(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

http://books.google.com/books?id=Xm...5Dw&sig=6cUrZKqmPMoe0QBRTSYNnipNRw4#PPA114,M1

For problem 6.5 I did a lot of manipulation and I get that the following holds at the endpoints of an open string:

[tex]c = |\frac{\partial{\vec{X}}}{\partial{t}}| [/tex]

This probably seems like silly intuitive queston, but I cannot explain why this implies that the string endpoints move at the speed of light. Since X is a four-vector not a three-vector, why can we draw that conclusion?

2. Relevant equations

3. The attempt at a solution

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# Homework Help: String theory question

Can you offer guidance or do you also need help?

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