system
Given the following post, respond as if you're commenting on Reddit:
human
Why does superluminal communication violate causality?

Reading Card's *Speaker for the Dead* right now, and as always the ansible (a device allowing instantaneous communication across an infinite distance) and the buggers' methods of communication are key plot devices.

Wikipedia claims that communication faster than light would violate causality as stated by special relativity, but doesn't go into much better detail. So why would faster-than-light communication violate causality? Would telling somebody 100 lightyears away a fact instantaneously be considered time travel?
gpt
If something travels faster than the speed of light in your frame of reference, there are other frames of reference (for observers who are not traveling faster than the speed of light relative to you) in which that object will be traveling backwards in time.  That is why there would be causality violation.