IF tachyons existed,would they violate causality?
Not necessarily: http://en.wikipedia.org/wiki/Tachyon#Causality
why is there so much doubt on tachyons? so a tachyon can be reinterpreted?
Because they haven't been detected and are thought to be to unstable to exist according to mainstream science I believe. I don't have anything to go off of other than that article though.
I was once crammed into a New York taxicab with Freeman Dyson and about six other people, and when Dyson was asked about the possible existence of tachyons he replied that there was an inconsistency in their thermodynamics. I believe he was referring to the vacuum instability, as described in the Wikipedia article on Tachyon Condensation.
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