Are there sources EXPLAINING why we have to make matzah within 18 minutes? I understand having a time limit if in fact the reason we eat matzah is to reenact the speed which we left Egypt (that explination is questionable as we ate matzah in Mitzraim anyway, but that is food for another question) But why 18 minutes, it seems a bit like an arbitrary number.
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The Gemara in Pesachim 46a (brought in the Rambam Laws of Chametz 5:13 and the Shulchan Aruch OC 459:2) says that if one leaves dough for the length of time that it takes to walk 1 "mil" (a Talmudic unit of distance) then it becomes Chametz. Opinions regarding the time it takes to walk a mil vary from 18-24 minutes, and here we are strict to take the shorter time.
The 18 minute mark is almost certainly a mistake. The source of this time limit is from Pesaḥim 46a
And later in Pesaḥim 93b the Gemara states that on an average day one day's journey (from sunrise to sunset) is 40 Roman miles. This would mean that on that 'average' day of 12 hours sunlight the time it takes to walk one Roman mile would be 720 minutes divided by 40 Roman miles. We have thus reached our 18 minutes.. But there's this tiny teeny little problem... the actual distance from what was the area known as Migdal Nunaiya to Tiberias is closer to 6 miles. Manuscript copyists of the Talmud Bavli wouldn't know the actual distance from Migdal to Tiberias, so this is an error that is easy to overlook for long periods of time; especially commentators who did not live in Israel nor knew archaeology might not catch it. Many would claim that this contradictory information isn't a problem and we should side with the Gemara and our commentators. Which is fine.... until you find out the Talmud Yerushalmi disagrees with the Talmud Bavli.
So we have a disagreement to the power of 4, and this is not a reconcileable difference. But hey, at least we are closer to 6 miles. But there are many who would say that we don't hold the Yerushalmi as being authoritative...but then you have variant manuscripts of the Talmud Bavli which don't mention what the actual distance between Migdal to Tiberias is. These manuscripts include the Yemenite copies of Pesahim, and there are more. Unfortunately i don't have copies of these manuscripts on hand, but they would read like the following:
If indeed the Talmud Bavli we have is in error, then the most reliable source we have is the Talmud Yerushalmi, which cites the distance as 4 mil. If this is the case, this would mean that you have approx 72 minutes rather than 18.
There are many divergent opinions regarding this topic. The Rambam in Halachot Chametz: Halacha 13 states:
The Rambam rules that as long as you are beating the dough, then even if you're beating it all day long, it will never become hametz. But he does agree with the 1 mil brought down by his manuscript of the Bavli, i don't know if he sources the Yerushalmi.
The RI'AZ is familiar with the statements of both Talmuds, and seems to try and reconcile them both by ignoring the Gemara and Rambam's statements that these times only apply to bread that is left sitting, according to his opinion:
For more information, including more precise citations of these sources, you can read Efraim Vaynman's article in Hakirah (volume 18): Chametz in Eighteen Minutes? An Inquiry into the Correct Text of the Talmud.