How the physics of light was used to theorise the curved trajectories of cannonballs

What is it about?

This paper explains how physical and geometrical principles of reflection, developed over centuries within optics, provided a new way of understanding the projectile motion of ballistics The upward path of a cannonball was treated as if it were the path of light's reflection/rebound, except that the ball was subject to gravity and so deviated from light's straight-line path. For the ball's entire parabolic curve, its upward motion was treated as if it were one of incidence and its downward motion was treated as if it were one of reflection/rebound from the parabola's high point. By conceptualising ballistic motion in this manner, Gaiileo was able to construct a novel, non-Aristotelian, theory of projectile motion based on the generalisation of core mechanical principles of optical reflection from light to matter.

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Photo by Riley McCullough on Unsplash

Photo by Riley McCullough on Unsplash

Why is it important?

In light of the fall of Aristotelian physics in the late 16th- and early 17th-centuries, this paper is important because it explains how Galileo employed the novel, non-Aristotelian, conceptual resource of optical principles of (rectilinear) projectile motion to create his new, non-Aristotelian, principles of (curved) projectile motion,. In so doing, this paper identifies for the first time the conceptual origins of Galileo's break with Aristotelian physics, which initiated the age of modern physics. as the cross-disciplinary use of laws of light.