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Are there laws of physics which allow you to tell left from right?

  1. Apr 24, 2009 #1
    If I consider a "geometrically symmetrical" physical system like maybe 2 identical masses connected by a spring. Now I stretch the masses apart and it is still symmetrical. So if I have a friend standing directly opposite me and facing me with the spring-mass system in between us the mass on my right is the mass on his left and similarly for the one on my left is the one on his right.

    If by some law of physics for the right mass causes the mass on my right moves in whatever fashion, that would mean that the mass on my friend's right moves in that same fashion. Since my right is his left and his right is my left, the conclusion is that both masses in the spring-mass system moves symmetrically. Which we know to be true from the conservation of momentum.

    This brings me to wonder if Newton's 3rd law is a result of this "symmetry", that we cannot tell left from right. Since if by some law of physics a force is exerted on B by A where A and B are identical, then a rotation of coordinates by 180 degree (like some sort of reflection in a mirror) would mean that B is now on the side where A used to be before the rotation. So if nature doesn't allow one to tell left from right, we would conclude that the force on A by B is exactly equivalent in magnitude but in opposite direction.

    Do you think my so called "analysis" is valid? Or is it just crackpot stuff? Thanks in advance! :smile:

    Small note: I am an amateur student in physics so it would be good if the replies are simple where they can be, but still address the root of the problem.
    Last edited: Apr 24, 2009
  2. jcsd
  3. Apr 24, 2009 #2


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    What what if the ball on your right was bigger than the ball on the left? There is no longer the left-right symmetry, but Newton's third law still applies.
  4. Apr 24, 2009 #3


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    Until the 1950s it was thought to be impossible. Then a particle decay was discovered that occurs more often in left than right handed forms.
    Lookup Charge-Parity-Time violation
  5. Apr 24, 2009 #4
    You are right, I realised that it was a problem too (which was why I initially have the situation for identical masses)

    However, I came up with the following idea in an embarrassing attempt "save" my previous argument, which I believe has failed.

    If I have a bigger mass A and a smaller mass B attached by a spring, and in one coordinate system A is on my left and B is on my right (I place the origin at their COM). If now the person standing exactly opposite looks at the same apparatus, A is on his right and B is on his left. For the law which governs what happens on the right, like maybe a force inwards, this force in on the smaller mass from my P.O.V but on the bigger mass from the other person's P.O.V. So the conclusion is Newton's 3rd law. And using Newton's 2nd law, I get a asymmetrical motion. From the looks of it, it seemed like geometrically symmetrical stuffs will stay symmetrical and similarly for geometrically asymmetrical stuffs will stay asymmetrical...but this sounds over generalising and silly.

    Also, I feel that I am, in some way, already putting Newton's 3rd law into the argument without realising it. Kind of like saying things tend to go from a higher potential energy state to a lower potential energy state to explain the motion of maybe two like charges which is quite the same as saying things move in the direction of the force, since F = - grad U.

    Oh well...it was a nice thought though.

    Wow that's very interesting. Seems very paradoxical (philosophically). Hopefully one day I'll understand it. :smile:

    Thanks for all the replies!
    Last edited: Apr 24, 2009
  6. Apr 24, 2009 #5
    The link between conservation laws and symmetry is a good catch for an 'amateur.' Google 'noether's theorem.'
  7. Apr 24, 2009 #6
    There are classical laws of physics that violate parity, meaning that if you look at the same event in a mirror, it is fundamentally different. One example is the Lorentz force law for charged particles moving in a magnetic field. The force F on a charged particle in a right-handed coordinate system is

    F = q(v x B)

    If you look at the same process in a mirror, and if you assume that the assume the coordinate system is still right-handed, the Lorentz Law is now

    F = -q(v x B)
  8. Apr 24, 2009 #7


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    CurlB=J also violates parity.

    F=vXB and CurlB=J together don't, which is probably why the classical laws are said not to violate parity.
  9. Apr 24, 2009 #8


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    I dispute that physicists would consider this a case of parity violation. Wouldn't the coordinate system in the "mirror world" be left-handed, and wouldn't the original law still apply using this coordinate system? I don't think you can halfway apply a transformation (to the mechanism but not the coordinate system) and then say that a parity violation has occurred.
  10. Apr 24, 2009 #9
    The third law is not derivable from the other two, if that is what you are asking. They are all independent.

    The first law tells us that inertial reference frames exist. It's a claim about the way the universe works. In other words, when an object has no resultant force acting upon it, then you can find a coordinate system in which that object's velocity is constant.

    The second law tells us how objects behave when we're using one of these blessed coordinate systems. It's incredibly useful because nature allows us to find formulas for F. (It is not a general case of the first law, despite the temptation to plug F=0 into F=ma and conclude that the first law follows from the second.)

    The third law offers the conceptual glue to tell us whether a coordinate system we've chosen is actually inertial or not. Remember the first law only tells us that they exist, not that we're in one. If an object accelerates, but there is no corresponding other object accelerating in the other direction, then by the 3rd law, we must not be in an inertial frame.
    Last edited: Apr 24, 2009
  11. Apr 24, 2009 #10
    Having said all that, why should the apparent symmetry of left and right be somehow more fundamental than Newton's 3rd law? In other words, why try to derive the law from it?

    The fact that you can turn your experimental apparatus around 180 degrees and get the same results is not an assertion of logic. It's an experimental observation. Nature didn't have to behave that way.

    Thanks for the thought-provoking post.
  12. Apr 24, 2009 #11


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