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Homework Help: Zwiebach Exercise 9.3b

  1. Oct 5, 2007 #1
    1. The problem statement, all variables and given/known data
    Apparently the answer is sqrt(2 alpha') *4a. But doesn't equation 9.10 imply that the 4a should just be a 1?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 5, 2007 #2
    It's kind of a coincidence that when I did that problem two years ago, that's what I wrote in the margin of my book. However it is not true. [itex]l_s[/itex] is just a unit of length like the Planck length. It is not necessarily the length of any string. It is useful as a unit of length when doing string theory as this problem shows. The answer is [itex]4\sqrt{2}a[/itex] in units of the string length.
    Last edited: Oct 5, 2007
  4. Oct 6, 2007 #3
    I see--we need to integrate differential lengths from sigma = 0 to sigma = 2 pi. Thanks.
    Last edited: Oct 6, 2007
  5. Oct 6, 2007 #4
    That's for closed strings. As this is an open string, the range is different.
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