Home » Science »

Physics Is About Rules, Not Facts [Uncertain Principles]


While in the library looking for something else, I noticed a book called The Trouble with Science by Robin Dunbar, whose description made it sound very much on point for my current project:

In The Trouble with Science, Robin Dunbar asks whether science really is unique to Western culture, even to humankind. He suggests that our “trouble with science”–our inability to grasp how it works, our suspiciousness of its successes–may lie in the fact that evolution has left our minds better able to cope with day-to-day social interaction than with the complexities of the external world.

Somewhat contrary to that description, the early bits (a potted history of science and the philosophy thereof, followed by some examples of research on science-y behaviors in other speices) were actually sort of in agreement with stuff that I’m saying in my book. The writing’s pretty dry, but I was basically okay with it until he starts into talking about the counterintuitive nature of modern science, and tries to illustrate this with references to physics. There’s a discussion of neutrinos that’s so awful I don’t even know how to correct it enough for it to be wrong, but even before that, he shows a deep misunderstanding of what’s going on with Newtonian physics. That part is at least based on misconceptions that are vaguely useful for the intro mechanics class I’m teaching this term, so it’s worth transcribing a bit and talking about why it’s wrong:

Strictly speaking, Newtonian physicsmust rank as the biggest confidence trick in the history of human learning: it makes all kinds of totally unrealistic assumptions about the existence of perfect vacuums, ideal gases, and frictionless processes, none of which ever occur in nature. Every experiment has to be carefully contrived to get it to work, otherwise extraneous variables are likely to produce results that bear no relationship to what the theory predicts. Try dropping a stone and a feather from the same height. Newtonian physics says that their rate of fall is governed only by the effects of gravity and is independent of their respective weights: they should hit the ground together. But, as everyone knows, they don’t: the stone will hit the ground before the feather. Aristotle concluded on the basis of everyday experiences of this kind that the rate at which objects fall depends on their weight (or mass, to use the proper physics term). Galileo had to cheat in a famous experiment to show that Aristotle was wrong: instead of dropping a feather and a stone from the Leaning Tower of Pisa, he dropped two cannon-balls of different weights. (Actually, he didn’t drop them from the Tower of Pisa at all: he rolled them down an inclined plane. But such is the mythology of science!) The effect is the same, at least in principle, since a light cannon-ball is standing in for a light feather. To show the same effect with a feather, you would have to carry out the experiment in a perfect vacuum, and that was impossible to do in the days before vacuum pumps (and even then isn’t all that easy). Instead, we assume that feathers will behave in the same way as cannon-balls and explain away the feather’s odd behavior as being due to air resistance.

Leave aside for the moment the dubious rhetorical tactic of telling the Leaning Tower story when you know it’s false. You don’t need a perfect vacuum to demonstrate the universality of free fall, just a physics textbook (the larger and heavier the better– any introductory text will do) and a dollar bill.

If you drop the book and the bill from the same height, the heavy book will, indeed, fall faster than the bill, due to air resistance. If you put the bill on top of the book, though, the book will clear a path for the bill, in a manner of speaking– the bill is in the lee of the book, and will not experience any significant force from the air. When you do that, the bill will remain right on top of the book all the way down to the ground, showing that they two fall at the same rate due to the force of gravity. No elaborate vacuum apparatus needed.

There’s a bigger problem here, though, which is the identification of Newtonian physics with a single thought experiment. Newtonian physics doesn’t consist of its most basic example; it’s a grand framework for understanding how interactions between objects affect their motion. Air resistance isn’t some extraneous force that Newtonian physics can’t hope to explain– it’s easily accommodated within the Newtonian framework. Which is why we know how to design aerodynamic cars, and efficient sailboats, and airplanes– the engineers working on those projects aren’t using quantum physics to do those designs, just Newton’s famous laws of motion.

The idea that air resistance forces somehow invalidate Newtonian mechanics is depressingly common, but it’s based on a common misconception of what physics is. Physics is not a collection of facts, it’s a set of rules for understanding the universe– in the specific case of Newtonian physics, rules governing the effect that forces have on the motion of objects. “All objects near the Earth’s surface fall at the same rate” is not a central idea of Newtonian physics, just one of the simplest predictions from it. The central ideas of Newtonian physics are the rules used to quantify the effect of interactions, chiefly the “second law of motion” which says that the rate of change of the momentum of an object is equal to the sum of all the forces acting on it.

The business with feathers and stones is a consequence of Newtonian rules, specifically the second law combined with Newton’s universal law of gravitation, which determines the strength of the gravitational force an object experiences. It’s explicitly only exactly valid in the approximation where there is no other significant force acting, but the presence of additional forces is not a problem– if you want to know the behavior of falling feathers in a room filled with air, you just add in the appropriate air resistance force, and you will get a prediction that exactly matches reality.

If Newtonian physics weren’t able to accommodate systems with more than one force acting, it wouldn’t be good for anything. You wouldn’t even need air resistance to invalidate it– I could drop one rock, and tie a string to a second rock of equal mass, and wind the string around a spool. The second rock will fall more slowly, as it also starts the spool spinning, and, hey presto, Newtonian physics is invalid. Other than, you know, the part where the falling rock tied to a rotating spool is a homework problem we assign in Newtonian physics classes.

It’s absolutely true that there’s a level of abstraction away from the everyday required to come up with Newton’s Laws in the first place. That was a big part of Newton’s genius– the realization that moving objects slow down not because it is in the nature of moving objects to slow down, but because there are other forces acting on them. To develop universal laws applicable to all motion, you need to abstract away everything complicated, and get to the imaginary simple case where only a single force acts (or even no force at all). For this reason, most of the examples we deal with in the first couple of weeks of class involve things like rocks floating in interstellar space, or simple objects sliding on icy surfaces with no appreciable friction. Those simple cases let you understand the universal rules, and see their implications.

The theory isn’t limited to simple cases, though. It’s vastly richer than that, and can accommodate basically any situation you care to throw at it, provided the speeds are low compared to the speed of light, and the masses are large enough to remove quantum effects from the problem. Though it should be noted that as stupid and abstract as they may seem, the simple cases can often be an excellent approximation to reality– see pretty much everything on Rhett’s blog.

Unfortunately, the fundamental misconception in this passage about the nature of Newtonian physics is sufficiently deep and wrong that it makes me question all the other stuff that I found sort of interesting in the book. And the utter gibberish he writes about neutrinos in the following pages even more so. I may manage to get some use out of this, in the form of pointers to other authors and some research within his own field (something in the biology/ psychology/ anthropology borderlands, apparently), but I’ll have to read anything he cites really carefully, because this does not inspire confidence that they’ll turn out to say what he says they do.


Related Posts

  • No Related Posts


You can be the first one to leave a comment.


Leave a Comment



This site is protected by WP-CopyRightPro