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Orbit and absolute motion

  1. Jul 8, 2003 #1
    First, picture a marble orbiting Earth.

    Now, get the idea of orbit out of your head. Put a huge ring around the outside diameter of a marble's potential orbit in space, and connect a mechanical hand to this ring, which holds the marble suspended above Earth. How does space know that this Earth/ring/marble system is not already spinning as a unit? If it is already spinning as a unit, then the marble will experience some outward force.

    The ability to measure relative motion by looking, visually, at other objects is not an answer to how space knows whether the marble is supposed to be in a geocentric orbit, or whether simply the Earth is spinning while the marble is carried round and let go to fall to the ground. The mechanical hand lets go of the marble. What happens?

    How does space and, or matter, know whether there is orbit or not?
    The fact that we do send objects into orbit does not answer the basic question.
  2. jcsd
  3. Jul 8, 2003 #2


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    I am tempted to respond that an object does not "know" anything, but answering in your terms: An object (and space) "knows" its position relative to the earth: that is basically what you are assuming in talking about a satellite orbiting the earth or being "held" there by a mechanical hand. The difference is the object's velocity: an object (and space) also "knows" its velocity relative to the earth. The motion of any object is completely determined by its mass, position (which determines external, i.e. gravitational, forces on it) AND it' velocity vector at each point.

    That's true because "force= ma" reduces to a second degree differential equation for position (acceleration is the second derivative of the position function) and has a unique solution for given position and velocity vectors.

    The point your "mechanical hand" is missing is the object's velocity at each point.
  4. Jul 9, 2003 #3

    First, a note on the term 'know'. I appreciate the comment. Some people will think I mean something that doesn't actually fit here. But, I prefer simple words used various ways, so that 'know' serves my purpose here. Unlike what it may appear to some people, I'm not bound by words: I use the words I already have, and mean by them what I choose in whatever context I use them. Some use the word 'experience' for the forces acting on non-living objects, so that it is said that, say, the moon experiences the gravity of earth. 'Experience'?? Thus, *'know'* works just as well for me for the thing *I'm* (talking) about here.

    If there is only one object in space, then how does space and, or, the object, know whether the object has velocity through space, has acceleration, is motionless linearly, is spinning - and in which direction and along what of any possible axis? I introduced the second object only to show the problem better, plus the problem of orbit vs. spin of the orbited (ed) body.

    I realize there must be a difference, but I cannot see what the difference could be except that space has absolute direction and coordinate. Am I missing part of the picture?

    Imagine a snapshot of a system of two objects (one massive and the other not). Either the massive one is spinning while the other is going to fall, or the other is moving round in orbit (whether or not the massive one is spinning). Simply deciding which of the two scenarios we wish to be talking about does not resolve the problem of how the objects themselves know which motion is occuring.

    It seems to me that if motion were itself relative between objects, then either orbit is impossible, or the scenario with the spin of the massive object is identical, gravitationally, to the scenario of orbit.
  5. Jul 9, 2003 #4


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    Sounds like you are treading on Zeno's paradoxes- one of which wss: at a given instant, an arrow has a specific position but NO speed: since motion requires a change in position and time, the concept of speed at a given instant makes no sense. Since, at any given time, an arrow has no speed it can't move!

    It required the development of limits and calculus to clarify that. An object does, in fact, have a speed at a given instant. An object has both momentum and energy. Space "knows" that the same way it knows an objects position.

    By the way, orbital motion requires constant acceleration and so is not "relative".
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