Physics Diary

Galileo’s Dialogues Concerning Two New Sciences

One afternoon, during a particularly difficult day, I found my way to Galileo’s Dialogues Concerning Two New Sciences. It was a PDF version of the 1914 edition translated by Crew and de Salvio that I had randomly stumbled upon. (It has been made freely available by

I’ve learned that studying is one of the only things that brings me comfort and pleasure, and I found great joy and satisfaction in reading this edition of Two New Sciences. As I proceeded toward the work without definite aim, there were so many moments that emerged worthy of memory. One of my many favourite parts of the dialogues was Galileo’s account of the speed of light using flickering lanterns. But the dialogues are filled with many incredible moments. Take, for instance, some of the geometrical demonstrations, such as the theorem of how “the volumes of right cylinders having equal curved surfaces are inversely proportional to their altitudes”. Or the theorem presented by Galileo on the area of a circle as “a mean proportional between any two regular and similar polygons of which one circumscribes it and the other is isoperimetric with it”. [As an aside,  isoperimetric inequalities and ratios are very interesting. So, too, is the isoperimetric problem, which I’ve just begun digging into].

I am beginning to feel that bringing everything back to geometry is an important recourse we too often take for granted. The dialogues are fascinating in that they weave together and connect so many concepts and theories, like any great book – from geometry, ballistics, and acoustics to astronomy, the dialogues flow in a way that seems so rare today. Galileo’s presence, or voice, also emerges through the pages, and I find the work offers a rare opportunity to spend time with one of the great masters. Perhaps it is the clarity of the edition, but it is easy to follow Galileo from thought to thought, as though sitting beside him pondering some of the pressing physical questions of the 17th century. I like it, too, because of momentary engagements with his compatriots, even the philosopher Simplicio, a fictitious straw man, created to perform the perfect mediate that keeps the discussion between Sagredo and Galileo (Salviati) unfolding. One of Simplicio’s great passages is as follows:

What a sea we are gradually slipping into without knowing it! With vacua and infinities and
indivisibles and instantaneous motions, shall we ever be able, even by means of a
thousand discussions, to reach dry land? – Simplicio

It is a marvellous moment in the context of the first day of the dialogues, in which Galileo ponders the role of infinite numbers and issues pertaining to the Aristotelian school mechanics, among other things. It makes me think of some of the theoretical issues currently facing us in contemporary physics, as though, in some way, we’re continuously having to search for and reach dry land, and then, once we find it, the tide comes in a little bit more and pushes us a little bit further.

As a whole, it is obviously one of the great works ever produced by a human being, and certainly a work that anyone interested in physics or is studying to become a professional physicist ought to read. Galileo is one of those masterful scientists and philosophers that we hear about as kids, along with Newton, Franklin, and others. But encouragement to actually read his and other’s work would seem rare, and that is unfortunate.