Perhaps it takes a philosopher to correct a disservice done centuries ago to Sir Isaac Newton.
The philosopher is Daniel Hoek, an assistant professor at Virginia Tech.
On Sept. 5, Scientific American ran a story on Hoek, calling attention to a paper he published earlier this year in the journal Philosophy of Science.
In “Forced Changes Only: A New Take On The Law Of Inertia,” Hoek (rhymes with “hook”) asserts that Newton’s first law of motion — memorized by every high school physics student —has been mistranslated and misrepresented since a few years after Newton’s death.
The law is written in Latin like the rest of Newton’s 1687 opus “Philosophiæ Naturalis Principia Mathematica.” Most modern translations go something like this: A body at rest remains at rest, and a body in motion remains in motion, at constant speed and in a straight line, unless acted on by an external force.
It’s that word “unless” that bothered Hoek, who, as a philosopher, is attuned to fine shades of meaning.
In fact, something about the first law has bothered Hoek since he was a high school student who took physics — as well as Latin — in his hometown of Utrecht in the Netherlands.
He pondered the moon. It doesn’t go in a straight line because the earth’s gravity is pulling on it.
“And then, at some point, you think, ‘Well, then, what does this law do? When does an object move in a straight line?’ It turns out that actually … never, at least never in the actual world. I think I actually talked to my teacher about that at the time and he said, ‘Well, the law says that if there were a body that wasn’t subject to forces, then it would go in a straight line.
“But I always thought that’s still strange, to make your first law of one of the most famous principles of physics about these imaginary bodies.” In other words, why bother to make a law — and make it the first law — about bodies that don’t exist?
Newton has taken some heat over the centuries from critics including physicist Sir Arthur Eddington, who, with tongue in cheek, rephrased the famous maxim as “Every body continues in its state of rest or uniform motion in a straight line, except insofar as it doesn’t.”
A modern wag might say, “Newton is the author of the law of inertia, except that he isn’t.” At least he didn’t say it the way many students have learned it, according to Hoek.
So what, exactly, did Newton say?
First of all, Newton wrote it in Latin, not English.
“At the time, English was not the language of science, the way it is now,” Hoek said. “Newton was writing for his fellow scientists in France and Germany.” Also there was “a perception at the time that Latin … was the language of science. If you wanted to say it really right, you needed to use Latin.”
But not everyone reads Latin. In 1729, Andrew Motte published his English translation of “Principia.” Newton, having died in 1727, was not available to review it.
Motte’s version runs:
“Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impress’d thereon.”
The “unless” seems to imply that there are at least some bodies not subject to external forces. In reality, all bodies are acted upon by gravity and other forces like friction.
But for hundreds of years, textbooks copied Motte’s translation. “So physicists don’t necessarily get access to the original version,” Hoek said, although they “maybe realized that something was a little bit odd.”
(If you happen to run across a first edition of Motte’s version — mistranslation and all — at a yard sale, make an offer anyway. In 2019 a copy sold for £43,750, or about $53,600.)
The issue continued to bother Hoek as an undergraduate at the University of Oxford, where he was studying math and philosophy. “I thought, surely people can’t be right that Newton overlooked something like that, because especially by the time I was taking these philosophy of physics classes, I was very impressed that Newton is a very philosophically careful person who really thinks about his words very, very carefully. And these are not just any words. This is like, the words in the first law. So the idea that he overlooked this struck me as very implausible.”
Around 2011, Hoek went to the library at Oxford’s Worcester College. There he discovered that “Principia” was available in a newer translation by I. Bernard Cohen and Anne Whitman, the first in more than 285 years.
This 1999 work listed a number of mistakes in the Motte edition but did not call special attention to the first law. It fell to Hoek, years later, to point out how the new wording — truer to Newton’s original — clears up a centuries-old misunderstanding of what Newton meant.
The Cohen-Whitman translation reads:
“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.” (“Forces impressed” means the forces pressing or acting on the body.)
The difference between Cohen/Whitman’s “except insofar” and Motte’s “unless” isn’t trivial, says Hoek, who specializes in the philosophy of language. The new wording, unlike the old, does not suggest that bodies untouched by external forces exist.
A good paraphrase, according to Hoek, would be: “Every change in a body’s state of motion is due to forces impressed from the outside.”
Hoek gave a real-world example of the first law. A hockey puck sliding on ice moves uniformly in a straight line, except insofar as resistance from the ice and air slows it down.
So the first law really applies to real-world objects. It is not just about imaginary situations.
This isn’t just a semantic tempest in a teapot, according to Virginia Tech professor of physics John Simonetti, who described Hoek’s paper as “a wonderful thing.”
“Students are always confused as to why the first law of Isaac Newton even exists,” Simonetti said, “because it seems to them that all of the action that we ever undertake is involving the second law, and the first law doesn’t seem to have any substance at all. And it looks like the substance that might be attributed to the first law is actually present in the second law.
“The second law basically says, ‘If a force acts on an object, then that object will accelerate in the direction of the force and proportional to the force that’s acting.'”
It is necessary to have both laws because, from a logical point of view, if-then statements only work in one direction, even if the laws are mirror images. The first law says if something is accelerating, it’s because a force is acting. The second law says if a force acts, then an object will accelerate.
Another rationale Newton had for writing the first law was to dispel the Aristotelian notion that objects might move because of some inner attribute of the object itself — an idea that’s foreign to the modern mind, but which seemed plausible to toga-clad ancients discoursing in open-air forums.
Hoek’s insight has educational value for students, Simonetti said. They learn that the first law really has meaning. They also learn that “Newtonian physics isn’t just some ancient subject, that, for example, physics majors have to get past in order to proceed to the more interesting things like relativity and quantum mechanics. It’s still being discussed. And in our case, I’m going to invite Dan to come to my class and explain this to my students.”
Newtonian mechanics is still relevant in the post-Einstein age.
“For very high speeds — a considerable fraction of the speed of light — relativity is more accurate than Newtonian mechanics, and on very small scales, quantum mechanics is more accurate,” Simonetti said. But for most everyday situations, like “cars moving, people riding bikes, balls being thrown through the air … Newtonian physics is perfectly adequate for describing all those things. So engineers learn a lot of Newtonian physics.”
Apollo 8 (Dec. 21-27, 1968) was the first crewed mission to orbit the moon. As the tired crew was heading back to Earth, Michael Collins, NASA’s communicator on the ground, chatted over the radio with astronaut Bill Anders. Relaying a question from his son Michael Jr., Collins asked Anders who was driving.
Anders famously replied, “I think Isaac Newton is doing most of the driving right now.”
That it was a philosopher rather than a scientist who discovered and corrected an ancient mistake is not as strange as it might seem. The study of nature was once called natural philosophy. Newton was a natural philosopher. His great work, in English, is called “The Mathematical Principles of Natural Philosophy.” Human knowledge was once less fragmented and specialized than it is now. Philosophy still takes a broad view.
“The thing that really appeals to me is making these cross connections,” Hoek said. “This paper is a little bit about history, it’s a little bit about Latin, it’s a little bit about language, it’s a little bit about physics. It’s not like these things have nothing to do with each other. You kind of need a little bit of everything to tell that story. And that’s what I love about philosophy.”
Thanks to alert reader (and writer) Daniel Hoek, perhaps Sir Isaac is resting easier in his grave in Westminster Abbey, which, being attached to the surface of the earth, rotates around the earth’s axis once every 24 hours, while also being propelled 1/365th of an orbit around the sun and wheeling through a bit of the solar system’s grand tour around the black hole at the center of the Milky Way. The galaxy’s moving, too, relative to other galaxies. It’s all moving.
By the way, Hoek believes there’s also a problem involving Newton’s second law. But that’s another story.