Ok, I should have asked a stronger question :-)
If two events A and B are *not* timelike, are they
spacelike?
or [I guess this is equivalent]
If there are two events A and B such that each line
joining A to B must have at least one arc that is
spacelike or two timelike arcs that are reversed in
time relative to each other, then is there any reference
system for which A and B are simultaneous?
What I am thinking is about abnormal situations, like
one event A inside a Black Hole and the other event B
outside it.
Alberto Monteiro
