Feynman diagrams in string theory

Question

I am beginning to study string theory, I have a beginner level doubt:

If we consider a Feynman torus diagram in string theory, it is a worldsheet. What does it represent? Does it actually mean that in the space time manifold, a closed string, for example, is moving forward in time, splitting into two closed strings and travelling along the two sides of the torus and then merging? Do the diagrams actually mean this?

My confusion is this: in quantum field theory a loop just shows an interaction. It doesn't actually say that the particles are going along the loops we've drawn on paper and then interacting. But the diagram in string theory seems to tell that the particles are indeed splitting, moving along the two sides of the torus and then coming together.

Answer 1

There is an excellent paper relevant to your question https://arxiv.org/pdf/1011.0456. What is usually seen as interactions between two close strings, or gravitons, which are D1 branes, moving forward/backward in time. Similarly point particles, D0 branes, the same concept is applied. For s-channel scatterings, the timelike momenta of intermediate particle implies the annihilation first and creation afterward, while t-u channel (exchange) the intermediate momenta is spacelike. That's what is reflected in the Time-ordering definition of the S-matrix. So, indeed for point particle interactions, we can have notion of particle coming and going. For strings or any extended object, the things are far more complicated, and incorporating interactions are difficult directly from second quantization.