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Galileo Galilei - and his mechanics and motion

Galileo Galilei graphic HOME .... William Gilbert . Rene Descartes . Isaac Newton . Albert Einstein ........ Johannes Kepler ........ General Image Theory Galileo Galilei graphic

picture of Galileo inclined plane motion

Galileo Galilei (1564-1642), was a good mathematician and early astronomer and in physics he mainly experimented in mechanics and in the workings of gravity and basically invented the telescope or at least its science use. He was under strong religious and legal requirement to restrict his publications, and from his Catholic Inquisition 1633 trial was put under house arrest till his death aged 77. Galileo's published works included his 1632 astronomy 'Dialogue concerning the Two Chief World Systems' and his 1638 mechanics 'Discourses(or Dialogues) concerning Two New Sciences' both in an ancient-Greek fictional logical argument dialogue style and with little real theory but the latter with some actual experimental proofs and with some sections written more in a Euclid or Newton style.

Galileo produced little theory and at times took a Newton black-box position as in saying the cause of gravity was of no immediate importance, maybe from fear of persecution or from failure to produce any satisfying gravity theory from the simple greek-Atomist push-physics theory that he adopted - but he like many early scientists saw experiment as more important. Galileo was an early outstanding experimental scientist and an early astronomer, yet studying gravity he wrongly rejected William Gilbert's attraction physics theory and moon-attraction tides theory as his own very wrong tides theory showed Galileo was really worse than useless on physics theory - but his big Catholic Inquisition trial did give him more publicity.

Galileo science graphic

'Dialogues concerning Two New Sciences' (1638),Translated by H Crew and A De Salvio

Galileo stated that he himself had not sought the publishing of this his major work on mechanics and motion. It largely deals with mechanical strength under gravity, motion generally and especially motion under gravity including motion on inclined planes, projectile motion and pendulum motion. Its dialogue style (between Salviati, Sagredo and Simplicio) made the presentation of its science somewhat more difficult for readers, but partly suited the nasty religious and legal requirement that Galileo was under to publish his ideas as 'only ideas'. Some key extracts follow ;

So on motion, Page 154 ;

"Uniform Motion.
In dealing with steady or uniform motion, we need a single definition which I give as follows.
DEFINITION. By steady or uniform motion, I mean one in which the distances traversed by the moving particle during any equal intervals of time, are themselves equal."

And Page 169 ;

"Sagrado.
... it would seem that up to the present we have established the definition of uniformly accelerated motion, which is expressed as follows ;
A motion is said to be equally or uniformly accelerated when, starting from rest, its momentum receives equal increments in equal times."

And Page 215 ;

"... Furthermore we may remark that any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of acceleration or retardation are removed ..."

And expressing a Newton-like blackbox view on different theories of gravity, Pages 166-167 ;

"Salviati.
The present does not seem to be the proper time to investigate the cause of the acceleration of natural motion concerning which various opinions have been expressed by various philosophers, some explaining it by attraction to the centre, others to repulsion between the very small parts of the body, while still others attribute it to a certain stress in the surrounding medium which closes in behind the body and drives it from one of its positions to another. Now all these fantasies, and others too, ought to be examined ; but it is not really worth while. At present it is the purpose of our author merely to demonstrate and to investigate some of the properties of accelerated motion (whatever the cause of this acceleration may be) - meaning thereby a motion, such that the speed goes on increasing after departure from rest, in simple proportionality to the time ..."

Of course Galileo basically took Earth's gravity as constant since its strength varies very little over moderate distances from Earth's surface, despite Gilbert having repeatedly noted that the strength of forces including magnetic and electric forces decreased with distance from their source.

And supporting the existence of a vacuum, Page 81 ;

"... in the previous experiment we weighed the air in vacuum and not in air or other medium."

Galileo claimed to have a Universal Law of Gravitation covering both terrestrial gravity and the motion of planets which he was afraid to discuss. But this looks more an aspiration than a reality, as he seems not to have considered gravitational force as decreasing with distance from its source, which was central to Newton's later Universal Law of Gravitation and had been considered earlier by others and was demonstrated earlier by Gilbert for magnetic and electrical forces at least. But Earth's gravitational force does not decrease much if the highest you test from is the top of a Pisa tower, and worse was his testing gravity by the acceleration it produced on bodies using a gravity clock to measure it. He used a water version of the sand hour-glass or egg-timer - but if gravity was weaker and actually produced less acceleration, then his gravity clock would run proportionately slower so that the acceleration, and gravity strength, would appear constant. Clearly a clock must be independent of the event it is measuring, so Galileo should maybe have used iron filings and a magnet horizontally for a magnetic clock - he is known to have certainly have been acquainted with magnetism although the copy of Gilbert's De Magnete that Galileo studied may have excluded Book 6, where Gilbert included a basically correct theory of Earth's tides (detailed later in his De Mundo), as Galileo then put much effort into producing his own quite incorrect mechanical theory of tides. And many kinds of clock are of course possible as using astronomical, physical, chemical, biological or possibly even mental processes of determinate duration, and time measurement was perhaps also an issue even later for Einstein ?

Yet on Pages 261-262 ;

"Sagrado.
(if each planet had started from rest at particular heights under gravity and) fall with naturally accelerated motion along a straight line, and were later to change the speed thus acquired into uniform motion, the size of its orbit and its period of revolution would be those actually observed.
Salviati.
I think I remember him having told me that he once made the computation and found a satisfactory correspondence with observation. But he did not wish to speak of it ..."

In his polemical 'Il saggiatore' (The Assayer) 1623, after Gilbert's death, Galileo vaguely supported greek Atomist mechanical push physics against Gilbert attraction physics in claiming that science should concern itself only with the size, shape and relative motion of objects - a clearly unreasonable narrow view but supported by some Jesuits and Rene Descartes and many others. Galileo like Kepler praised Gilbert while opposing his attraction physics with no actual disproofs or even discussion being provided, both seemed to use bits of trickery at times and especially to try to avoid the scary Catholic anti-science Inquisition. Some may have felt that attacking a Protestant scientist might help placate the then very scary Catholic church. But Galileo also unreasonably rejected Kepler's proof that planet orbits are elliptical and are not circles. And in the 1623 Assayer he also publicly condemned the German astronomer Simon Marius wrongly as being a plagiarist, but also as being a Protestant not a Catholic !

Galileo's 'invention' of the telescope (really more its improvement and use in astronomy) maybe led him to seeing astronomy as more important than physics, and to wanting to advance that quickly despite astronomical evidence at the time being to many less convincing than experiment evidence. He produced an awful wrong mechanical push theory of Earth tides as later did Descartes, and Kepler produced a mechanical-field push theory of Earth tides - but all were easily disproved later by Newton who correctly developed Gilbert's better attraction theory of Earth tides (useable field or continuum mechanical push explanations of tides may well be possible but seem hard to find). Kepler, but not Galileo, correctly had gravity decrease as the square of the distance from its source and a better mathematics of planet motion within his own pseudo-Gilbert physics. But Galileo's motion under gravity experiments did basically show how planet ellipse type motion in nature could derive from linear motion. He was just not himself very strong on such theory. And 2017 still sees some of Galileo's writings not yet translated from their Italian to English.

Science and churches.

Early science in Europe faced sometimes fierce opposition from churches that often dominated governments, with Bruno being burnt at the stake in catholic Rome in 1600 (the year that Gilbert after much hesitation finally published his work in a then slightly less intolerant protestant England under Queen Elizabeth who however died just months before Gilbert's death in 1603). But churches generally preferred to control dissent and science more often by reasoning and by nasty threats and less often by extreme nasty action. The catholic church executing Bruno in 1600 acted as a strong threat to all dissidents and emerging scientists, and its Jesuit Order soon began pushing an acceptable greek-Atomist physics that was basically taken up as a self-perpetuating mainstream physics that Newton considered 'prejudice'. The catholic church also ensured that Galileo faced legal restriction and also pressured Descartes strongly, maybe encouraging them both to attack protestant Gilbert's physics. In Galileo's 1633 Catholic Inquisition trial the one book condemned besides his 1632 'Dialogue Concerning the Two Chief World Systems' was Gilbert's De Magnete, though the Catholic Church had already banned that and some other books including Copernicus and some Galileo. When Copernicus published his astronomy it was a fairly good theory backed by only a little evidence and, though Galileo and others did then add some further supporting evidence, it was only later that Newton was to tie together such evidence with a stronger theory which really proved it. The catholic Jesuit 'scholars' who had strongly opposed William Gilbert were also involved in Galileo's catholic inquisition trial really directed perhaps more at Copernicus.

It has been noted that "Soon after 1600, when William Gilbert published his famous book on magnetism, a copy was given to Galileo by an Italian professor of natural philosophy at Padua, probably Cesare Cremonini … Galileo remarked that the professor seemed afraid that Gilbert's work might infect the other books on his shelves (or that Galileo believed he wanted to free his library of its contagion)." Galileo was probably joking in a somewhat insulting manner, but the more theory inclined Cremonini may well have thought De Magnete of more interest to the experimentalist Galileo and may have given him friendly advise that the catholic inquisition might investigate anyone caught owning it. Cremonini himself was an atheist Aristotlean and had survived several investigations by the catholic inquisition and he did have a healthy fear of them and considered limited accommodation necessary. Galileo generally thought likewise. see 'Galileo at Work, His Scientific Biography' Stillman Drake 2003 p.62-63 - or - http://www-spof.gsfc.nasa.gov/earthmag/demagadd.htm Around 1602, on studying De Magnete, Galileo did some magnetic experiments with Sarpi and Sagredo but produced nothing new on magnetism beyond what Gilbert had - and later Newton did the same with the same null result. Hence basically Galileo did Gilbert's magnetic experiments but did not interpret the results as Gilbert had, while Newton did Gilbert's magnetic experiments and did interpret the results as Gilbert had. Experiments can be interpreted differently, and Galileo was probably less well acquainted with Gilbert's interpretation than Newton. It seems that Galileo's copy of De Magnete contained just one note by Galileo but many underlinings relating to Gilbert's experiments, indicating that he studied Gilbert's experiments but not Gilbert's theory as many at the time probably did. He certainly was not the 'honest reader' to which Gilbert's preface addressed De Magnete, which was written chiefly as new science for the more intelligent reader but to entertain and hold all readers included also a deal of entertaining chat not intended to be taken seriously, so it was easy to take his experiments as the only serious science though Gilbert undoubtedly hoped that much of his basic physics theory would also be taken seriously. It is not clear if Galileo and friends also replicated Gilbert's electrical experiments though others did, but it was to be only after the 1820 Oersted discovery that electric currents also produce magnetism that real further progress was made in magnetism or electromagnetism.

Churches being inclined to the view of God as the cause of everything, led many early scientists to omitting causal theory from their science. Yet churches generally really saw God as at least largely an imaginary unseen that science would never be able to fully prove or disprove. And the churches were in fact less concerned about what caused day to day events, than with science contradicting some particular words in their holy books. So their real opposition was to science claiming that the Earth is not the centre of the universe but is just one planet of several orbiting the Sun, and to science claiming that humans were not specially created but evolved from apes.

So early scientists even claiming that almost everything was caused by God, could still be in trouble. Of course Descartes basically did just get away with claiming that everything was caused by God AND that nothing was caused by God. But in the end physics survived, in a maybe highly prejudiced form, chiefly because the power of religion in Europe gradually weakened and science was increasingly seen as being of practical use - especially for war weapon development.

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