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The importance of the relative movement

  1. Jul 5, 2013 #1
    Relative movement or Observer knowledge about motion
    Are these things important about special relativity
    If its important,why and how?

    Thank you
  2. jcsd
  3. Jul 5, 2013 #2
    It's VERY important.....SR tells us everything is ' relative'...the only constant is the speed of
    light. So all inertial observers record the same 'c' for lightspeed. This happens because space and time 'conspire' together, change together in such as way that while they are dynamic [changing] depending on relative speed, good old 'c' remains the speed of light. This is called 'time dilation' for time changes and 'length contraction' for distance changes.

    As a simplistic example, if you are right alongside a house and a friend is a mile away how do you compare observations about the size of the house?? You think the house is' big', she thinks both you and the house are 'small' ...what is correct? what is real? both views are ok but different.*

    *You need some sort of a transform, an adjustment, to reflect your different positions. To adjust for such heights, you can figure comparisons using the tangent and separation for the distant observer relative to the adjacent observer. In relativity this gets more complicated since distance and time are not constant...so Lorentz transforms have been found to be the appropriate corrections at high speeds, but less than c, when space time is flat [no gravity].

    Different observers at high relative speeds are in fact separated not by fixed time and fixed distance but by the Lorentz transforms. Lorentz transforms are the mathematics to make ‘adjustments’ to bring distant observers together: their apparently different individual measurements [ observations] will then agree. The Lorentz transform is the relativistically-correct transform from one flat-spacetime reference frame to another when the relative velocities of the two frames is smaller than c....so they apply to particles with mass, not to light..which is massless.
  4. Jul 5, 2013 #3


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    What's important is movement or motion of an Observer (or any other object) relative to an Inertial Reference Frame.

    An Observer only has knowledge of the motion of another Observer or object after the fact, that is, sometime later when he can actually make a measurement of his own motion relative to some other Observer/object or the motion of another Observer/object relative to himself. He needs to use radar to make those measurements which means he has to send out his radar signals long enough prior to their reflection off the other Observer/object that they can make the round trip and get back to him so that he can make his calculations about the positions of the Observer/object as a function of his time and thereby conclude their motion. This is especially easy to do when we limit the positions of all Observers/objects to be in line.
  5. Jul 6, 2013 #4
    Why is it important to know that we do not move in special relativity?
  6. Jul 6, 2013 #5
    What do you mean? We do 'move'. One type of motion is constant velocity, another is acceleration where speed or direction of velocity changes. SR handles both.

    Are you referring to the fact that in SR there is no absolute velocity, no absolute inertial reference frame.? If so, the importance is that my velocity relative to you is the reciprocal
    of you velocity relative to me. In other words, you attribute relative motion to me, I attribute
    it to you. Using the term inertial reference frames, IRF, we are each at rest in our own IRF but see the other person and his IRF in motion. So I attribute kineticenergy of that motion to you, you attribute it to me.

    Example: A rocketship whizzes past earth....which is in motion, the ship or the earth? It depends on which IRF you choose to use, no one IRF is special...none is 'priviliged' is language sometimes used to describe that. A ship observer sees earth moving, an earth observer sees the ship moving, a moon observer sees both moving. [This example suffers a bit because especially the earth and moon, for example, are accelerating around each other as the orbit...that is not really an inertial frame of either.]

    So an issue in SR is always 'what frames are inertial' [constant velocity] and when not, are the effects of non inertial motion small enough to ignore?

    PS: If you SEARCH in these forums for 'special relativity' [top of this page] you'll find other very similar discussions. And most importantly, you'll find different people explain things using different language and different perspectives. THAT's when you can begin to get a really good feel for what is being posted.
  7. Jul 6, 2013 #6
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