Robert Waldmann

Isaac Newton inspired many people to try to be the Newton of various fields. Unfortunately many tried the shortcut of using the same words as Newton but changing their meanings. This meant that those words had high status, because of Newton, but nothing to do with Newton’s success in fitting facts. One such word is still doing damage. It is “friction” and it is used as a general argument that empirical rejection is not a big problem for a theory. My thoughts after the jump.

Isaac Newton presented four simple equations (three laws of motion plus his law of gravity) which made it possible to predict where the planets would be to well within a small fraction of measurement error. He seems to have believed that each equation was just plain true. Certainly after 2 centuries almost all physicists believed that.

To get from the 4 hypotheses of interest to testable predictions, two auxiliary hypotheses were needed — that no force but gravity affects the location of the planets and that the Sun and the planets are rigid spheres. These are good approximations and, as far as I know, they haven’t been disproven by observations of the locations of planets.

Newton hypothesized that the four equations were true natural laws. Clearly the two auxiliary hypotheses were not true statements about planet Earth. Newton hypothesized another force which he called friction. We would now consider friction one of the effects of the electromagnetic force.

Newton inspired many people to try to be the Newton of this that or the other field of inquiry. Unfortunately, they tended to try to do this by keeping the form of Newton’s work and changing the substance. Hence we have von Clausewitz advising that one strike at “the center of mass” of the adversary and Rousseau talking about “solidarity” and assuming that solidal polities act like solid bodies. The same words were used with different meanings based on trust in their magical powers. Sometimes, this was reckless but fruitful.

Unfortunately, one of the magical words was friction. In the Principia, the Latin word appears roughly in the context of a confession that what was written so far is clearly inconsistent with masses of easily available data. Newton wrote something along the lines of, that’s because I left out friction, nonetheless the chapters on motion without friction were useful.

Thus the magical power of “friction” was its ability to make refutation of hypotheses not matter at all, because models which were wildly inconsistent with the data might be like the earlier chapters of the Principia. Granting a word power over our minds is always a mistake and this was a very serious mistake whose damaging consequences still harm economic research (which is getting much better very quickly thank you (no thanks to me)).

However, note two things. Friction doesn’t appear in the laws of motion. The first law describes motion with no forces and hence neither friction nor gravity. The second and third describe motion with forces including gravity friction and presumably other forces. Newton’s law of gravity does not assert that there is no other force. In Newton’s work on the solar system, there was a clear distinction between the hypotheses of interest and the two auxiliary hypotheses added to get testable implications.

Also the testable implications were accurate to well within measurement error.
Better measurements have since been made and Newton’s law of gravity and f=ma are both known to be false. Newton merely asserted that, since we have excellent reason to believe the 4 equations apply out there (we now know they don’t) we should hypothesize that they apply down here too.

The Newton’s of this that and the other thing don’t have any example of their model without “friction” working. The magic word “friction” is invoked the instant the theory confronts any data at all.

Now I think the real lesson is that if one takes a true statement and changes the meanings of the words, one is likely to get a false statement, that words don’t have magical powers and that the forms of theoretical physics do not have value if it is separated from the substance. But the magic word “friction” has done more damage than the others and it’s past time to insist that people rephrase arguments without using it before they are taken seriously.