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Motion relative to what?

  1. Jul 4, 2013 #1
    As it's known, one of the most fundamental aspects of relativity is that all motion is relative.
    But when we apply lenght contraction, in our reference frame the object that we move relative to changes, so the distance may become shorter.
    So my question is, when said that motion is relative, is it meant relative to the rest frame of the object or relative to the object that we measure, with the length that it's assigned to it in our reference frame.
     
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  3. Jul 4, 2013 #2
    It would have helped had you framed your question on rest frames in a relatively clear manner. However, we'll try anyway.


    What is the difference? They are the same thing unless you stipulate that there is some difference, which you didn't. "You" are in your inertial or "rest" frame, and the object moving relative to you is in its (object) own inertial rest frame. You apply the Lorentz transformation for length contraction by plugging in the value of the velocity the object is moving relative to your rest frame and then you get the amount by which the object has contracted relative to your perspective.
     
  4. Jul 4, 2013 #3

    I'll sum up this.

    a) The object in its rest frame has its proper length.
    b) The object viewed from my frame is length contracted.

    If I'm moving relative to it, am I moving relative to the length contracted object or the object that has its proper length? Or both? The question really cannot be formulated simplier.
     
  5. Jul 4, 2013 #4

    Dale

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    The phrase "X is relative" simply means that the value of X depends on the reference frame that we choose.
    You ask the question as though you think that the length-contracted object and the proper-length object are different objects. They are not different objects, they are different ways of describing the same object.

    You are moving relative to the object in any reference frame. The object's length is relative, meaning that its value depends on the reference frame.
     
  6. Jul 4, 2013 #5

    ghwellsjr

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    I guess I'm having a problem understanding why you think this question is worth asking. Help me understand. The only way I could understand why you would ask this question is if you thought the two ends of the object were traveling at different speeds which would require that the object would be stretching or contracting. Is that what you're thinking?
     
  7. Jul 4, 2013 #6
    lol, there is such a thing as too simple aka vague.

    anyways I think I see what you are asking. From a physical perspective you are moving relative to a (the) length contracted object. Not both because..you are moving and that contraction is the physical reality from a causation perspective. It is not "proper length" because it is in motion compared to you. ( i think motion is the only cause of length contraction in SR)

    You do have to note the difference between the concepts of proper length like a meter stick held in your hands & calculated length...or length measured in an "instant". Like a meter stick flying by you at some "relativistic speed". Also note how RoS applies to the "ends" of the object (as determined by direction of motion)
     
    Last edited: Jul 4, 2013
  8. Jul 4, 2013 #7
    Great answer, thanks.

    So is it possible that the direction of motion varies between an observer and the contracted objected or the object with its proper length?

    Let me explain before the question becomes misunderstood. For instance, I am moving towards a length contracted object, as viewed from my frame. Is it possible that I am moving in some other direction relative to the object in its rest frame? Or does the relative motion of the length-contracted 'version' of the object correspond to the motion of the rest frame 'version' of the object?

    Sorry for my fuzziness of words, I hope you understand my question.
     
  9. Jul 4, 2013 #8
    I already wrote why I asked it, the only thing on my mind was basically the question 'does length contraction affect the properties of the motion of an object, direction or something else'.
    For instance the object may be closer to us in our frame than in some other, and I'm wondering can this affect the motion in some way.
     
  10. Jul 4, 2013 #9

    Nugatory

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    If you are moving at velocity v, as measured in an inertial frame in which I am at rest, then I am necessarily moving at a velocity -v as measured in an inertial frame in which you are at rest. Same speed, opposite direction.

    Example: I'm standing by the side of the road watching you in a car drive by, traveling due east at 100 km/hr. If you choose to consider yourself and the cart be at rest, you'll say that I and the scenery are moving due west at 100 km/hr.
     
  11. Jul 4, 2013 #10

    ghwellsjr

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    I still cannot understand your concerns because you are not following the advice I gave you in your other thread. Pick one frame. Describe the motions of all object/observers. Then if you want to switch to a different frame, use the Lorentz Transformation. There will be no confusion if you do it that way.

    In this post, you said that you are moving in your own frame. What does that mean?

    It's real simple. Objects that are moving in a frame are length contracted. Pick another frame that is moving with respect to the original one and the motions of all object can change. They can change to a new direction. They can change to a new speed. Some may be at rest. The ones that are at rest are not length contracted. The ones that are moving are length contracted along their direction of motion. The faster their speed, the more the contraction.

    I showed you some of these concepts in your first thread and in the thread that was linked to in that thread. You haven't put closure on our first thread. Are you going to keep starting new threads ignoring the answers that you have been given just like Durant did in that linked thread and got himself banned?
     
  12. Jul 4, 2013 #11

    Nugatory

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    You'll hear people saying "my frame", "his frame", "the frame of <something>" all the time. You have to be a bit careful with that terminology - we use it because it's convenient, but it's not very precise. Usually when someone says "my frame" they mean "a frame in which I am at rest" and it's no more specially theirs than any other frame.
     
  13. Jul 4, 2013 #12

    I don't understand why you criticize somebody who's just joined this forum and who has little knowledge to this? Have you every wondered that your method of showing things is wrong? Dale Spam gave me a perfectly clear and straight-forward answer in his post while you keep telling me to do stuff that you know, but I'm not good at them and I need more time to understand. You talk about constructing space time diagrams, while I talk about a completely different approach on this which for some reason isn't a good one and you keep 'being angry' at me for reason. I mean, you keep talking about these mind-boggling concepts like everybody's familiar with them and then criticize me for not following your advices for which I have no knowledge to begin with. It seems to me that you don't know how to show something with an example, or by a definition, which is what I asked for.
     
  14. Jul 4, 2013 #13

    So, basically if I'm moving away from you as viewed from a frame in which I'm at rest, you're moving away from me in a frame at which you're at rest. This has nothing to do with length contraction, right? And all observers will agree upon relative motions of two bodies relative to one another?
     
  15. Jul 4, 2013 #14

    ghwellsjr

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    Does length contraction affect motion? No. Motion of an object in a frame results in length contraction of that object. Transform to a different frame and you can get a different motion and a different length contraction. Is that such a hard concept?

    Are you thinking that transforming to different frames causes measurable, observable or otherwise noticeable differences in objects? I showed you in the link in your other thread that that is not the case. Did you read it? Are you just going to ignore my help? Are you not going to respond in your other thread?

    Again, for the umpteenth time, let the Lorentz Transformation answer these question for you. Define a scenario according to an Inertial Reference Frame (IRF) and draw a diagram, like I did in your other thread. Then transform to another IRF and draw another diagram, like I did in your other thread. Look at the diagrams. See the Length Contraction. It's so easy. Why do you refuse to accept the answer provided by Special Relativity?
     
  16. Jul 4, 2013 #15

    ghwellsjr

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    How can you move in a frame in which you are at rest?
     
  17. Jul 4, 2013 #16
    Lapsus. I meant if an object is moving away from me viewed from my frame.
     
  18. Jul 4, 2013 #17

    I don't refuse them, in fact I want them. But I believe that I'll be able to learn much easier through examples like the Einstein's train, and then relate it to Lorentz Transformations to completely grasp the concept. Imagine if you were me and somebody offers you to do maths that's mind boggling for you. It's not so easy for me as it is for you. :/
     
  19. Jul 4, 2013 #18
    Ghwellsjr, may I ask you another question that seeks a straight-forward answer?
    The object that we have in our frame gets length contracted if it's moving, but can we consider that the object in our frame is not the same object as the one in its rest frame?

    By this I mean, do we always have a 'cross-sectional object' in a moving frame, that is composed of future and past points of the object as viewed from its rest frame? I hope you understand my question.
     
  20. Jul 4, 2013 #19

    ghwellsjr

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    No, I don't understand your question.
     
  21. Jul 4, 2013 #20

    ghwellsjr

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    I think I used to be just like you trying to understand Length Contraction and Time Dilation in isolation until Dalespam finally convinced me to always use the Lorentz Transformation and then it took me a long time before I started drawing diagrams and now I have written a computer program to make it trivial. So if you specify an in-line scenario according to an IRF, I will draw it along with any additional transformed IRF's, within reason, you desire. Deal?

    But you should at least do the math for a couple events, just to convince yourself that the process is legitimate.
     
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