?? signature 1+1 (?? or more generally n+n ??) ... ??? isometry switching time and space .... ??? ....

??? usual (?? ) level slip confusion here ??? .... isometry from time to space vs auto-isometry of space-time ... ??? ....

?? tachyon ... two places at once .... all points along line at once ... ???? "space-like" .... ???? ... ?? ordinary particle appears to have speed zero in some frame, while tachyon appears to have infinite speed in some frame .... ???? ....