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The absolute nature of motion

  1. Feb 26, 2012 #1
    I think the experiment of Newton's bucket is wrongly criticized.They deny the absolute velocity,it is right ,but they agnore the absolute nature of motion,which is not relative to other system,but is relative to the thing itself at the previous adjoining instant.Such as the expanding of universe and radiation of the sun.

    The expanding of universe can't be described by coordinate system,because it is a motion which is relative to itself(at the previous instant),and our universe is different every instant,it can't be threated as a inertial system.And because it is the biggest one whick contain everyting existed in it,so we certainly can't find another coordinate system to describe this motion.

    So,we deny the absolute motion for which we must find an absolute coordinate system,and we confirm the absolute nature of substance which is relative to itself.Some people will say that if we use the thing itself as reference,we can't get any movement,because: A-A=0.But A(t+Δt)is not equal to A(t).So the thing itself can and must be the really most meaningful reference.Because the reason of motion of substance is not out of the thing but in the thing itself.Inertia is an inner reason.
  2. jcsd
  3. Feb 26, 2012 #2
    If I perceived you correctly, then how do you determine whether the particle is different from its "previous self"? Unless you specify a reference frame, it is impossible to tell.

    I think that your dogma is based on the fact that a coordinate system is embedded in the fabric of space, where we can say that the particle is displaced from the previous instant. But I don't think that we are able to find such a universal reference frame.

    Even some people (someone called Mach?) proposed that inertia is also relativistic. But I cannot quite perceive the idea though, since under non-inertial reference frame, the objects seem to have an inertial force, but the rest observer experience a real useful force.
  4. Feb 26, 2012 #3


    Staff: Mentor

    That doesn't prevent it from being described by a coordinate system nor from using other coordinate systems. The usual metric is the FRW metric which can be easily expressed in several different coordinate systems:
  5. Feb 26, 2012 #4
    If I recall, Einstein liked Mach's ideas too but could not reconcile those with his own relativity with it so he moved on. I never understood its inherent appeal myself, but maybe that's because I was exposed to relativity first.....
  6. Feb 26, 2012 #5
    If I understand you correctly, you attempt to criticise Newton's bucket in an original way; however, your argument sounds like that of Descartes against which the rotating bucket was a counter example. If the motion is only related to the thing is itself, and inertia is an inner reason, then what would you predict for the water that is at rest relative to the bucket: will its surface be flat or not? And what would you predict for the twin scenario?
  7. Feb 26, 2012 #6
    Can we imagine an experiment in a weightless envirement like astronaut's training room?
    At that room ,if we can rotate a drop of water ,I think the surface of it must be concave below,it is suspending in the room,and rotating around it's own axis,so it is moving relative to itself ,and this cause the concave surface.

    The sun is also rotating around its own axis,and we can't deny that everything is moving relative to itself.
  8. Feb 27, 2012 #7
    Sorry I can't follow your reasoning: all the molecules of the water and the bucket are stationary relative to each other.
  9. Feb 28, 2012 #8
    Relative to the fixed point in spacetime of the axis of spin, does the water not gain momentum in a direction perpedicular to that axis, outwords; the farther from the axis, the greater the momentum (of each water molecule) resulting in a concave shape.

    I cannot see how the water molecules would be at rest compared to the rest of the "system" (at rest compared to spinning). In that they do move outword, but are forced up the wall of the bucket (moving up the bucket, [as opposed to down] because it's easiest). I'd guess the comparative momentum of each molecule is proportionate to the severity of the concave surface.

    Said differently why is the axis not the point from which all other motion is derived (since its the only "at rest" part of the system, when compared to the momentum [angular], for this analysis what does it matter that it is spinning in circles)?

    The bucket scenario must be more complicated, what am I missing?
    Last edited: Feb 28, 2012
  10. Feb 28, 2012 #9
    You are using a concept ("spacetime") that resembles a bit Newton's concept of "absolute space"; Descartes apparently held that only motion of objects relative to other objects can play a role in physics (that is nowadays called "relationist" theory). Thus following Descartes you are not allowed to relate to "the fixed point in spacetime": that is not an object. I think that Newton's bucket experiment (perhaps it was just a thought experiment) was meant to disprove that. After a water bucket is spinning for some time, the water should come to rest relative to the bucket thanks to friction. He could also have used a merry-go-round (if they had them): when you are sitting in a merry-go-round, you are not moving relative to the apparatus but your body is pressing against the support.

    Einstein tried to achieve, influenced by Mach, a replacement for Descartes by means of GR. However, it appears that he didn't fully succeed (despite its impressive success as theory of gravitation).
  11. Feb 28, 2012 #10
    Thanks for the reply harrylin & the clarification. I see now what was meant with at rest, yea the water would be at rest compared to the bucket.

    I'm missing your point about the concept of spacetime. I can ignore the concept of "absolute" spacetime and still the axis is a "fixed point" compared to the angular momentum. The flow of energy from axis to water cannot be ignored. Said different, cause/effect pin points the location of the axis, from which all other motion "came from", and is invariant. (would the speed of one full rotation be calculated to the same value regardless of relative [inertial] motion of an observer?).

    If the bucket is "at rest" and I move around the bucket, the water doesn't move up the wall of the bucket. I get the feeling this is a mix mash of relative motion, which here seems to lose meaning in the case of the bucket if the axis is ignored because it is not an object.

    There is a speed difference the farther out from the axis that is "absolute" motion, is it not? Axis of rotation is invariant right? The spinning bucket has a pretty deffinitive cause/effect, which every obsever would agree on. Clearly I need to read more about Mach. Among other things, thinking of displacement in this scenario and I have no idea how to "look at it"; what would displacement be measured from?

    So in Mach's view the universe spins about the bucket's axis or vice versa because of relative motion? In the scenario, the rope spins the bucket. I'd say relative motion is for things that can be measured ( I think this is a point Einstien has touched on).

    I'm gunna read about the Lense–Thirring effect wiki says Einstien thought of it as proof of "Mach's Principle". Clearly I'm not inturpreting this properly. Just read a bit about the effect, I cannot at all see how Einstien sees this as a proof of Mach's Principle or as the orgin of inertia between two bodies. It all is of course over my head, but suspect some bias from Einstien.

    Ah now I think I get it;
    the summation of the Lense–Thirring effect from all the masses in the universe amounts to water going up the wall of the bucket, figure skaters arms flinging outwards from spinning ect. Is that right?
    Last edited: Feb 28, 2012
  12. Feb 28, 2012 #11
    Angular momentum relative to what? For example, you're sitting in merry-go-round. How much angular momentum does it (the merry-go-round incl. its axis) have relative to you?
    Right. :smile:
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