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Physical representation of direction

  1. Jun 10, 2012 #1
    Please forgive such a simple question, but how does a particle 'remember' what direction to travel in from instant to instant?

    I mean, say we take 2 particles way out in space, we kick one say along a +x axis, and the other with the same force along the -x axis....

    How is the Direction information physically represented in each particle, I mean, we give each the same energy, only difference is the vector, where is that direction vector physically stored from moment to moment?

    Thanks, apologies again for the basic question.

  2. jcsd
  3. Jun 10, 2012 #2


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    It's the momentum vector, which in the absence of forces is a constant.
  4. Jun 22, 2012 #3
    Yes, the momentum vector is the mathematical representation of the direction, my question is, what is PHYSICALLY different between the two particles? How is that mathematical vector represented?
    My guess is some kind of spacetime curvature storing the vector info??
    From each particles perspective, it can be viewed as 'stationary' and the other 'moving'...but having both particles have this view does not represent the manner in which the direction of motion is 'stored' by the universe...and stored it must be, for from moment to moment forever (without outside forces), the particles will continue on their path....so perhaps when you impart motion to one, it somehow stores its direction vector into the curvature of space around it??? i.e. energy given (and vector direction) is stored into a 'field' or 'virtual direction particle' around the moving particle??

    I know it seems a senseless question at first...and perhaps thats more due to the senselessness of the person asking the question :-)

  5. Jun 22, 2012 #4
    Well the simple answer is the two particles are different because one is moving one way and one is moving the other. There is no 'knowing' or 'storing' of information in the universe. If you push it one way it goes that way because (simply) Newton's first law.
  6. Jun 23, 2012 #5
    There appears to be a void of how 'particles' move, esp, in vacuum. I don't think anyone can explain how this technically happens. The deep nature of motion is a mystery but if you use Newton's laws and/or the equations of SR(say in the LHC), you'd be able to predict their behavior.
  7. Jun 23, 2012 #6
    This comes from right out of nowhere, so it may be total nonsense:

    With reading your mention of a curve I thought that perhaps you might be thinking of a General Relativity formula for the path of a massive object in a gravitational field (does GR cover that?), but solved with a value of zero for the gravitational field. Sounds like taking a long way to get back to Newtonian as mentioned above.

    Perhaps your question is "What causes inertia?"

  8. Jun 24, 2012 #7


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    The picture of 2 particles moving in opposite directions on the x-axis is more classical that QM.

    In QM, there is no knowing of info from "moment to moment". If, at one time, you measure the momentum of the above 2 particles precisely then their position (along the x-axis) is completely unknown, i.e. they do not "know" where they are.

    Also, for individual particles traveling freely, you get wave packet spreading. So, in a sense, they are traveling in many directions at the same time.

    If you want to also consider Feynman's Path Integral Formulation of QM, in the absence of other info, particles travel all possible paths between 2 (space-time) points. An equivalent statement is that the particles don't "know" what direction their traveling in.
  9. Jun 24, 2012 #8
    Sorry for the strange question, I was curious if the direction information could be modified without the magnitude being effected....so the vector 1,0,0 could somehow be reflected to be -1,0,0 without the magnitude of the motion being effected. Understanding how direction is stored would give some insight into the ability to do 'inertial reflection'

    If we ever want to really travel in space, we need some way to change the direction of incoming particles without having to absorb and re-emit their energy, finding a 'cheap' way to modify the direction vector alone would do this, though the how is tied to the how direction being represented....

    Last edited: Jun 24, 2012
  10. Jun 25, 2012 #9
    Science doesn't work like that. The magnitude of two equal and opposite vectors is the same, but that's purely a mathematical similarity, the two vectors are completely different. This 'inertial reflection' you speak of simply isn't possible.
  11. Jun 25, 2012 #10


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    You can't get away from the conservation of momentum, it is built into the homogeniety of space. Anything that moves and has energy has momentum. Change in direction = change in momentum. There is no "cheap" way to change the direction of an incoming particle. The "cost" is always equal to the net change in momentum (direction and magnitude).
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