Thursday, November 19, 2009

Motion Sensors on Shabbat

[Our "Toronto Torah" for this week is now available here.]

I am mulling a post on Embarrassing Ringtones and Personal Creativity... but while I think that one through, here's an article from this week's Toronto Torah, a summary of a shiur I delivered last night on "Motion Sensors on Shabbat."

Note: This abbreviated discussion is not intended as a halachic ruling. Ask your posek for guidance.

May one knowingly enter a motion detector’s field on Shabbat, if he does not intend to turn on a light, cause a door to open, etc?

We discussed four different types of technology – photodetector, ultrasonic, microwave and passive infrared – in order to familiarize ourselves with the relevant halachic considerations. We then discussed four arguments for leniency in cases where one does not benefit from the light, the opened door, etc:

I am not performing an act of melachah
The Talmud Yerushalmi (Shabbat 13:6) permitted closing the door of one’s house for the sake of normal security, even if a deer is inside the house and will be trapped when the door is closed.

The Rashba (Shabbat 107a) explained that one only violates Shabbat if he performs a melachah via an action ordinarily associated with that melachah, and closing a house’s door is not an act of trapping.

Based on the Rashba, Rav Wosner, in a series of responsa (such as Shevet haLevi 3:97), permitted triggering a motion detector, arguing that walking is not an action normally associated with the melachot triggered by the detector.

However, Rav Wosner would not permit an action performed with actual intent to execute the prohibited melachah, though. Therefore, one may not walk through a sensor’s field with intent to thereby turn on a light.

[Someone asked whether one could use the same rationale to permit opening a refrigerator even if the light bulb remains lit, since opening a door is not an action normally associated with turning on a light. This is an interesting question.]

I do not care about the light
A gemara (Shabbat 103a) ruled that one is not biblically liable for improving someone else’s field on Shabbat by picking grass. This is understood to mean that one is not biblically liable for prohibited melachah if he has no personal stake in the outcome, even if the prohibited melachah is an inevitable result of his actions (פסיק רישיה דלא איכפת ליה).

Rav Moshe Stern (Beer Moshe VI Kuntrus Electric I #82) contended that there would be room for additional leniency if the prohibited melachah involved was itself prohibited rabbinically, rather than biblically.

Based on this argument, one could walk through a sensor’s area on Shabbat, so long as he did not benefit from the light that was triggered.

The light is not an inevitable result of my action
The Tur (Orach Chaim 316) wrote that one may close a box on Shabbat even if flies might be inside, without concern for the prohibition against trapping. The Taz (316:3) explained that (a) Flies might escape when the box is opened, and so they are not truly trapped, and (b) The person closing the box does not know with certainty that flies are inside. Regarding this latter argument, trapping is not considered an inevitable result (פסיק רישיה) of closing the box, and since one does not intend to trap, one may close the box.

Rav Moshe Kessler (Or Yisrael אדר ב תשנז) contended that walking through a motion detector’s field is like closing the box – one does not know whether any given step will trigger the detector. As with the leniencies we cited previously, though, one could not rely on this leniency if he intended to turn on the light.

I only trigger the light indirectly
A gemara regarding extinguishing a fire on Shabbat (Shabbat 120b) rules leniently regarding indirect action (גרמא). In a case of potential loss, or in a case in which one does not intend to perform the prohibited melachah, indirect causation is permitted.

Rav Shlomo Zalman Auerbach (Minchat Shlomo 1:10:6) considered this in discussing opening a refrigerator door on Shabbat, allowing warm air to enter and, ultimately, triggering a motor to cool the refrigerator. Rav Shlomo Zalman noted that an action is termed “indirect” if there is a lapse of time between the action and the resulting melachah, and so one may open the refrigerator if he does not intend to turn on the motor.

Given that motion detectors allow time to pass between their stimulus and their reaction, the one who walks through a sensor’s field has only performed melachah indirectly, and therefore there are grounds for leniency, assuming that there is no intent to set off the detector.


  1. Does anyone think that if these were not pervasive and the likelihood of avoiding them were becoming ever smaller, that noone would be entertaining kulot?

    Joel Rich

  2. I've actually been wondering about the refrigerator thing -- could a blind person (ie one who will not benefit from the light) open a fridge door 1) if he doesn't know if the light has been disabled and 2.) if he's pretty sure it hasn't?

    Not asking for a p'sak, just raising a related question.

  3. I just asked someone about the motion detcctor on Shabbos, and they just shrugged. thanks for the great answer!

  4. Joel-
    Usually the complaint is that the poskim don't pay attention to the pervasive needs, no?

    For anyone other than Rav Moshe Stern, the problem of inevitability (פסיק רישיה) may remain.

    As always, though, please do ask your rabbi...

  5. Agree, so it's always interesting to me to see what "the poskim" feel needs attention and where "our hands are tied".
    Joel Rich