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For a point particle we have light cone: enter image description here

String Theory- with it's extended body concepts- however will not admit a light cone such as this. In particular the most problematic causal issue would be the following illustration:enter image description here

From the illustration the past light cone from the endpoint of the string leading in it's direction of motion is seen as overlapping the future light cone from the opposite end.

It is the rhomboid region that I wonder about- here it seems the past and future of the string would continually interfere with one and another.

Qmechanic
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1 Answers1

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Two comments, which I think add up to an answer:

0) String theory has no off-shell formalism. (Indeed, it's not even clear that string theory has equations of motion in the traditional sense.) There are no observables measuring quantities strictly localized in the spacetime. Diagrams like the one you have above only appear inside the sum over virtual histories which defines string theory's S-matrix elements. So you can't necessarily make any physical conclusions by thinking about the position of strings in spacetime.

1) Inside the sum over histories, yes, the past and future continually interfere with each other. This always happens in Feynman integrals, even in ordinary quantum field theory. It's why antiparticles play such a crucial role in field theory; the sum over histories isn't necessarily causal, but it is when you have antiparticles and the right statistics. (In popular science, people talk about antiparticles being regular particles going backwards in time.)

user1504
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