OK, I accept that, so how can we simplify the adjudication? If we can't go by the direction the ball travels over the ground, we need to look at something else. So, what if we say that the bunker can rule on forward passes in the same way that they rule on offside plays. That is, if the passer throws the ball to another player who is in front of the passer at the moment of release, then that is deemed a forward pass; indeed it means that the receiver is offside. All it takes is for the replay to be frozen at the point of release and the relative positions of the passer and receiver checked just as happens now with kicks.
I saw in the Australian Rugby video that a truly forward pass would be almost impossible to catch but I also noted that they used the example of two players some metres apart and running at near top speed. I don't disagree with that but if the players were closer together, for instance with a short offload, this no longer applies; the ball can easily be caught and it would show up in the replay whether the receiver was offside or not.
If this was done when the referee goes to the bunker for confirmation of the try, a penalty should be given when the receiver is in front of the passer, just the same as any other offside play. I'd be interested to know what others think of this, my never ending quest for truth, justice, the American way and the detection of forward passes and offside play.
I understand where you are coming from, I just don't think it's feasible when you get to the nitty-gritty of it.
Offsides for kicks are one thing, because momentum of the kicker is irrelevant, as is the kick trajectory. But for passes, momentum of the passer is important, because as you noted, it is still possible to throw a pass backwards and have someone catch it in front of where you threw it.
So you are right, if the players were close together and not running at top-speed, it would be easier to use feet position as a marker of whether the pass went forward or not. But the question is how to make that call? How close do they have to be together and how slowly do they need to be running? Becomes pretty technical.
As I noted before, as far as I am aware, the most probable way of tracking forward passes is to have a GPS football with triangulation, so the ball itself can tell you where it moved from and take away the existing momentum. Ideally the football would constantly self-measure, be aware of the difference of being held / passed / dropped / kicked, and alert the ref when it went forwards.
But it hasn't happened yet. Maybe the physics of it is just too complicated or the measuring equipment too expensive / difficult to set up at every field?