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STR 'rotation'.

  1. Aug 24, 2009 #1

    cos

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    In his book 'An Introduction to the Special Theory of Relativity' (46, East-West, 1964) Robert Katz presents Weisskopf's depiction (Physics Today, 13, 24 1960) of length contraction showing that a cube that is moving past a stationary observer will be 'interpreted' as being tilted toward the observer and raised upwards in its direction of travel whilst a photograph that he takes will not show the cube as being 'bent and rotated'.

    Is this what STR shows will take place or is it just Weisskop's 'interpretation' of what STR shows?
     
    Last edited: Aug 24, 2009
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  3. Aug 25, 2009 #2

    George Jones

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    I'm not sure, but I think Weisskopf's article is a semi-popular exposition of papers published independently in 1959 by Terrell and Penrose. Look up Terrell rotation, or Terrell-Penrose rotation, or Penrose-Terrell rotation.
     
  4. Aug 25, 2009 #3

    A.T.

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    I think it is the other way around: The cube will be measured/interpreted to be just Lorentz contracted along it's movement direction. But a camera/eye will see it rather bent and rotated, but not contracted:
    http://www.spacetimetravel.org/bewegung/bewegung5.html
    http://www.spacetimetravel.org/fussball/fussball.html
     
  5. Aug 25, 2009 #4
    i think this can vizualised in 3D view since the dimension of any object are relative to speed of the object i will think on it again if u send that picture
     
  6. Aug 25, 2009 #5

    George Jones

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    Like A.T., I think the wording in the original post could be better.
     
  7. Aug 26, 2009 #6

    cos

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    In that article http://18.181.0.31/afs/athena/course/8/...sskopf.pdf [Broken] Weisskopf (as does Katz' dia c Fig. 2-8.1) depicts the observer's view as well as his photograph showing a square face of the cube A B C D with, as a rectangular extension, the left face A B E F fig.1 Relativistic.

    Katz wrote that the photographer then interprets that photograph as representing a rotated cube. Weisskopf, above, wrote that objects appear rotated although his fig.1 Relativistic shows that it is not.

    On what basis does Katz' photographer interpret a cube that is not rotated as being rotated?

    On what basis does Weisskopf's observer see an non-rotated cube as being rotated?
     
    Last edited by a moderator: May 4, 2017
  8. Aug 26, 2009 #7

    A.T.

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    Doesn't work for me (404 File Not Found)
    These statements seem contradictory. It depends what they mean by "interpret" and "see", but the second one seems more correct to me, Although 'rotation' is not exactly correct. It's more a perspective distortion (see links in post #3)
     
    Last edited by a moderator: May 4, 2017
  9. Aug 27, 2009 #8

    cos

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    It connects in other groups but perhaps try 'The Visual Appearance of Rapidly Moving Objects' V.F.Weisskopf, (Physics Today, 13, 24, 1960).

    Having printed Weiskopf's article I find that his diagram 'Relativistic Fig. 1' shows that the side of the cube facing the observer, ABCD, is not a square as Katz depicted (and as I wrongly assumed on the basis of the screen image) but is a rectangle.

    Weisskopf (and Katz) include in their diagrams the left-hand face of the cube ABEF also as a rectangle however the 'rotation' is obviously nothing more than a perspective distortion created by the aberration of light.
     
  10. Jan 20, 2010 #9
    In STR an observer in a given reference frame is actually a group of many observers all at rest with respect to each other and all with synchronised clocks. There are so many observers, that at any given time there would be one observer right next to a given part of the cube, eg a corner. If they plot the locations of the observers that happen to be right next to a corner at a given time, they would construct a composite measurement of the cube on a map, to be length contracted in its direction of travel, but otherwise undistorted or rotated. This is a measure, rather than what any individual sees. This method of measuring the moving cube elliminates all distortions due to light travel times.
    Ok, we should be clear that what a single observer "sees" and what a camera records is essentially the same. In the original Penrose-Terrell analysis, they went so far as to specify a camera with a curved backplate to better simulate a human eyeball. What the eye or camera sees at the time the image is recorded on the back of the camera or eye, is a composition of points on the cube at different times. Light from the trailing edge of an aproaching cube left at a time when the cube was further away, than where the cube was when light left the leading edge of the cube. The end result of the distortion due to light light travel times, is that a moving object appears rotated and stretched. This stretching distortion is largely cancelled by actual length contraction, so that so that in the case of a small sphere, the sphere appears neither stretched nor compressed (only rotated) to a single observer. It should be noted that the normal appearance of a moving sphere to a single observer or camera, requires the sphere to be length contracted in the first place.

    It would appear that you have misinterpreted Weisskop's 'interpretation', which is consistent with STR.
     
    Last edited: Jan 20, 2010
  11. Jan 20, 2010 #10
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