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How to distinguish left vs right

  1. Mar 16, 2012 #1
    Hi All,
    This is a question that has been bugging me for a few years now. How can we tell left vs right without using something else as reference? The same applies for clockwise vs anti-clockwise.
    For example, the way I can tell left and right is by relating to my body. If I was a point particle, how do I tell left vs right and clockwise vs counter-clockwise?
    Particles have the concepts of chirality and helicity (which I don't understand by the way) that seem to be related to knowing left vs right. How do they "know"? Similarly, how does the magnetic field "know" which way to align based on the direction of the electric current?

    Can anyone shed some light on this? Any comments are much appreciated.
  2. jcsd
  3. Mar 16, 2012 #2
    Well certainly if you were a particle, you would have to reference off something to be in a clockwise or counter clockwise motion. Or that is, there must be some reason for this induced motion. So I don't think of it so much as knowing but rather it is simply reacting with its environment. We can then describe the particle with a clockwise or counter clockwise motion, which is definitely relative to you (look from the other side it will be the reverse.) Of course, then, this only has meaning when we can interpret it and build from it in a consistent way.
  4. Mar 17, 2012 #3


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    That's a good question. There is no difference between right and left other than the point of view. And clockwise becomes counterclockwise depending on if you look at the front or the back. Helicity is whether it is spinning clockwise or counterclockwise in the direction it is moving, so it depends on the observer. On the other hand, chirality is the difference between your right hand and your left hand. You can't transform one into the other, no matter which way you observe, without doing some mirror flip, which an observer can't do. An observer can look forward or back or move in whichever direction, but it can't mirror itself.

    Also, the right hand rule is purely a convention. The direction of the magnetic field and spin are based on this convention, but there is no physical meaning to it. We could use a left handed system if we were consistent about it. It's merely a bookkeeping system. You should not think of the "z" component of the spin or magnetic field as having anything to do with the "z" direction, but rather having everything to do with the xy plane. Similarly, "y" is actually referring to the xz plane and x is referring to the yz plane. There are actually 3 components to the magnetic field: yz, xz, xy. Physicists have just found it convenient to relabel these to -x,y,-z. Same deal with angular momentum.

    A more correct and symmetric way of handling the direction of the magnetic field is by using the electromagnetic tensor, which has 6 components, in which the electric field is given by the xx, yy, and zz components and the magnetic field is given by the -yz, xz, -xy components.
  5. Mar 17, 2012 #4
    I believe Richard Feyman answers this question in Surely You're Joking Mr. Feynman.
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