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Magic Cord?

  1. Aug 1, 2010 #1
    I had a thought that's been bugging me and I wanted to ask some people who have more than a two semesters of physics about it.

    Let's say I have a cord made out of a hypothetical material that can't bend, stretch, or condense. Now, I take that cord and span it across one light year. If person A on one end of the cord pulls it, would person B on the other end instantly see the cord getting pulled, or would it take a year for person B to notice it?
  2. jcsd
  3. Aug 1, 2010 #2


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    Given your premise, B would see the effect instantaneously. This contradicts relativity, so we conclude that your premise is not consistent with relativity. According to relativity, it is not possible to have a perfectly inelastic cord.
  4. Aug 1, 2010 #3
    Hi 77. Mechanical force propagates at speed of sound through object. In the case of material that 'can't band, stretch, or condense', speed of sound would be infinite, so person B would instantly feel pulling. Of course, such material does not exist, nor it is theoretically possible to exist.
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