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If there is a (entirely hypothetical) light which turns on automatically if there are at least two people nearby, is it forbidden to walk up to someone standing underneath it on Shabbat? And are you obligated to walk away from the area if someone approaches you (and therefore away from the automatic light)?

My intuition would be that it's forbidden to walk up to someone standing underneath it, since this would be turning it on, but you wouldn't be obligated to walk away from it if you were underneath such a light (whilst it is off) and someone was approaching you, since you wouldn't be the one turning it on. Although that then begs the question can you then walk away if the light is on, since that would turn it off.

  • Probie, welcome to Mi Yodeya, and thanks for bringing your question here! Could you edit in a link or something to indicate that such technology exists in the wild, or is this purely a theoretical question, meant to tease out some point of theory? In either case, but especially in the latter case, could you edit in more on why you suspect the proposed prohibition/obligation might exist? – Isaac Moses Apr 24 '18 at 14:56
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    It's entirely theoretical, the thought just came to me because a friend of mine uses a camera to detect whenever a second person enters his room, causing his computer play a sound. – Probie Apr 24 '18 at 15:15
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Assuming the person walking is Jewish, that the light always turns on and that the person walking benefits from its light, then it is forbidden to walk towards it intentionally. This is called psik reisha, i.e., something that you know will happen, and it is forbidden when done intentionally.

R Eliezer Melamed writes in Peninei Halacha

One may not enter a room where doing so automatically turns on the lights or air conditioning. While one might claim that he did not intend to turn on the lights or air conditioning by entering the room, the fact is that everyone knows how the system works in such places.

If the person doesn't care for the light (e.g., a streetlamp that adds light to a path you want to walk on during Shabbat, but you would have walked there anyway), then it becomes psik reisha de lo nichalei (an expected event you don't want) and many are lenient. R Melamed continues

What if the hotel guest is inside such a room when Shabbat begins, and he knows that if he leaves he will cause the lights or air conditioning to shut off? If he can easily stay inside until after Shabbat, or if a non-Jew is due to come shortly to disable the system, it is preferable to wait inside. However, if doing so causes him anguish, he may leave the room or bathroom because the purpose of this system is to save the hotel money by conserving electricity. The hotel guest does not care about that, so it is a case of psik reisha de-lo niĥa lei regarding a rabbinic prohibition (since all agree that the prohibition of turning off the lights or air conditioning is rabbinic). When necessary, in such a case, one may be lenient.

I do not see however why someone would have to walk away from the area. It would be a good idea to call out to the second person she cannot walk there (do not place a stumbling block in front of the blind) but staying there is not forbidden.

Of course, should this become a real scenario, ask a rav and don't trust Internet strangers.

  • This answer seems to apply to numerous common technological scenarios, today. DO you think you can address, or provide a link to - driveway / pathway / home lights that sense when a person arrives? Automatic doors? Fridge lights (you forgot to disable it, but you're not interested in the light at all.) A.C.'s. I think are a bit more complex. They work on thermostats and when more people are in the room it senses a higher temp. that may make it go on; maybe not. It's not the movement that causes the problem. And, here you specifically are benefitting from the A.C. – DanF Apr 24 '18 at 16:13

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