Is a forward pass measured relative to the passer or the pitch?

Question

In Rugby, if the ball carrier is running at high speed, and then passes the ball to a player behind them, the ball might still actually travel forwards due to the the momentum of the passer, but end up behind them, and the recipient still be behind them when they receive it.

If this happens, is this considered a forward pass? i.e. does the referee look at the flight of the ball relative to the pitch, or relative to the player who passed the ball?

And if it's the passer, what if the passer stopped running when they released the ball? Now it would appear to move forward relative to them, and appear as a forward pass even though the pass would be the exact same if they had continued running.

Answer 1

For Union, "Forward" is defined in the definitions of the Laws:

Forward: Towards the opposition’s dead-ball line.

The relevant law is then 11.7 Throw Forward:

A player must not intentionally throw or pass the ball forward. Sanction: Penalty.

From this, it is clear that any pass which is towards the opposition's dead ball line is a forward pass, so "relative to the pitch" in your terminology.

League simply defines "Forward Pass" in the glossary:

Forward Pass is a throw towards the opponent's dead ball line.

Again, note no reference to the player, it is all to the pitch.

Answer 2

NB: This answer is based around Rugby Union

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While it seems pretty clear-cut in the Laws, as pointed out in Philips's anwser, there is actually some contention around this - with referees appearing to make the decision based on whether the ball leaves the passer's hands backwards.

This is because, as you pointed out in the question, the pass could be made backwards relative to the passer and receiver, but travel forwards because of player momentum.

Additionally, in a seeming contradiction of their own Laws, World Rugby have a video on their official YouTube channel explaining this:

This video is admittedly a little old (from 2011), however in an article on World Rugby's website, published before the Quarter Finals of the 2019 World Cup, referee Wayne Barnes says:

[...] the forward pass is the most straightforward to explain. As English referee Wayne Barnes, [...] put it: “It’s not about the direction that the ball eventually ends up, it’s about the direction it’s travelling in as it leaves the hands. So, when a TMO comes in, the referees are looking at the hands and the ball.”