Does the psak of Beis Din affect reality?
The Wolf has a post where he claims that Psak does not affect reality. I commented there and would like to expand on what I wrote.
There is a gemara which is quoted l'halacha which supports the idea that the psak of Beis Din affects reality. The Gemara in Nidda 45a states that a girl under 3 who loses her virginity, the virginity (hymen) will grow back. The Yerushalmi (Kesubos 1:2) comments that even if when she had relations she was over 3 but then the Beis Din made a leap year which in doing so made her at the time that she had relations under 3, it will grow back. The Yerushalmi bases this on a pasuk in Tehilim. The Pnei Moshe explains the Yerushalmi and states: אף הטבע מסכמת עליהן. Even nature agress with the psak. This is explicit that the psak changes reality. Before Beis Din declared a leap year her virginity would not have grown back, now that they declared a leap year it will grow back. This Yerushalmi is quoted l'halacha in the Rama Even Haezer Siman 20 sif 1 (relating to this halacha of a girl under 3, see the Gra there) as well as by the Acharonim in Orach Chaim Siman 55 sif 9 (relating to a boy who becomes Bar Mitzva in a leap year, again see the Gra there). We see clearly that the Beis Din declaring a leap year changes reality. If they hadn't she would not be a virgin (the hymen would not grow back), since they did she is a virgin (it does grow back).
One of the mefarshim on the Yerushalmi (Kesubos 1:2) brings another example that psak affects reality from the Tosefta in Rosh Hashana (1:10). The Tosefta assumes that the מן did not fall on Yom Tov. The Tosefta says that how long the מן that fell on erev Rosh Hashana lasted depended on the psak of Beis Din. If Beis Din made the 30th Rosh Hashana, then the מן lasted 2 days (the 29th and Rosh Hashana). However, if Beis Din made Rosh Hashana on the 31st, then the מן had to last a third day (29,30 because it didn't fall because it could have been Yom Tov, and Rosh Hashana). Again, we see that the psak of Beis Din affected the reality of when the מן rotted away.
The Ran in his Derashos in Derasha 11 seems to disagree with this. The Ran asks why is psak given to חכמים and not נביאים? He gives the following example. What happens if the חכם makes a mistake in pask and he allows you to eat non-kosher food. The Ran states that you will be harmed because non-kosher is poison. Therefore he says, why wasn't psak given to Neviim who would not make mistakes (see the derasha for his answer). We see clearly from the Ran that the psak that the food is kosher does not affect the reality that non-kosher food is poison.
The resolution of the contradiction may be as follows. The Ran is discussing a case where the חכם made a mistake and paskened against reality. The 2 examples above are where the Beis Din paskened correctly and as a result of their psak reality changed. When the psak is in error reality doesn't change.
It comes out that a psak of Beis Din which contradicts reality (and therefore is wrong) does not cause reality to change, however, a correct psak whose outcome is that reality needs to change does change reality.