reasonable approximation of the variety of tidal phenomena actually observed at different places and seasons.

VI

GALILEO AND THE INQUISITION

A Pisan professor, in Galileo’s absence, told the Medici—the ruling family of Florence as well as Galileo’s employers—that belief in a moving Earth was contrary to the Bible. Galileo immediately wrote a pamphlet for private circulation, Letter to Castelli, sketching his views on the relation of scripture and science. In December 1614 a Dominican friar denounced “Galileists” from a Florentine pulpit, and early in 1615 the Florentine Dominican convent of San Marco sent criticisms of Galileists to the Inquisition in Rome.

Galileo enlarged his Letter to Castelli into a Letter to the Grand Duchess Cristina on the correct use of biblical passages in scientific arguments, holding that the interpretation of the Bible should be adapted to increasing knowledge and warning against the danger of treating any scientific opinion as an article of Roman Catholic faith. This remarkable work of amateur theology was not published in Italy in his lifetime and had little influence on the course of events.  

A

The Copernican System and the Church

Early in 1616 Copernican books were subjected to censorship by the church’s Index of Forbidden Books, and Jesuit cardinal Robert Bellarmine instructed Galileo that he must no longer hold or defend the opinion that Earth moves. Following a long tradition that hypotheses in astronomy were merely instruments or calculating devices, Cardinal Bellarmine had previously advised him to treat this subject only hypothetically and for scientific purposes, without taking Copernican concepts as literally true or attempting to reconcile them with the Bible. The ruling of 1616 similarly laid down that Catholics could use Copernicanism as a calculating device but could not say that it was the true system of the universe.

Galileo remained silent on the subject for years, working on a method of determining longitudes (see Latitude and Longitude) at sea by using his predictions of the positions of Jupiter’s satellites, resuming his earlier studies of falling bodies, and setting forth his views on scientific reasoning in a book on comets, The Assayer, which is a classic of polemical writing.

In 1624 Galileo began a book he wished to call Dialogue on the Tides, in which he discussed the Ptolemaic and Copernican hypotheses in relation to the physics of tides. In 1630 the book was licensed for printing by Roman Catholic censors at Rome, but they altered the title to Dialogue on the Two Chief World Systems. It was published at Florence in 1632. Despite the book’s having two official licenses, Galileo was summoned to Rome by the Inquisition to stand trial for “grave suspicion of heresy.”

B

Galileo’s Trial

Although Galileo had made considerable efforts to conform to the letter of the ruling of 1616, he had clearly written a pro-Copernican book. He had occasionally also slipped up by explicitly treating the Copernican system as “probable,” meaning that, although it was yet unproven, sooner or later it could well be shown to be true. Such a position was incompatible with the ruling of 1616, as was pointed out at his trial: Catholics were allowed to use Copernicanism as a helpful calculating device, provided that they did not treat it as having any truth. 

The charge against Galileo was grounded on a report that Galileo had been personally ordered in 1616 not to discuss Copernicanism either orally or in writing. Cardinal Bellarmine had died, but Galileo produced a certificate signed by the cardinal, stating that Galileo had been subjected to no further restriction than applied to any Roman Catholic under the 1616 edict. No signed document contradicting this was ever found, but Galileo was nevertheless compelled in 1633 to abjure (formally renounce his beliefs) and was sentenced to life imprisonment (swiftly commuted to permanent house arrest). The Dialogue was ordered to be burned, and the sentence against him was to be read publicly in every university.

VII

GALILEO’S IMPACT ON THOUGHT

The condemnation of Galileo did have some effect on universities and colleges in countries where the Catholic Church exercised control over teaching and publication, although the permission to treat Copernicanism as a useful, though false, calculating device meant that heliocentric (Sun-centered) ideas could always be made familiar to students. The ideas contained in the Dialogue could not be suppressed, and Galileo’s own scientific reputation remained high, both in Italy and abroad, especially after the publication of his final and greatest work.

Galileo’s final book, Discourses Concerning Two New Sciences, was published at Leiden in 1638. It reviews and refines his earlier studies of motion and, in general, the principles of mechanics. The book opened a road that was to lead Newton to the law of universal gravitation, which linked the planetary laws discovered by Galileo’s contemporary Johannes Kepler with Galileo’s mathematical physics. Galileo became blind before it was published, and he died at Arcetri, near Florence, on January 8, 1642.

Galileo’s most valuable scientific contribution was his part in transforming physics from a plausible framework erected on casual observations of complex everyday experiences into a method whereby selected experiences were so simplified that their underlying structures or patterns could be explained in geometrical terms and thus become susceptible to precise measurement. Galileo’s law of falling bodies, for example, disregards the resistance of the medium and concentrates solely on the relationship between distance fallen and time elapsed in a vacuum. If this simplified law proves to be only approximate, then the approach is repeated to find what refinement is needed to account for how an actual body falls through a medium—for example, through air. 

Galileo abandoned the key Aristotelian ideas according to which rest is a natural state and only motion needs explanation, and got so near to understanding the nature of inertial motion that Newton credited him with the discovery. More widely influential, however, were The Starry Messenger and Dialogue on the Two Chief World Systems, which opened new vistas in astronomy. Galileo was an outstanding popularizer of his own work and is recognized as a master of Italian prose. 

Galileo’s lifelong struggle to free scientific inquiry from restriction by philosophical and theological interference is also remembered as a major contribution to the development of science. Since the full publication of Galileo’s trial documents in the 1870s, entire responsibility for Galileo’s condemnation has customarily been placed on the officials of the Roman Catholic Church. A fuller picture would include the role of the professors of philosophy who first persuaded theologians to link Galileo’s science with heresy, although the responsibility for the ruling of 1616 and for the condemnation of Galileo must remain with the officials of the church and their advisers. 

An investigation into the astronomer’s condemnation was opened in 1979 by Pope John Paul II. A papal commission, set up in 1982, produced several scholarly publications related to the trial. In October 1992 the commission acknowledged the error of the church’s officials. In a speech accepting the report John Paul, alluding to Galileo’s views on scripture and science, said that Galileo, “a sincere believer, showed himself to be more perceptive in this regard than the theologians who opposed him.”