Fthe sun blows up at some instant and four minutes later we on earth sit down to eat lunch, those two events are separated by a spacelike interval. the sun's blowup cannot possibly have influenced us at the time we sat down, because it takes 8 minutes for light to reach us from the sun. and since the interval is spacelike, there must be a reference frame in which the two events are simultaneolls. find the speed of this frame relative to the sun-earth rest frame.