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Galileo’s defence of Copernicus

What were Galileo’s views on an infinite universe? Just nine years before Galileo began looking through his telescope, in 1600, Giordano Bruno was burned at the stake by the Inquisition, after being tortured for nine years for preaching that the universe was infinite. He refused to recant. So, perhaps not surprisingly, Galileo did not express, or at least did not publish, any views on whether the universe was infinite or not. But he did discover additional contradictions which the idea of infinity leads to, (anticipating some of the concepts of the late nineteenth century mathematician George Cantor, who we discuss very briefly below) and elsewhere notes that infinities “transcend our finite understanding… In spite of this, men cannot refrain from discussing them… we attempt, with our finite minds, to discuss the infinite, assigning to it properties which we give to the finite and limited; but I think this is wrong…” (Galileo, Dialogues Concerning Two Sciences, First day, p418)

The ‘corruptible heavens’ which Galileo viewed through his telescope implied – by Aristotle’s own admission – a beginning and end to the universe, and this was bound to disturb all accepted dogmas. In order to defend the theory of Copernicus that the sun, not the earth, was the centre of the solar system, Galileo demonstrated that motion was relative, not absolute, as Aristotle had taught. In the section, Problem Not Resolved, Woods says:

“Einstein was determined to re-write the laws of physics in such a way

that the predictions would always be correct, irrespective of the motions

of different bodies, or the ‘points of view’ which derive from them. From

the standpoint of relativity, steady motion on a straight line is

indistinguishable from being at rest.” (p161)

The law that steady motion in a straight line “is indistinguishable from being at rest” is an essential feature of Newton’s first law of motion. This was no “re-write” of the laws of physics. Here are Newton’s words, published in 1687:

“Every body perseveres in its state of rest, or of uniform motion in a

right line, unless it is compelled to change that state by forces

impressed thereon.”

Principia, Axioms or Laws of Motion

We shall show that, from the point of view of Newton’s laws of motion, rest and uniform motion are equivalent. Woods mistakes what became known as Galileo’s principle of relativity, enshrined in Newton’s first law of motion, for a supposed ‘re-write’ of the laws of physics by Einstein in his theory of relativity, which in fact takes Galileo’s principle of relativity as its starting point. Woods even uses the same terms, “rest” “motion” and “line” and yet does not recognise this elementary physics.

 

 

Furthermore, Woods seeks to undermine this law of motion, Galileo’s principle of relativity, all the time believing it to be a facet of Einstein’s relativity, asserting instead the existence of absolute space and time. This leads him to take essentially the same position as the supporters of Aristotle in their dispute with Galileo in his defence of Copernicus. Copernicus, we should add in passing, briefly anticipates Galileo’s arguments. (On the Revolution of the Heavenly Spheres, book one, Introduction, point five, p23)



What is this fundamental law of physics, the principle of relativity, on which Newton based his first law of motion, and from which Einstein took the name ‘relativity’? Let us take a glimpse at what Galileo and Einstein said.

Galileo’s thought experiment

The followers of Aristotle’s orthodoxy in the early seventeenth century thought that if the earth was travelling round the sun, or rotating, this would cause many very visible effects:

“How would birds find their nest again after they had flown from them?

Why does a stone thrown up come straight down if the earth underneath it is rotating rapidly to the east?”

The Galileo Project, http://galileo.rice.edu/sci/theories/copernican_system.html)

Aristotle himself provided arguments against the notion that the earth moved, since one school of ancient Greek philosophers, led by Pythagoras, proposed that the earth did move. For instance, Aristotle asks, why do “heavy bodies forcibly thrown quite straight upward return to the point from which they started” if the earth has moved in the meantime? (On The Heavens, book II, chapter 14)

In the same passage Aristotle also argued that if the earth moved, one would surely see the stars pass by: “… there would have to be passings and turnings of the fixed stars. Yet no such thing is observed. The same stars always rise and set in the same parts of the earth.” This was a very powerful argument, not experimentally refuted until stellar parallax was measured by powerful telescopes in the 1800s. (Stellar parallax is the apparent movement of a star caused by viewing it from different positions, for instance, when the earth has moved a sufficient distance in its orbit round the sun.) Galileo could only suppose (correctly) that the stars were at too great a distance for parallax to be observed with the naked eye.

 

Galileo suggested experiments to prove the followers of Aristotle wrong. Adopting a popular, accessible style and writing in the native language rather than the scholars’ Latin, Galileo begins by asking his audience to imagine they are shut up “with some friend in the main cabin below decks on some large ship”. While the ship is stationary, Galileo suggests conducting a number of experiments designed to test motion in space, such as throwing and jumping, and setting a bottle to drip into a jar below. Then, he suggests:

“… have the ship proceed with any speed you like, so long as the

motion is uniform and not fluctuating this way and that. You will

discover not the least change in all the effects named, nor could you

tell from any of them whether the ship was moving or standing still.”

Dialogue Concerning the Two Chief World Systems

The water still drips directly into the jar below – it does not fall behind the jar as the ship moves forward steadily. Many people have been below decks on a ship or car ferry, where you cannot see out, and experienced something similar: you cannot be sure if the ferry is moving or not, so long as it is going at a constant velocity. Galileo’s point is that a scientist, conducting experiments, could not determine by any experiment whether the ferry or, of course, the earth, was in constant motion or stationary either. This was termed Galileo’s principle of relativity.

 

 


Date: 2015-01-11; view: 885


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