A previously unnoticed misinterpretation of Isaac Newton’s first law of motion, which went unnoticed for three centuries, is shedding new light on the thoughts of the pioneering natural philosopher when he laid the groundwork for classical mechanics.
Newton’s first law is often summarized as “objects in motion tend to stay in motion, and objects at rest tend to stay at rest.” However, the history of this seemingly straightforward axiom about inertia is complex. In his 17th-century book “Philosophiae Naturalis Principia Mathematica,” written in Latin, Newton stated, “Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by the forces impressed.”
Over the centuries, many philosophers of science have interpreted this wording as referring to bodies that are not subject to any external forces, according to Daniel Hoek, a philosopher at Virginia Tech. For instance, in 1965, Newton scholar Brian Ellis paraphrased him as saying, “Every body not subject to the action of forces continues in its state of rest or uniform motion in a straight line.” However, Hoek finds this puzzling because there are no bodies in the universe that are completely free of external forces. So why would Newton formulate a law about something that doesn’t exist?
In a recent paper published in the journal “Philosophy of Science,” Hoek argued that Newton did not intend for the first law to apply solely to imaginary, force-free bodies. He suggested that Newton’s use of the Latin phrase “nisi quatenus” was not meant to specify that the law only applied to such bodies but to emphasize that motion changes only in response to a compelling force. In other words, Hoek proposed a more comprehensive paraphrase that applies to all bodies: “Every change in a body’s state of motion is due to impressed forces.”
While this distinction may seem somewhat academic, it has significance because misinterpretations of Newton’s first law have been used to argue that there are fundamental philosophical disagreements between Einstein’s general theory of relativity and Newton’s theories. Some have criticized Newton’s first law as circular, stating that it asserts force-free bodies move in straight lines or remain at rest, but how can you determine they are force-free? Well, because they move in straight lines or stay at rest.
Robert DiSalle, a historian of the philosophy of physics at Western University in Ontario, noted that Hoek’s paper makes it clear that this viewpoint is incorrect. Not only did Newton not intend to formulate a law concerning imaginary force-free bodies, but his contemporaries did not interpret it that way either. According to George Smith, a philosopher at Tufts University and an expert in Newton’s writings, the primary purpose of the first law was to infer the existence of force. During Newton’s time, it was not universally accepted that objects required a force to set them in motion; various old theories proposed that objects had their own inherent motivating force. Newton was rejecting these older ideas and asserting that there was no such thing as an object unaffected by external forces.
The confusion regarding Newton’s intent likely persisted due to a translation from Latin to English by Andrew Motte in 1729, which used the word “unless” instead of “except insofar.” This subtle difference made it appear as if Newton was discussing force-free bodies instead of explaining why all bodies respond to forces, according to Hoek.
Ramón Barthelemy, a physics education researcher at the University of Utah, praised this new interpretation as more comprehensive. He highlighted the importance of the words scientists use to convey their ideas, particularly for students, and expressed excitement that people are still discussing and debating these concepts in physics. Barthelemy emphasized that providing different interpretations can engage students and promote their involvement in physics.