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What is the true description of motion?

  1. Jun 9, 2004 #1

    Ian

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    Imagine two bodies moving together through space; one body (the larger) moves in a more or less straight line, but the smaller body moves in a sort of zig-zag pattern weaving in and out of the larger bodies path.
    The distance separating the two masses remains more or less constant.

    how would you describe the pattern of motion, mathematically or otherwise?
     
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  3. Jun 9, 2004 #2

    Chi Meson

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    The fundimental difference between the motion of these two objects is that the smaller craft is NOT mantaining a constant velocity. Even if it keeps the same speed, it will continuously have to change direction. THis means it must accelerate.

    THe smaller craft will observe the larger craft to be zig-zagging, since the point of view of the smaller craft keeps changing, but only the smaller craft will experience the acceleration, so the people inside it will be the ones that will be "thrown" back and forth each time the shift starts to chage its direction of motion. Anyone inside this ship who tries to drink a cup of coffee will get scalded as the liquid spills over the side (just the way it does if you take a quick turn in a car).

    In the large ship, it maintains constant speed in a constant direction, so as the captain sips his (or her) cup of coffee, there will be no sloshing. So even though you cannot tell an absolute speed for either ship, you can detect and calculate an absolute acceleration for the smaller ship.
     
  4. Jun 9, 2004 #3

    turin

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    This sounds likely Bishop Berkeley's paradox. If you look at the large object with respect to the small object, then the large object will demonstrate a zig-zag whatever behavior. So the question is, which one is actually moving. You don't even have to go there; you can just think about simple rotates of the two bodies and a simple revolution of one about the other. Then, when in the frame of the other, it is the one that is rotating.

    Mathematically, there is no problem. You choose some coordinate system and then describe the motion with respect to it (I'm assuming that we are allowed to "mark on the bodies"). The paradox comes in when you try to describe it physically. Then, you will find a prefered coordinate system, at least a prefered state of orientation, against which all others are physically rotating.
     
  5. Jun 10, 2004 #4
    Essentially, placing yourself in a vehicle that is accelerating is problematic in terms of describing the dynamics of the other body. You can describe its kinetics (motion), but once you start talking about how the passengers in the other vehicle are feeling because of the forces you imagine they must be experiencing, you run into trouble.
     
  6. Jun 10, 2004 #5

    Ian

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    I posted this thread hoping to see a description of planetary motion similar to Turin's idea - 'just think about simple rotation of the two bodies and a simple revolution of one about the other'.

    My point is;
    Ptolemy insisted that the earth was fixed and all the other solar system bodies revolved around it. He was shown to be false by Copernicus, kepler, Galileo etc who showed that the sun is the fixed body and that all else revolves about the sun in elliptical orbits.
    We now know that the sun is not 'fixed' but is moving with a velocity a great deal faster than its' fastest satellite (mercury). Therefore the true path of any solar body must resemble an epicycle and not an ellipse. The elliptical orbit we see and describe is simply an illusion of sorts.

    To press home the issue;
    If we drop a ball from a 100m height above the earth we would naturally say that the ball travelled 100m in a time of t seconds. However, if we view the motion from the surface of the moon we would notice that the earth is also rotating with a velocity v and conclude that the ball travelled a total of 100m plus the distance vt metres. If we viewed the motion from the surface of the sun we would see that the earth is also orbiting the sun with a velocity v' and conclude that the ball had travelled 100m plus vt metres plus v't metres. If we viewed the motion from outside the solar system we would include the sun's velocity through space in the balls motion.
    By this, if we insist on an elliptical description of planetary motion we will never understand truly what gravitation is all about.
     
  7. Jun 10, 2004 #6

    turin

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    I find this to be an excellent (albeit slightly naive) observation. No one that I know who is serious about understanding gravity truly believes that orbits are ellipses. At the very least, the ellipses should precess (and so they aren't really ellipses, are they). But relative motion in itself is not a sufficient consideration, for it is obvious that there is some sort of prefered motion (prefered frame) in any given physical consideration.

    Out of curiosity, how much do you know about general relativity?
     
  8. Jun 10, 2004 #7

    reilly

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    True orbit?

    There is no such thing as a true planetary orbit. As Einstein pointed out, all coordinate systems are equivalent -- I'm not going to go to Mach's Prinicple --. This is pretty much the same as the notion: a discussion of the orbit in English can also be conducted in French or Russian or Japanese. The event is fundamental, the description is relative. Both Ptolmey and Copernicus are correct: the choice between the two modes of description was and is made on purely practical grounds. And, the idea of elliptical orbits in a solar centric system is well known to be an approximation, quite a good one in fact.
    Regards,
    Reilly Atkinson
     
  9. Jun 11, 2004 #8

    Ian

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    Reilly & Turin,

    To say that 'all coordinate systems are equivalent ' is a longwinded way of saying that everything exists within one universe. It is quoting the obvious, e.g, an apple is an apple, but it tells nothing apart from what is obvious.

    The problem I have with 'approximations' is that although our descriptions of motion can predict positions, times and distances to a great accuracy they tell very little of the why of physical phenomena. That essentially is what physics is all about isn't it, so that we can understand why things happen.

    Take the problem of perihelion advance; If we view the motion from the earth then we assume that the orbital motion is elliptical (a closed loop) and like Einstein we conclude that the advance is a 'rotation' of the orbit and are satisfied with expressing the rotation as an angular motion. From there we continue to look for reasons why there is a 'rotation'.

    However, it is clear that the motions of the planets are not elliptical but wavelike through space. Therefore the path is not a closed loop and the changing positions of the perihelia cannot properly be described as 'rotations' in angular units but must be an advance along the length of orbit.

    The danger in thinking like Einstein can be seen in the fact that he may have correctly predicted the magnitude of the angular advance but in his naivety he went on to say that the advance must be greater for planets nearer the sun. This is certainly true for the 'elliptical' scenario (which isn't correct) but in the planets true 3d wave motion through space the advance is the same for all planets, ~ 27833 metres along the length of orbit. In fact, if you remove the correction ( r(1-e^2) ) for elliptical orbits from Einsteins expression for the advance the units reduce to metres and predict the same value of advance of 27833 metres for all the planets regardless of distance from the sun.

    What do I know about GR? it may be an excellent number cruncher but it is terrible for theoretics - its' a blue litmus test that uses green paper, i.e, all coordinate systems are mathematically but not theoretically equivalent. :confused:
     
  10. Jun 11, 2004 #9

    turin

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    Ian,
    I don't recall saying, "all coordinate systems are equivalent." In fact, I don't think many things could be further from the truth, not even mathematically.

    What approximations are you talking about?

    I don't believe that it is the goal of physics to explain "why."

    I don't at all follow what you're talking about when you bring up this wave business.

    I don't believe Einstein thought the way you believe he did. Einstein didn't just fit his theory to angular deviations and frequency shifts. He came up with a formalism that went way deeper into explaining "why" than anyone else at the time, it is an extremely theoretical formalism that commands the evidence on the basis of reason, not a mere number crunching mechanism (perhaps you are confusing GR with QM). When he checked his theory against the known orbital precession - agreement and cheers. The other two famous tests of his theory were only predictions at the time the theory was introduced, so they are two huge jewels in the crown of GR. I don't understand how you have become so confused about GR. What books and such have you read about it?
     
  11. Jun 12, 2004 #10

    Ian

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    Turin,
    I replied to both you and Reilly (the poster immediately before you) and thought you had also read his post. Hence the quotations you are confused about.

    Think very carefully about the path of the earth through space and you will see that we do indeed trace out a wave path through space. That wave motion is what I am talking about.
    But although Einstein correctly calculated the amount of advance (and was acclaimed for it) he did not explain why the perihelion changes position.

    I have no problems as such with much of GR's calculations, only the explanations.
     
  12. Jun 13, 2004 #11

    turin

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    I think I see what you're saying now. The position of the Earth traces out a wave against some imaginary background that is fixed with respect to the other galaxies? If you, then I suppose I agree.




    What is the difference between the advance and the changing position of the perhelion?
     
  13. Jun 14, 2004 #12

    Ian

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    Turin,
    You're almost there, but what you said (about the path traced by the earth)will suffice. My point is this; the 'ellipse' that Kepler described doesn't actually exist. That may seem the case when we view our orbit from the earth but it simply isn't correct.

    Let me rephrase the other quote you referred to;
    Einstein correctly calculated the angular change in the position of Mercury's perhelion but he didn't explain why the change came about. He simply said that this was due to the fact that the gravitational effect didn't precisely obey the inverse square law except at very large distances from the sun. To be fair to him I think he meant that the variation was always there but we would only be able to observe it at close solar orbits. In the case of Pluto the deviation is still there but so small we shouldn't bother trying to see it.
    Einstein gave no other explanation than to say that his maths differed from Newtons, which tells us absolutely nothing of gravitation.

    In answer to your earlier question, I have read Einstein's three original papers from the beginning of the last century on GR, SR and the Photoelectric effect. I've also read a lot of his letters to other physicists.
    After reading his work I realised that he understood the math but not what he was describing. Consequently his errors are not mathematical but interpretational and in my view he has done more harm to the understanding of gravitation than anyone else in history.

    If you think Newton's equations for gravitational force obey the inverse square law think again, Newton didn't finish his work properly. there are further terms in his force equation that he didn't see.
    Don't you think it is logical that if Einstein expanded the classical kinetic energy expression (1/2mv*2) by the binomial theorem then the same existence of extra terms should also apply to Newton's force equation. If you add an extra term to the classical force expression (mv*2/r) in the same way as Einstein treated the k.e expression the third term in the force equation becomes (3mv*4/c*2r) which accounts for the additional force required to advance the position of perhelion. If you understand this then you will see that Newtons laws do not obey the inverse square law either.
     
  14. Jun 14, 2004 #13

    turin

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    Do you believe that our view from Earth isn't valid? What from what perspective do you believe our view would be correct?

    We definitely disagree about Einstein. I don't want to argue with you about it; I respect your opinion. (BTW, there was way more than just 1 paper on GR. 1915 is considered by most people to be the definitive one. There were at least two other widely recognized papers on GR prior to 1915, one in 1907 and then one in 1911. Einstein admitted that both of these earlier works were more like intellectual spring boards than anything else. He wrote a lot about GR after his 1915 paper, probably for the very reason that he didn't want to be misunderstood. Frankly, I don't really try to study GR from Einstein's writings. I don't at all see what this has to do with the photoelectric effect; perhaps you could enlighten me.)
     
  15. Jun 14, 2004 #14

    Ian

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    Turin,
    I only included the photoelectric effect because that was one of Einstein's papers I had read, among others. It has nothing to do with our current discussion though. OK, I agree, I don't want to argue with anyone who's kind enough to talk to me.

    I honestly do believe that our view from the earth is not valid, except for an acknowledgement of our bound solar system. If we viewed our motion from any point outside of the solar system we would do a lot better for ourselves.

    There is so much that we are blind to because we view our motion from the earth.
     
  16. Jun 14, 2004 #15

    turin

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    I think I agree with you there.
     
  17. Jun 16, 2004 #16

    Chi Meson

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    I maybe missing your point, but it seems you are arguing that there is no absolute reference frame for determining motion, that all motion is relative, and that any moving object could be thought to be motionless when observed from a different point of view.

    This is a very basic principle of physics; its one of the first things you learn in a physics class.
     
  18. Jun 16, 2004 #17

    Integral

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    Do we really view our motion from the earth frame? I think not! this was the case up till the work of Copernicus, he forced us to change to a SUN centered view of the motion. Meanwhile we can and do understand the motion of the SUN with respect to the center of galaxy. So one could argue that we have all the information needed to see the motion of the Earth with respect to the center of galaxy, since it is simply a superposition of the earth and suns motions.

    Now the motion of galaxy can only be gauged against other Galaxy's or perhaps the Cosmic Background Radiation.



    What is to be gained? You will find that on the scales necessary to measure the Earths motion against the other Galaxy's, the earths motion about the sun, and even the suns motion about our galaxy are negligible, if even detectable. What is interesting and useful is the Earths motion about the sun, perhaps someday we will have use of knowledge of the suns motion in the galaxy, till then it is interesting but not useful in a practical sense.
     
  19. Jun 18, 2004 #18

    Gza

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    From either frame, earth or sun, the laws of physics are just as valid. It just makes the calculations of the other planets trajectories easier when we consider a sun centered solar system.
     
  20. Jun 21, 2004 #19
    it's not the why, it's the how


    I have never been acquainted with a theory that talks about the why of a phenomenon. Theories (for me) only try to prove that observed phenomena
    are consistent with its set of postulates. For example, the observed trajectories
    of planets agree with Newtons first law (which is a completely counter-intuitive postulate), provided we consider that the interaction of the planet with the objects around it are inversely proportional to the square of the distance separating them. I think it is a mistake to think that physics persues to answer why type questions. Sometimes an author says " this is because such and such", and the reader might think the author has pointed out the cause. What the author has really done is shown that the phenomenon in question is consistent with the postulates at the begining of the book.

    When I first started studying physics I thought I would be tought the brilliant and fundamental insights into why-type questions, and that the super-class theoretitians that would be teaching me them, spent their research time trying to answer the ultimate why-type question. This is false. All they do is test artificious theories (but that is not necessairily a bad thing, of course). It's actualy quite fun, and that's the only reason why it's done.
     
  21. Jun 22, 2004 #20

    Ian

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    That is exactly the problem I have with physics. The theories may or may not work, some are more complex than others, but because we remove the 'why' from physics we can never actually learn anything by it. That essentially says that one day we will 'press a button' so to speak and annihilate ourselves quite unexpectedly - **it happens explains it very well.

    We have come to rely on mathematical descriptions in all theory, but maths is only the framework on which we base our understanding and does not govern why phenomena occur. You know, Newton first asked 'why' the apple fell, not 'how' it fell. There is a very subtle difference between the two and one is the answer and the other is not.
     
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