Lecture - Galileo (2/12/96)

Seeds: Chapter 4

1. Galileo Galilei
   • born in Pisa, Italy (1564-1642)
   • known for his telescopic observations, physics experiments, and trial for heresy
   • the telescope was invented around 1608 in Holland, and Galileo was the first to make systematic astronomical observations and apply them to the Copernican versus Ptolemaic debate
   • Sometime in the 1590's Galileo accepted the Copernican hypothesis. As one might imagine, this was not a popular philosophy in Catholic Italy at that time. It was primarily because of Galileo that the Church issued the Decree in 1616 that the Copernican doctorine was "false and absurd" and not to be held or defended.
   • Galileo discovered many astronomical things that supported the heliocentric hypothesis and refuted the geocentric hypothesis.
   • It was prohibited to hold this view, but he still hoped to convert his countrymen to his view. He prevailed upon his old acquaintance Pope Urban VIII to allow him to publish a book explaining all the arguments pro and con, without supporting the banned doctorine, in order to show that the Italians were not ignorant of the latest theories.
   • The book appeared in 1632 under the title Dialogue on the Two Great World Systems. The book was written in Italian for the mass audiences, not Latin.
   • The Dialogue was a forceful and persuasive argument for the Copernican cosmology. It was written in the form of a trialogue between three philosophers: Salviati, the brilliant mouthpiece of Galileo himself; Sagredo, who is dubious but quick to see the truth of Salviati's arguments, a sort of straight man; and Simplicio, a thick witted Aristotlean who brings up all the usual objections to the Copernican system, and is of course shot down with ease.
   • It was pointed out in the preface to the Dialogue that the arguments therein were merely mathematical fantasy and that divine knowledge assures us the immobility of the Earth. Of course, this thinly cloaked irony went down poorly with the Church and Galileo was duly called before the Inquisition.
   • After a trial, Galileo was forced to plead guilty and deny his own doctorines. His life sentence was commuted to house arrest in Arcetri, where he spent the final 10 years of his life. He was seventy at the time of the trial.
   • Naturally, the Dialogue joined Copernicus' De Revolutionibus on the infamous "Index of Prohibited Books". It was removed from the Index in 1835.
   • In 1980, Pope John Paul II ordered a re-examination of the evidence against Galileo. He was exonerated in the Verdict of 1992.
   • As much as we might fault the Roman Catholic Church in hindsight for its obstinate defense of the Aristotlean doctorine under the guise of divine knowledge, it is hard to be totally sympathetic with Galileo. Galileo was outspoken and tactless and arrogant. He did himself no service with the galling Dialogue. It is interesting to compare this with Charles Darwin's rectitude in the 19th century on the much more inflammatory debate on evolution.
2. Galileo the Physicist
   • Galileo's greatest contributions were in the field of Physics known as mechanics. Mechanics is the study of motion and the action of forces, such as gravity, on material bodies.
   • Aristotle's mechanics was still believed at this time, and Galileo set to doing ingenious experiments with pendulums, balls rolling down inclined planes, light & mirrors, falling objects, and many others. At this time, the practice of doing experiments to learn the nature of physical laws was not accepted as standard.
   • Aristotle maintained that heavy objects fall faster than light objects, like a rock falls faster than a feather. Galileo argued that if a heavy object and a light object were dropped together, even from a great height, both would hit the ground at the same time. Any difference can be attributed to the action of wind and air resistance - if a stone and a feather were dropped simultaneously inside a vessel in which the air had been pumped out, they do indeed hit the bottom simultaneously also. Galileo demonstrated that he was right, and Aristotle wrong, by the famous experiment of dropping two cannonballs of differing weight from the Tower of Pisa.
   • Galileo discovered the Law of Inertia, which later became Newton's First Law of Motion. It was well known to Aristotle, and is apparent to all, that bodies at rest tend to stay at rest - objects do not spontaneously move on their own! Aristotle the philosopher reckoned that this showed that rest was the "natural state of matter", and that bodies in motion would return to rest after you stopped pushing them. Galileo showed that bodies in motion in fact tend to stay in uniform motion with constant velocity unless acted upon by a force - this property of matter was called inertia.
   • Through a series of experiments where he pushed an object on a rough floor, a smooth floor, and ice, and found that friction tended to stop the body quickly while when friction was reduced on a polished floor or ice, it went much farther when pushed. He made the conjecture that if he were to remove the friction entirely, then the body once put into motion would continue indefinitely on a straight line at constant speed.
   • Galileo also studied the acceleration of bodies, that is, how they change their speed when they fall freely or roll down an inclined plane. He show that if a constant force is applied, like gravity at the surface of the Earth, then they accelerate uniformly gaining equal increments of speed in equal increments of time.
   • Galileo formulated these newly-formed laws of nature in precise mathematical terms that enabled one to predict the motions of objects and test against experiment and observation. Newton incorporated these into his own Laws of Motion.
3. Galileo's Cosmology
   • Once he had established the Law of Inertia, it was easy to accept that the planets remain in their perpetual orbits without any special celestial or heavenly property, but posessed interia as all of matter.
   • Galileo did not pursue the question of why the planets follow closed orbits instead of straight-line motion, as one would expect purely under the influence of inertia. It remained for Newton to work out that the force of gravity acted to keep the planets on their ellipses forever in motion.
   • Galileo did manage to answer the common objection to the notion of a moving and spinning Earth - why did we not fly off. I does seem unsettling to think that at this very moment we are whirling eastward at over 300 meters per second and streaking through space at nearly 30 kilometers per second, yet cannot feel it! Galileo pointed out that we share the Earth's own inertia, and thus move with its motions. He pointed out that a stone dropped from the masthead of a moving ship does not fall behind it, but lands at the foot of the mast just as if the ship were at rest. We can see this easily by dropping a pencil on an airplane. It falls down, not backwards at hundreds of miles per hour. In fact we only notice that we are on a moving plane when it accelerates - speeds up or slows down - otherwise inertia keeps us moving with the plane.
   • The physical laws were applied to astronomical bodies the same way in which they were applied to terrestrial objects. The universe was thus unified.
4. Galileo the Astronomer
   • The first telescopes in Europe to attract much attention were made by the Dutch spectacle-maker Hans Lippershey in 1608. Galileo heard of the invention in 1609 and without seeing an assembled telescope made his own!
   • His first telescope had a magnification of 3 times (objects appeared 3 times larger than with the naked eye). His most powerful telescope had a magnification of 30 times.
   • The usefulness of telescopes for terrestrial observation (like spying) were well known, but the idea of training a telescope on the heavens was less obvious. There was a long tradition that the human eye was the best possible measure of truth, while lenses and mirrors produced distorted and upside-down images.
   • Galileo conducted extensive tests to convince himself that the magnified images produced were faithful representations of distant scenes, before turning his instruments on the sky.
   • Late in 1609 he began his astronomical work. In 1610 he startled the world by publishing a list of his remarkable discoveries in the book The Sidereal Messenger.
   • He found that many stars too faint to be seen by the eye alone would become visible in the telescope.
   • He found that the Milky Way, which appeared as a blurred band of diffuse light to the eye was actually made up of very many faint stars.
   • He also found that some smally blurry patches in the sky were actually clusters of faint stars close together.
   • He found that Jupiter had 4 moons of its own (!) that revolved around it with periods ranging from under 2 days to 17 days. This showed that the Earth itself could move through space without leaving the Moon behind, since Jupiter could manage it.
   • He observed that Venus goes through phases just like the moon! This really demonstrated that the Ptolemaic model was incorrect, since it would have to revolve around the Sun to show phases (or around the Earth without the Sun like our own Moon does, which it clearly does not). If it circled between us and the Sun as Ptolemy would have it, it would always appear as a crescent. The same applied to Mercury.
   • He showed that the surface of the Moon was "uneven, rough, and full of cavities and prominences, being not unlike the face of the Earth." He suggested that the prominent dark flat areas were seas (Maria) of water. This showed that heavenly bodies could be "flawed" and were similar to our own Earth.
   • He found, in his most disturbing observation, that the Sun was blemished, with sunspots that could be seen to move across its face day to day. The sunspots were temporary, lasting a few months at most, but new ones would appear as old ones would vanish. He deduced that they were near or on the surface of the Sun and that the Sun rotated with a period of about a month.
   • Galileo compiled a great deal of evidence to support the Copernican system, and refute the Ptolemaic. In addition to the Sidereal Messenger, he published his arguments for the heliocentric system in the Dialogue, which appeared in 1632.
5. Midterm #1 This Wednesday 2/14/96
   • Here are some Review Topics for the midterm.

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