A: At the U.S. Open, Carl Pettersson was taking his backswing when his ball was hit by the ball of someone playing another hole. What would have happened if he had whiffed or made contact?
Q: Petterson would have been in the clear.
There is a specific rule for this in the Rules of Golf. It is Rule 18-1/1, which states, "If a ball at rest is moved by an outside agency, there is no penalty and the ball must be replaced."
Clearly in the case of Petterson at the Open, there was no question that the ball had moved because of an outside agency, in this case another ball from a player in another fairway.
But what if Petterson had argued that there was some outside agency that moved the ball but which no one else for sure saw? Well, that could be a different ruling. A decision in the Rules states it must be known known or virtually certain that an outside agency has moved the ball. Without that knowledge, the ball must be played as it lies. That mean Petterson could have whiffed and been charged with a stroke
Since television cameras caught the strange moment of Petterson's ball being hit by another ball rolling down the fairway, there was no question Petterson would suffer no penalty the second his ball was hit by the other ball.