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Perception of motion in a void

  1. Mar 18, 2010 #1
    Here's a little thought experiment for the absolute/relative debate over the nature of space:

    If I am in a void, and I perceive a small sphere some distance away from me, and it appears to be moving from left to right, is it really moving or could I be rotating to the left? How can I be sure if there is nothing else in relation to that object to judge its movement or even my own movement? I could be standing on a platfrom that is slowly rotating to the left while the small sphere remains stationary in the distance. It's kind of like when you see the sped up videos of the night sky and the stars seem to be moving around Earth when in reality we know we are the ones moving. I suppose if there were more than 1 object, and if one appears to move past the other, then we could say it was actually moving. But if they both moved at an apparent equal distance from one another and in perfect synchrony, I suppose we once again cannot be sure.

    Any thoughts?
  2. jcsd
  3. Mar 18, 2010 #2
    To be honest, this has always bugged me, with wormholes and all, supposedly these are shortcuts through space. But space has no 'location' as far as I know, matter inside space has, but there's no way to identify the location of a 'piece of space', that I know of, so it doesn't really strike me, how can a wormhole then be anchored to some piece?

    And if it is, I mean, the galaxy rotates and and moves through the cosmos and all that crap right? The wormhole itself should be some piece of matter with a location that is subservient to gravity to stay in pull in the galaxy and orbit its centre with the same speed as the objects around it for it to have any use, right?
  4. Mar 18, 2010 #3
    If it appeared you or the ball was going left to right to left, you would know, as there is the Higgs Field and thus you would feel inertia.

    But if you were sleeping, and you woke up with you or the ball spinning one way with no changes in direction (no acceleration), you wouldn't know. But, IN PRINCIPLE, you could know, as whichever is moving has more mass than it did before, BUT THEN AGAIN, it's just you and the ball existing.

    If this scenario was happening, one of you would ACTUALLY be moving, and the other still, because Newton showed in his Bucket argument that things can be said to be relative to Absolute Space.
  5. Mar 18, 2010 #4
    Doesn't the bucket argument only apply to rotation or am I missing your point here?

    Moving in a curve means accelerating, means change in motion, means change in energy.

    As far as I know, the deal is that you can detect a change in motion in the absolute sense, because a change means a change in energy. However, since there's only void, you, and the ball, the energy for the ball to change must some how come from you, and I'm not sure what that would exactly mean for you detecting the ball's change.
  6. Mar 18, 2010 #5
    I was vauge, I mishmashed different points.

    Yeah that's exactly what I meant :)

    I was also saying that if you and the ball existed in empty spacetime, and ONE of you were changing directions, you would know, as you would be able to detect inertia. (That ISN'T the example in the OP, but I thought I'd state it anyway)

    I was also adressing the OPs reference-frame questions, stating that Newton's bucket argument shows that Absolute Space can act as a reference frame.
    Last edited: Mar 19, 2010
  7. Mar 19, 2010 #6
    But still, how does the ball change velocity if only the ball and you exist in infinite and empty space?

    Well, let's say that the ball and I orbit around our common centre of gravity. We constantly accelerate then and gain energy for that from ourselves as we lose energy in another direction.

    However, since we move around each other in a circular motion we can't detect (I think) that either of us moves.

    However, if we move in an ellipsoid motion around each other, then the distance is not fixed and we can observe that we move.

    So yes, with empty space, and only you and a ball, a configuration is possible were we can perceive that we both move. But of course, that was already true as the ball could simply move parallel to you and faster than you.
  8. Mar 19, 2010 #7
    Agreed, agreed, agreed.

    This is so wrong, please ignore. Reason: it assumes more parties than (A) absolute space (B) ball (C) person.

    ALTHOUGH, it can be said that either [or a combination of] (B) and/or (C) are moving. It is, in principle, calculable, if you replace either (B) or (C) with a machine that measures its own mass. BUT, if these two parties were spinning for all infinite, this feat would be impossible as the machine would have no pre-spinning mass to refer to.

    (As you said, the act of spinning over being motionless adds energy and thus mass to the object.)
  9. Mar 19, 2010 #8
    if its just you and a ball the ball appears to be moving left or right, more than likely your both moving. you said small sphere so im assuming it has less mass than you, the fact that if either one is moving makes you find a comin center of mass which will make the other one move, even if its slightly, but if u c the ball and its too far away to have any gravatational influence on you, theres no way to tell whos actually the one moving. speed would be hard to find 2 considering its just 2 objects in total blackness
  10. Mar 19, 2010 #9
    You can't.

    This principle is called relativity. It's been around since Galileo. You can only describe an object's motion in relation to yourself. Things you see as being "at rest" could in fact be moving relative to someone else. Things you see as being "in motion" could be seen as at rest to someone else. Neither party is correct. Two people talking about an object in space will not agree on its motion unless they account for the other person's motion relative to (themself) as well.

    There ARE ways of identifying a piece of space. You just point at it. You can label points in space as long as you pick a frame first. Once you've done that, you simply draw up a map and draw a circle around your wormhole. There is nothing that prevents you from doing this.

    What you need to consider is who you share your map with. If your friend is in a different frame (they are in motion relative to you), they can still figure out where the wormhole is by taking your map and your frame into consideration.

    The harder part to do is determine what counts as the wormhole. The wormhole is not an object in space. It's not even a mass-less object like a photon. But it's position is encoded in the gravitational field defined on it.
  11. Mar 19, 2010 #10


    Staff: Mentor

    You can determine if you are rotating or not by using a gyroscope or a ring interferometer. Rotation is not relative in this sense, since it is non-inertial.

    What you cannot distinguish is inertial motion, i.e. you cannot distinguish between the small sphere moving from left to right in a straight line at constant velocity and you moving from right to left in a straight line at a constant velocity.
  12. Mar 20, 2010 #11
    I disagree, because if you were moving in an elliptical orbit around a sphere, similar to the way our planets orbit the sun, the sphere would just appear to change sizes. Yes, if it were a perfect circular orbit, it would appear as if the ball just sits in front of you, since you would have no other external objects to judge your own movement by. But if it were elliptical, your distance would change slightly at two ends and so the ball would just appear to change size a bit.

    Disregarding a gyroscope or an interferometer since the only objects in this void are you and a sphere, you still have questions concerning rotation. Again, if you were orbiting around one sphere that had enough mass/gravity to cause you to orbit it, you could not sense any movement at all, you would just perceive a sphere in front of you. Say another smaller sphere and you are orbiting the large sphere: then it would appear as if only the smaller sphere was in orbit. It would be impossible for you to tell that both you and the smaller sphere are in orbit. So it seems in a void, rotation always presents 2+ possibilities
  13. Mar 20, 2010 #12


    Staff: Mentor

    No, rotation is unambiguous. The laws of physics take a distinctly different form in a rotating coordinate system than in an inertial coordinate system. You do not even need to look and see another object, you can tell through purely local measurements whether or not you are rotating.
  14. Mar 20, 2010 #13
    And what local measurements would you have available to you inside emptyness?
  15. Mar 20, 2010 #14


    Staff: Mentor

    Gyroscopes, ring inteferometers, inner ear, spit and see if it goes in a straight line, etc. If you are measuring the position of the other object then you must have some measuring devices anyway, so don't be too over-insistent on the "emptyness" thing.
  16. Mar 20, 2010 #15
    That makes sense
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