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I read in Polchinski's book, "String Theory", page 56, that for the open string the energy momentum tensor satisfies equation (2.6.26) at a boundary $$ T_{ab}n^a t^b=0 \,, $$ where $n^a$ and $t^a$ are respectively a normal and tangent vector to the boundary of the open string, which is well represented in Fig. 2.5a (page 57).

Questions:

  1. Where does this equation explicitly come from?

    I am even more confused because I see on Tong's review at page 105 that these are Neumann boundary conditions, but I don't really see how this is related to the Neumann boundary conditions that one imposes on the end points of the open string when studying the equations of motion of Polyakov action (see equation (1.2.29) on Polchinski's book for example).

  2. How do you get equation (2.6.27)?

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