Galileo

Galileo Galilei: Subterman's portrait, circa 1640

Galileo Galilei (1564-1642), often referred to simply as Galileo, was and Italian physicist and astronomer who improved the telescope and had a famous dispute with the Catholic Church.

In June or July of 1609, word reached Galileo about the telescope, which had been invented in Holland. Without knowing the technical details of the construction of the device, he managed to create one for himself. With it, he was able to witness a supernova, observe our Moon, and document the phases of Venus. He also located sunspots. Some of these discoveries helped support a heliocentric system.

Until the sixteenth century, the prevailing view was that the Sun, Moon, stars and planets circled the Earth (the geocentric system, based on the second century work of Ptolemy). Puzzled astronomers noticed that Mars, Jupiter and Saturn sometimes seemed to move backwards, but their motions were well predicted by Ptolemy's theory.

A Polish astronomer, Nicolaus Copernicus (1473-1543), published a model of the solar system De Revolutionibus in 1543, in which the earth and other planets circled the sun. The Catholic Church endorsed the book. The book’s preface (which was not written by Copernicus) argued that astronomical models have merit if they correctly predict observations, even if the underlying hypotheses cannot be proved.

Some Dominican friars in Florence sent criticisms of Galileo’s work to the Inquisition in 1615. Galileo later wrote a privately-circulated pamphlet which argued that the Bible should be interpreted in the light of increasing knowledge, and warning that scientific opinion should not be treated as an article of faith.

By 1616, the Church’s Index of Forbidden Books required that nine sentences in Copernicus's book be corrected, and Galileo was told by the Inquisition that he must only use Copernican concepts as a hypothesis for the sake of calculation, without claiming that they had been literally proven true. Because Galileo had overstated the scientific case for the heliocentric theory and given some dubious theological arguments, the Catholic Church decided against Galileo in 1616 and said that 1) the immobility of the Sun at the center of the universe was absurd in philosophy and formally heretical, and that 2) the mobility of Earth was absurd in philosophy and at least erroneous in theology.

Galileo tested Aristotle's theory that heavy objects fall faster than light ones and found it was a consequence of air resistance, not gravity. By making careful measurements he found he could use mathematics to predict how things fell and the movements of pendulums and tides. His 1632 book Dialogue on the Tides, which discussed both the Ptolemaic and Copernican hypotheses, was originally given permission to be published by Catholic censors, but the title was changed to Dialogue on the Two Chief World Systems. This led to Galileo’s summons to Rome by the Inquisition. Galileo had erred by advocating heliocentrism as scientifically proven, which was not compatible with the Inquisition’s 1616 ruling. It also didn't help that he mocked the Pope in the form of a fictitious character named Simplicio. In 1633 Galileo was forced to renounce his beliefs. At first he was sentenced to life imprisonment, but this was commuted to permanent house arrest with a church pension. This sentence was read aloud in public in every university and the dialogue was ordered to be burned. Galileo went back to studying motion and mechanics.

His theory of tides turned out to be mistaken.

Dialogues Concerning Two New Sciences

Galileo’s last book, the 1638 work Discourses Concerning Two New Sciences, dealt with motion and mechanics. This work helped inspire Isaac Newton to create his theory of gravity, which linked Galileo’s mathematics and physics to Kepler’s laws of planetary movement.

Galileo's drawing of how the dimensions of different-sized bones need to vary to make them equally strong

In Two New Sciences, Galileo revealed a sophisticated understanding of engineering and what now would be called dimensional analysis, the way in which the proportions of an engineering structure need to change as the size of the structure is scaled up. He applied this to biology, explaining why the bones of a large animal need to be thicker in proportion to their size than those of smaller animals.

In an age before stopwatches, Galileo was able to make quantitative observations of uniformly-accelerated motion, using his pulse as a time reference and watching the slow acceleration of objects moving down inclined planes.

This book contains his famous discussion and thought experiment, in which he shows both by logic and by appeal to experiment that light and heavy objects fall at the same speed (contradicting Aristotle). One of his discussants says "But I, Simplicio, who have made the test can assure you that a cannon ball weighing one or two hundred pounds, or even more, will not reach the ground by as much as a span ahead of a musket ball weighing only half a pound, provided both are dropped from a height of 200 cubits.".

Another discussant points out that this result is logical, by positing a large stone that moves at a speed of eight and a smaller one which moves at a speed of four, and showing that this leads to a contradiction if the stones are tied together. Since the smaller stone tends to fall more slowly, it ought to retard the motion of the larger one, so the combination should fall more slowly than the large stone by itself. However, "the two stones when tied together make a stone larger than before," and hence ought to fall more quickly than the large stone by itself.

It is interesting to see how Galileo was able to obtain quantitative results in physics before algebra and calculus were invented. The book uses the methods of geometry, and lays out "proofs" of physical relationships using a style and terminology similar to Euclid's Elements.

In 1741, a century after Galileo's death in January 1642, the Church under Pope Benedict XIV bid the Holy Office grant an imprimatur to the first edition of the Complete Works of Galileo.

Stillman Drake of the University of Toronto [3] was for the last decades of his life the most original and important scholar to study this seventeenth-century physicist. == References ==