“Why There’s No Such Thing as North and South”
September 23, 2011 Leave a comment
This article from Degrees of Freedom points out that people often take certain man-made conventions for granted as actual, physical realities, using the example of north and south, and the right-hand rule (which, if you’re like me, you may vaguely remember from high school physics – here‘s a teacher’s explanation of it, here are videos showing it with respect to a magnetic field (around a wire) and magnetic force).
The human mind often confuses familiarity with understanding.
You’ve learned the basics of a field. You’ve memorized the rules and used them so many times they have become second nature, or “common sense”–which means that you have stopped asking yourself why they should be true. And now it’s often harder for you to learn a new concept than it would be if you were to start from tabula rasa.
… In EM class, students learn that an electric current generates a magnetic field. That field swirls around the space surrounding the wire similar to how the pattern of wind velocity in a hurricane wraps itself around the eye of the storm. To remember which way the field goes, students are taught something called the right-hand rule.
… Then someone comes along and claims that electromagnetism has nothing to do with handedness after all. You listen to their words but all the while your brain keeps blocking them out and instead visualizing the picture of the magnetic field around a wire. How could it ever be, your inner voice keeps repeating, that the theory of the right hand rule cannot tell left from right?
The reason is simple: the idea that the magnetic field itself points in a well-defined direction–the idea that there is a north and a south–is purely a convention.
… But is the orientation of the magnetic field really arbitrary? After all, doesn’t a bar magnet point in a definite direction? In fact, one way to define the magnetic field is by observing its effects on bar magnets, in particular on a compass. You place the compass at a point in space and you take note of which way the “N” points. If you walk your compass around the electric wire the S->N direction is always that of the magnetic field, as defined by the right-hand rule.
There is one small problem, though. Our very definition of magnetic north is itself a convention. It would not be any easier to explain to an alien what we mean by north than it would be to explain our concept right-handed.
We are accustomed to looking at maps in which north is up and south is down (although the North Pole of maps does not quite coincide with the North Magnetic Pole, which complicates things a bit). Maps point north perhaps because they were invented by people in the Northern Hemisphere, who may have found it convenient because they used the North Star for navigation. If you look at the North Star while holding up a map in front of you, it helps to be able to read the labels on the map without having to tilt your head. According to some, the tradition of putting north up and south down dates back to Ptolemy.
But there is a perfect symmetry between the north and south magnetic pole of the Earth. Nothing moves preferentially from south to north–or from north to south–except in our imagination. Auroras don’t happen any differently at the South Magnetic Pole than they do at the North Magnetic Pole. An alien arriving at Earth would certainly be able to measure the geomagnetic field, but from that he could never guess which way we have conventionally decided to point the arrow on our compasses.
While the fact that North and South are arbitrarily defined is an interesting thing to think about, this article is more interesting to me because it brings up the idea that there are some things that we just take for granted that aren’t based on reality. I would’ve liked if the author had had some more examples, because I’m sure there are other big ones – but maybe he doesn’t realize them either?