Galileo (1564 - 1642)

When one thinks of the controversy which surrounded the heliocentric theory of the Universe, one thinks of Galileo.  The three other important players in this game (Copernicus, Brahe, and Kepler) attracted surprisingly little controversy: Copernicus's book was written in Latin and aimed at specialists; it also contained a preface (written after Copernicus's death by a nervous publisher) claiming that the model was only for computational purposes and wasn't supposed to be taken literally.  Brahe was not a Copernican at all (he favored his own model, the Tychonian model, wherein the Sun and Moon orbited the Earth, but all the planets orbited the Sun), so the Church had no quarrel with him.  Kepler was shy, and he set down his discovery of elliptical-shaped orbits in a very dense book which almost nobody could understand, so he also attracted little attention.

Galileo, on the other hand, was a brash and highly opinionated man who did not hesitate to let people know where he stood, and who took a positive glee in dismembering Aristotelian arguments in print.  In 1610 he was 46 years old and a respected but basicially unknown professor of mathematics and astronomy.  Then he heard that one could combine two lenses to make things that were far away appear near, and within months he was the premier telescope maker in the world.  He turned his telescope to the heavens and he saw:

  • Spots on the Sun
  • Mountains on the Moon
  • Venus going through a full cycle of phases
  • Four moons orbiting Jupiter
  • That Saturn has an odd shape
  • The Milky Way consists of innumerable stars
The first two items dealt a heavy blow to Aristotelian beliefs about the heavens, because Aristotle had taught that the heavens were different from the Earth, i.e., they were perfect and unchanging.  A rough and craggy Moon made out of mere rocks and a Sun with spots drifting across it did not compute.

The Ptolemic model said that Venus was always either in front of the Sun or to one side of it, so it wasn't possible for Venus to show a full-moon phase.  But Galileo in fact saw a full-moon phase, which meant that Venus had to be circling behind the Sun, in agreement with Copernicus.

The four moons of Jupiter were a real surprise (they are called the Galilean moons today, and are about the same size as our Moon).  They proved there were objects in the Universe which didn't orbit the Earth.  Within itself, this didn't prove whether the Sun orbited the Earth or vice versa -- but it certainly dealt a blow to the notion that the Earth held an exclusive priviledge with respect to having bodies orbit it.

The odd shape of Saturn (Galileo's telescope couldn't resolve the rings of Saturn; he thought the planet was cigar-shaped) and the way the Milky Way breaks into a cloud of stars under a telescope were further proof that there was more in the heavens than was dreamed of in Aristotle's philosophy.
Galileo published his observations in a book called The Starry Messenger, and it made him an instant sensation all across Europe.  He was wined and dined by everyone who was anyone in Italy, including the Pope, and offered a new position at three times his current salary.  Galileo urged the Pope to have the Church openly adopt the new, Copernican views, but the Pope demurred.  It was acceptable for scholars to debate these things, the Pope thought, but the people wouldn't understand.

Galileo was disappointed, but he accepted the Pope's decision.  With his new-found fame (and with the many new scientific enemies which his sharp tongue was earning him) Galileo found that he was now at the center of the heliocentric controversy.  Scientific opposition to the theory generally centered around one great objection: how could the Earth move?  Aristotle taught that things in the heavens moved in unending, uniform motion in perfect circles because this was their natural motion.  They were made of heavenly aether.  But things on the Earth tended towards rest unless pushed (which is entirely obvious to anyone who has ever tried to move an ox-cart over muddy roads).  So what was pushing the Earth?  Also, if the Earth was moving, why weren't there gale-force winds as the air rushed by?  Why did a ball thrown from the top of a tower fall straight down, rather than being left behind, if the tower was supposedly moving so fast?

Galileo's greatest contribution to physics is this: he recognized that the arguments of the Aristotelians must be wrong, somehow -- but he also recognized that the various counter-arguments put forth by his fellow pro-Copernicans were just as bad.  (Classic example: The air is being pushed along by the mountains.)  Galileo realized that no one really understood motion at all, and that the Earth-centered theory of the Universe could never be demolished until the Aristotelian theories of motion were overthrown.

As a consequence, Galileo spent a substantial fraction of his life observing simple motion:  balls rolling down inclined planes, objects falling under different conditions, and so forth.  (The story about him dropping balls off the Leaning Tower of Pisa is pure fiction, however).  Watching objects roll down planes at different angles might seem like an utterly trivial exercise -- but it helped Galileo discover truths about motion that had completely eluded everyone since the time of Eudoxus, over 2000 years before.  Among his discoveries:

1)  In the absence of air friction, all things fall at exactly the same acceleration, regardless of mass, size, shape, or composition.

2)  In the absence of friction, anything set into linear motion will stay in motion forever.

3)  If one combines the two principles above, then it must be true that an object dropped from a moving platform (or tossed in the air) does not fall directly towards the Earth, but in fact moves along a curved path such that it remains abreast of the (still moving) platform.

From the viewpoint of a person on a moving wagon, a ball tossed in the air goes straight up.  But from the viewpoint of a person on the ground, the ball moves in an arc such that it always remains above the moving wagon.

These laws constitute the basis of what is today called Galilean Relativity.  They explained how it was that the Earth could move without anything "pushing" it -- in so many words, Aristotle had it exactly backwards.  Once in motion, the Earth (or anything else) will continue to move forever unless some force intervenes to stop it.  Friction stops everyday objects, but there is nothing to stop a planet in its orbit.  Furthermore, once an object acquires a velocity, it keeps it.  A ball dropped from the Leaning Tower of Pisa already shares any velocity that the Tower has as the Earth rotates.  As the ball drops, the Tower, the Earth, and the ball all move together.  Their "absolute" motion cannot be measured.  Only the ball's relative motion with respect to the Earth and the Tower can ever be detected.

Or to phrase it another way, to say that something is "moving" is almost meaningless.  You can only say that something is moving with respect to something else.  This is the essence of Galilean Relativity.  One way of thinking about relativity is this: if you are placed inside a closed box, with no way of detecting anything outside the box, then there is NO experiment you can do inside the box, NO measurement you can possibly make, which will tell you if you are moving (at a constant velocity) with respect to anything outside the box or not.  Everything inside the box shares your velocity, and measurements on those can only tell you how you are moving with respect to them -- not how they (and you) are moving with respect to something else.  This is very different from the physics of Aristotle, who held that objects had an absolute tendency to move straight towards the center of Earth, no matter what the circumstances.

In 1623 the old Pope died and Maffeo Barberini, an intelligent and educated man whom Galileo practically knew on a first-name basis, was elected the new Pope.  Galileo immediately asked for permission to write a book discussing the merits of the Copernican and Ptolemic theories of the Universe.  He was given the go-ahead, and the book was published in 1632.

But....the book was not at all what the Pope had expected.  It was written in Italian, not Latin, and was aimed at a general audience, not a handful of scholars.  It was elegantly written, it was interesting, and it was understandable to anyone.  It was also written in the form of a logical debate, with fictional characters discussing the opposing theories.  But worst of all, the character upholding the traditional views was named Simplicio (i.e., the Simpleton) and was obviously very stupid.  As the Pope himself held some of these same views, he became completely convinced that Galileo was trying to ridicule him.  Historians are certain that Galileo never considered such a thing, but that is neither here nor there.  The Pope was convinced.  And Galileo's sharp tongue had begun to catch up with him.  He had made bitter enemies of several highly-placed men in the Vatican, by ridiculing their Aristotelian physics, and now they were only too happy to venomously exaggerate any rumor about Galileo that might make the Pope angrier.

In short, the Pope eventually forced Galileo to recant his views about Copernicanism.  (The legend that Galileo muttered "Nonetheless, it still moves" upon arising from his recantation is pure fiction, however.)  Galileo was placed under house arrest for the rest of his life (about eight years; Galileo was 70 at the time of his trial), and almost every book advocating Copernicanism was banned.  However, when the Pope's soldiers went into the streets of Rome to seize the banned books from the booksellers, there were no copies of Galileo's book to be found.  They had all been sold almost instantly after they were printed.

It should be noted that Galileo was the last person of any importance to be persecuted for advocating the heliocentric theory, and in fact, within only twenty years or so it had become completely obvious to anyone with any education in the Vatican -- including the new Pope who succeeded Barberini -- that the Church could only make itself look dogmatic and foolish by opposing the heliocentric theory.  But they could not openly endorse it, not after the full authority of the Pope had been used against it, so they just kept silent and hoped everything would blow over.  The Church did not officially apologize for its treatment of Galileo until the 1980's.

In the last years of his life, while still under house arrest, Galileo wrote his most important book, Dialogues Concerning Two New Sciences.  This was an exposition of his research into moving bodies, and it was smuggled out of Italy by friends and published in Holland.  Galileo died in 1642.

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