Saturday, June 2, 2012

Asymmetric conventions

In the blog post "Left or right?" I talked about different names for left or right.  Sometimes we need to make a choice between these 2 options in designing a system or defining a standard.  For instance, most countries drive on the right side of the road with the steering wheel on the left side of the car.  Typically screws tighten clockwise.  The hands of a clock turn "clockwise".  Faucets open by turning counterclockwise (at least in places where I live).  These choices are arbitrary as one choice is as good as another choice. Sometimes the choices are dictated by the application.  There are occasions where screws are tightened counterclockwise to prevent it from being loosened by the natural motion of the apparatus.  For instance, the left pedal on a bicycle uses this kind of reverse threads.

In electrical engineering, there are several arbitrary choices that were made.  For instance, electricity flows from the positive node to the negative node, which as it turns out, is opposite to the flow of electrons.  Similarly, magnetic field lines flow from the North pole to the South pole.  These choices result in "right-hand" rules of electromagnetic induction being part of the electrical engineer's armamentarium:  a counterclockwise current generates a magnetic field coming out of the page and a current coming out of the page generates counterclockwise magnetic field lines.  There is also a "right-hand" rule for generators and a "left-hand" rule for motors due to Fleming that tell you the direction of current and the direction of motion respectively.

Sometimes nature has inherent asymmetry.  For instance, our bodies are not symmetric.  Almost all of us has the heart slightly in the left side of the body, with the right half pumping blood to the lungs to replenish it with oxygen and the left half pumping it to the rest of the body.

As I learned in high-school chemistry, chemical compounds can also be asymmetric (this concept is called chirality), i.e. a molecule is chiral if it is different from its mirror image.  In particular amino acids are chiral.  Certain molecules can only be absorbed by the body if it is of a certain chirality.  I remember reading the science fiction story "Technical Error" by Arthur C. Clarke many years ago where a man accidentally travels through the fourth dimension and back and this resulted in his molecules having the opposite symmetry as us. His body cannot absorb the nutrients in normal food, and he is kept alive with engineered food of opposite chirality that is very expensive to manufacture.