Galileo (scientist)

I

Introduction

Galileo (scientist) (1564-1642), Italian physicist and astronomer, who pioneered the scientific revolution that flowered in the work of the English physicist Isaac Newton. His main contributions to astronomy were the use of the telescope in observation, and the discovery of lunar mountains and valleys, the four largest satellites of Jupiter, the phases of Venus, and sunspots. In physics, he discovered the laws governing falling bodies and projectiles. In the history of culture, Galileo stands as a symbol of the battle against authority for freedom of inquiry.

Galileo, whose full name was Galileo Galilei, was born near Pisa, in Tuscany, on February 15, 1564. His father, Vincenzio Galilei, played an important role in the musical revolution from medieval polyphony to harmonic modulation. Just as Vincenzio saw that rigid theory stifled new forms in music, so his eldest son came to see both the currently dominant physics of the Greek philosopher Aristotle and the Roman Catholic theology influenced by it as limiting physical inquiry. Galileo was taught by monks at Vallombrosa and then entered the University of Pisa in 1580 or 1581 to study medicine. Although the syllabus was uncongenial to him, it did give him a useful introduction to current versions of Aristotelian physics.

Aristotelians made a sharp division between the 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 subject to its own distinct laws of natural motion. Fire, for instance, had lightness, which made it rise vertically, away from the centre of the Earth. Earthy objects fell naturally downward towards the centre of the fixed 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. See Chemistry: Greek Natural Philosophy; Philosophy, Greek: Plato and Aristotle.

II

Galileo’s Work in Physics

The key to Galileo’s new physics lay in mathematics. Although he was still registered as a medical student, he increasingly devoted his time to the extra-curricular study of mathematics, with the encouragement of the court mathematician Ostilio Ricci. He left the university without a degree in 1585. For a time he tutored privately and wrote on hydrostatics, but he did not publish anything. In 1589 he became Professor of Mathematics at the University of Pisa.

The celebrated story of Galileo dropping objects from the Leaning Tower of Pisa to demonstrate to assembled professors 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. Viviani has Galileo simultaneously dropping two objects of the same material but different weights to refute the Aristotelian belief that speed of fall is proportional to weight. That much Galileo could show even at this early stage of his career. However, his manuscript works show 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. He considered himself a follower of the ancient Greek scientist Archimedes and abandoned Aristotelian notions of heaviness and lightness in favour of the more useful notion of density. He made his first attempts at producing simple mathematical comparisons of how bodies of varying densities fall in various media 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 approximation to inertial motion and a measure of how he was distancing himself from Aristotelian ideas of natural and forced motions.

Galileo’s contract was not renewed in 1592, probably because he contradicted Aristotelian professors. In the same year he was appointed to the chair of mathematics at the University of Padua in the republic of Venice, where he remained until 1610.

At Padua, Galileo invented a calculating “compass” for the practical solution of mathematical problems. He was much impressed by the practical knowledge of mechanics displayed by the foremen of the world-famous shipyard, the Arsenal of Venice. In his own work he combined an ability to discern simple mathematical patterns underlying familiar occurrences, such as the free fall of objects to the ground, with a knack of devising controlled observations in which the looked-for mathematical relationships presented themselves as obvious and measurable with precision. His fundamental conviction was that the universe is an open book but, as he wrote later in The Assayer, “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 ... .”

A

Projectiles and Pendulums

This conviction led to important discoveries in the first decade of the 17th century. Galileo not only recognized that the acceleration of any body in free fall was uniform but he expressed this in a simple law: the distance travelled in free fall is proportional to the square of the time elapsed; that is, in 2 seconds a body will fall 4 times as far as it will in 1 second; in 3 seconds it will fall 9 times as far; and so on. Alternatively expressed: the distances moved in successive equal intervals of time are as the odd numbers: 1, 3, 5 ... .

This same law led to an understanding of the motion of projectiles. Galileo could look at the fall of an arrow or cannon ball 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 parabolic. The practical consequences for efficient gunnery were deduced from this seemingly abstract geometrical account.

In similar vein, Galileo investigated mechanics and the strength of materials. In his studies of pendulums he discovered that for a given pendulum the swing of the bob takes the same time for arcs of different sizes, though others soon pointed out that this was true only provided that the swings did not become too large.

One of the greatest contrasts between Galileo’s ideas and Aristotle’s is in their underlying models of motion. Galileo considered that an object moving uniformly on the Earth’s surface without meeting any resistance would continue to do so without needing to be kept moving by any force, whereas Aristotelians would look for a force to cause the continuing motion. It is true that the surface of the Earth is a spherical surface, but it is reasonable to see Galileo’s ideas as approximating to 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 (see Mechanics: Newton’s Three Laws of Motion). At the least, Galileo made the advance of not treating rest as a state more natural or privileged than motion.

III

Astronomical Research

During most of his Paduan period Galileo showed only occasional interest in astronomy, although in 1597 he declared in private correspondence that he preferred the Copernican theory that the Earth revolves around the Sun to the Aristotelian and Ptolemaic assumption that the planets, the Moon, and the Sun circle a fixed Earth (see Ptolemaic System). Only the Copernican model supported Galileo’s ingenious but mistaken theory of the tides: according to this theory Earth’s rotatory motion is alternately added to the orbital motion and subtracted from it, with the effect that the seas are set sloshing backwards and forwards. 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 to the variety of tidal phenomena actually observed at different places and seasons.