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A question on length contraction

  1. Jun 30, 2015 #1

    rede96

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    Just wondering if someone could check that I am understating this aspect of length contraction correctly...

    I am in a space ship moving at say a speed of 0.6c relative to some imaginary ruler in space which has increments set 1 meter apart. (1 meter in the ruler's rest frame.) I am moving along its length.

    If I look forward along the length of the ruler I assume I would see these 1 meter increments reduced in length due to length contraction. But if I was to fix my gaze on a fixed point somewhere in the distance, say the 100,000 meter mark for example, then I assume that as I start to reach this point and the angle I am looking at it starts to move towards 90 degrees, then as I pass perpendicular to this 100,000 meter mark, I would see the meter increments either side as being a meter too (almost a meter) in my frame. Then as I move past this point I would start to see the meter increments behind me start to reduce again.

    Is that correct?
     
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  3. Jun 30, 2015 #2

    Dale

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    What do you mean by this? Are you talking about actual visual perception (eg the angle sub tended in your visual field). Or are you talking about what you calculate after correcting for optical effects (eg the distance in your reference frame).
     
  4. Jun 30, 2015 #3

    Orodruin

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    No. In your frame, the marks on the ruler are always closer than they are in the rest frame.
     
  5. Jun 30, 2015 #4

    A.T.

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  6. Jun 30, 2015 #5

    rede96

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    I know that if I was at rest wrt to the ruler that I would see the increments in the distance shorter just through visual perception, so assumed the visual perceptions would be compounded by length contraction.

    But as I understood length contraction happens in the direction of travel. So if I was at rest wrt to the ruler and I looked out at 90 degrees then I would measure the increments to be 1 meter in my frame too. But I was wondering if I was moving parallel to some imaginary ruler with increments set at 1 meter intervals in its rest frame, if I look at the ruler at 90 degrees to my direction of travel, would I still measure the increments as seen in my frame as less than 1 meter?


    Thanks, I will give that a good read.
     
  7. Jun 30, 2015 #6

    Dale

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    Can you be clear? Are you asking about visual appearance or about distance measured in your reference frame.
     
  8. Jun 30, 2015 #7

    rede96

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    As regards looking at the ruler at 90 degrees in my direction of travel, then distance measured in my FOR.
     
  9. Jun 30, 2015 #8

    A.T.

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    Your looking direction is irrelevant for measurements of these distances. You are confusing visual impression with measurement.
     
  10. Jun 30, 2015 #9

    Dale

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    The distance measured in your FOR is 0.8 m between adjacent marks, regardless of whether those marks are in front, to the side, or behind you.

    The anisotropy of length contraction has to do with the direction of travel, not the direction of looking.
     
  11. Jun 30, 2015 #10

    rede96

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    Thanks, that is what I wanted to know, and that it is a constant. (EDIT: Constant assuming no change in relative velocity.) As A.T. stated I was getting a bit confused between visual impression and measurement.
     
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