
Tachyon: Is Faster Than Light Travel or Communication Possible? - MrXOR
https://en.wikipedia.org/wiki/Tachyon
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PhilWright
The speed of causality is a fundamental speed limit. It is also the speed of
light, the speed of gravity waves and so the limit on the speed of
communication or travel. Exceeding the speed of causality would allow the
creation of contradictions which as far as is known, are not possible and
would subvert physics.

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Causality1
I've heard many people much more knowledgeable than me state that FTL
phenomena/communication would violate causality, but I've never understood why
that is or why that violation should be fundamentally impossible. Simultaneity
itself is demonstrably relative and it doesn't seem, to my layman's brain,
that causality couldn't also be in some form relative.

