How to fix Mamzeirim

Question

There is a Gemara in Kesuvos (2b–3a) where it says that if someone divorced someone on condition and then didn't fulfill the condition, the woman doesn't become divorced, she becomes un-married retroactively.

There were many discussions how to solve mamzeirim problems. Why not have the man give his wife a divorce al tnai and then not fulfill the condition BeOnes (like tell his wife on 1 Adar that she will be divorced if he doesn't come back by the 1st of Nissan and then be so far out of town then that he won't make it back on time).

From the fact that this idea was never proposed, I suspect that it isn't a valid trick.

Why not?

Answer 1

Fist of all, making women un-married retroactively is possible due to gzeira (decree) of Hazal (the rabbis), and gzeira has strength because "כל דמקדש אדעתא דרבנן מקדש" ("anyone effecting kidushin (marriage) does so intending it to be effective only to the extent instituted by Hazal") and the like.

Tosafot in Gittin 33a ד"ה ואפקעינהו proposes this, and says that in such a case the women doesn't become un-married retroactively becuase Hazal didn't make the gzera in such a case where women would willingly sin with intention of retroactively annulling the get as this would cause moral depravation.

Citation from Tosafot:

ומה שהקשה אם כן יחפה על בת אחותו ויכולין ממזרין ליטהר אי ידעינן שלכך מתכוין לא מפקעינן קדושין מיניה דלתקנה עשו חכמים ולא לתקלה שמתוך כך יהיו בנות ישראל פרוצות בעריות

Answer 2

I don't know about the above case, but my Rav was commissioned by the gedolim in eretz Yisrael to cleanse a mamzer in the U.S using a similar tactic:

The mishna says that a man threw a get to his wife in a public domain: If it lands "close to her", she's divorced. "Close to him", she isn't divorced. ""1/2 by 1/2", "she's divorced and not divorced.

The Rosh says on this doubt that the rabbis revoke their (original) marriage.

My rav explained to me that "close" means control. The case of doubt is where one party did not have control over the document more than the other party. I don't know every detail, but he set up the case where the get was thrown over a car in such a way where none of the parties had control over the get more than the other. The son was then permitted to a bas yisrael.

(Btw, my rav said he never would have done this on his own without told to do so by the gedolim.)