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Shabbat 39a says: "With regard to heating food in the sun itself, everyone agreed that one is permitted to place food in the sun to heat it, as it is certainly neither fire nor a typical form of cooking".

However, today we know that sun and fire, from physics point of view, are actually quite similar (they consist of matter in form of plasma). So the statement "it is certainly not fire" can be questioned. That brings me to the following question:

Can halacha be changed/adjusted, if a scientific research shows that some of the argumentation behind it was actually not valid? Has it happened, does anyone know some examples?

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I think that your final question is an excellent one, which is the subject of much debate by recent and contemporary authorities. However, the particular example you use to get to it is not a great one, since you're assuming that the word "fire," used in Talmudic discussion of Jewish law, means the same thing as the word "fire" used to describe a particular thermochemical reaction in contemporary scientific literature. –  Isaac Moses Feb 3 at 19:57
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Actually, cooking through the sun and fire are different. The sun heats the food through thermal radiation, while fire usually heats the food through conduction or convection. See enwp.org/heat_transfer. –  Ypnypn Feb 3 at 20:04
    
Thanks for the comments, yes, I'm aware of this difference between sun and fire. However, the similarity between them which was probably not considered before, brought me to the final question I posted. As Isaac says, maybe it's not the greatest example; I'd be happy to know about some other (better) ones. –  Mariana R. Feb 3 at 20:26
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The most oft-discussed example of this issue is probably that of the permission to kill lice on Shabbat based on the understanding that they are spontaneously-generated. Here's one discussion thereof. –  Isaac Moses Feb 3 at 20:47
    
On a more basic level as a bachur (foolish that I was) I received a psak from 2 Dayanim in Jerusalem that it was forbidden to smoke on Yom Tov since in our day and age it's known how harmful smoking is and the trend is to quit thus not making it "Shave L'Kol Nefesh". –  eramm Feb 4 at 10:11
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2 Answers

In the translation of Michtav M'elyahu (vol. 4 p 355), R' Carmell quotes R' Dessler that halacha does not change even if the reason given for the law seems to be untrue. He says that there may be other reasons other than the one given for the said halacha, and only the most obvious reason was the one stated, so the halacha stands without the fact associated with it.

R' Moshe Feinstein (Igros Moshe - Choshen Mishpat 2:73) says that the halacha depends on the situation at the time of the original ruling of the Sages, whether science (or our understanding of it) has changed. R' Y.B. Soleveitchik (in a recorded discourse) said a similar idea about the chazaka of "tav l'meitav ten du" - that a woman wants to get married - regarding the feminist movement and ramifications of women "becoming independent," namely that the reality at the time of Chazal is the binding reality.

However, the Pachad Yitzchok (not R' Hutner - R' Lampronti of the 18th century) concludes that we cannot kill lice on Shabbos, as the allowance to do so was based on the science of their times which Chazal accepted but we have discovered otherwise.

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Notwithstanding my agreement to the above comments about the example of the sun. –  YEZ Feb 3 at 20:50
    
Yeyasher Kochacha! This would be even more valuable with links to information about the cited authorities as well as links to source materials, where available. –  Isaac Moses Feb 3 at 21:14
    
I added some links. Can you add a link to the discourse of RYBS that you mention, or at least a more precise citation? Same goes for the Pachad Yitzchok citation. By "translation of Michtave M'elyahu," do you mean Strive for Truth? –  Isaac Moses Feb 3 at 21:33
    
Wow thanks for picking up my slack. I don't remember what the title of the recording was, but I could probably find out. –  YEZ Feb 3 at 21:37
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Harav Yishak Yosef Shelit"a, our current Rishon Lesion writes in Yalkut Yosef Kitzur Shulhan Aruch that we should never chose what science says over Hazal. There are a few reasons as to why. One of the most accepted answers is "Nishtanu HaTvaim"- nature has changed and therefor Hazal said what they said because what they said was true at their time.

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"Nature changed," by itself, would seem to be a good reason to make new rulings that apply Halacha to the new laws of nature. –  Isaac Moses Feb 4 at 1:28
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Indeed that reason says that currently chazal are not right and science is. In any event, sourcing any of this would be invaluable. –  Double AA Feb 4 at 2:48
    
Right, @DoubleAA, but it doesn't say we should accept what science says for practical halachic purposes over what chazal do. Anyway, hachamgabriel, can you cite where he says this more precisely? –  msh210 Feb 4 at 2:49
    
@msh210 it doesn't say we shouldn't either... –  Double AA Feb 4 at 2:49
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No, no, @DoubleAA, "nishtane hateva" itself doesn't imply one way or the other. But this is an answer: it says Rabbi Yosef said "we should never chose what science says over Hazal". –  msh210 Feb 4 at 2:51
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