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A narrative commonly taught in schools and propagated in books is that Galileo Galilei "dropped two spheres of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass" (Wikipedia). According to the same article, "it is accepted by most historians that it was a thought experiment which did not actually take place," but their references (here and here) are less than scholarly, to say the least.

Is this consensus of historians real? If so, who's saying it and where?

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    If you have access to it, the article Galileo and the Tower of Pisa experiment in the American Journal of Physics from 1978 demonstrates that it was almost certainly a thought experiment, and that he could not have obtained the results presented in his paper by actually performing the experiment in the real world. – sempaiscuba Nov 12 '18 at 19:53
  • Galileo never made such a claim, thus there is no support for it. IIRC, Stevinus (Simon Stevin: en.wikipedia.org/wiki/Simon_Stevin) is the one that actually did a similar experiment, dropping balls down a mine shaft, years prior to Galileo's thought experiment. – Peter Diehr Nov 12 '18 at 19:57
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    @BenCrowell I wasn't aware such a site existed, but thanks for letting me know. The link is here, as the previous comment links only to the site. – Zenon Nov 12 '18 at 20:07
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    @PeterDiehr: Galileo never made such a claim, thus there is no support for it. Galileo describes such an experiment in fictionalized form, through the mouth of the character Simplicio. Also, Galileo's pupil Viviani describes such an experiment as having been done by Galileo, so there is certainly a historical source. The question is really how reliable the sources are and whether the claimed result makes sense and has context that makes sense. – Ben Crowell Nov 12 '18 at 20:13
  • @BenCrowell: Viviani: galileo.rice.edu/sci/viviani.html – Peter Diehr Nov 13 '18 at 13:01
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The paper, Galileo and the Tower of Pisa experiment, by Carl G. Adler and Byron L. Coulter (published in the American Journal of Physics 46(3), March 1978), demonstrates that it was almost certainly a thought experiment, and that he could not have obtained the results presented in his paper by actually performing the experiment in the real world. Unfortunately, the full paper is behind a paywall, but the abstract to the paper observes:

A study of Galileo's book On Motion shows that many modern textbooks misrepresent what he would have been trying to prove in the legendary Tower of Pisa experiment. Recently performed experiments and calculations based on the current theory of falling bodies also show that it is doubtful if Galileo could have obtained the results claimed in the legend.


Another paper, Laboratory Test of the Galilean Universality of Free Fall Experiment, by Christensen et al. of the Department of Physics and Astronomy, Aarhus University, Denmark, involved an experimental approach to determine whether Galileo could have obtained the results claimed. In the paper, they note that:

"Most historians do not believe that Galileo actually performed the free fall experiment, but see it rather as another thought experiment illustrating Galileo’s point."

and then state that, based on their experimental data,:

"... it can be questioned if Galileo actually did perform the experiment (or at least that he did not do it very accurately), since two objects with different mass, but otherwise identical, dropped from the Leaning Leaning Tower of Pisa would hit the ground at quite different times."

So he was either a rather poor scientist, or else never actually carried out the experiment as claimed. His inclined plane experiment suggests that he was absolutely not a poor scientist!


In the chapter about Galileo's alleged experiment at Pisa in Newton's Apple and Other Myths about Science (Harvard University Press, 2015, Edited by Ronald L. Numbers and Kostas Kampourakis), the American historian John L. Heilbron observes:

"There are several reasons for suspecting that Viviani’s story is a myth: No member of the large and literate audience supposedly present and shocked by Galileo’s disproof of received truth seems to have written a word about it. The theses ascribed to Aristotle (384–322 BCE) are taken out of context. Galileo would have been as surprised as his audience had his repeated experiments showed that all bodies fall with the same velocity."


As for the views of historians more generally, and the consensus that Galileo never actually carried out the experiment, most recent papers seem - disappointingly - to also be behind paywalls. However, the arguments put forward in the 1956 thesis: Galileo, falling bodies, and the Tower of Pisa, by Charles Phillip Fogg of Boston University give a reasonable explanation of why historians have come to that consensus.

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