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This wiki page [1] explains why whether two events happen at the same time depends on your reference frame. If you can send signals faster than light then you can send a signal to an even far away and cancel that event. In another reference frame the sending of the signal would have happened after the far-way event has already happened. Hence, due to the "relativity of simultaneity" FTL communication is equivalent to sending messages to the past.

[1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity




By sending a message in two opposite directions, we can achieve 2c speed of information transfer. How that is possible?


No, we can not, but the reason might seem subtle. S sends the same information in opposite directions to A and B. So A learns what B learns, but A can not use that to tell something to B, that S did not already know. A can not influence B, rather it is S that influenced both A and B.




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