Thursday, June 20, 2013

Hadran massechet Eruvin - Introducing The Laser Eruv!

This Friday Daf Yomi will be finishing Massechet Eruvin - according to many, one of the hardest sections of the Babylonian Talmud.  I am proud to say that this is the second time that I have finished Eruvin with Daf Yomi.  The last time was on the 19th of Av in 1998.  

Learning Eruvin this time around was a completely different experience.  Last time I was a Yeshiva student and everything was theoretical.  This time I am a community Rabbi who had the rare privilege of being able to oversee the establishment of an eruv in my community.  (For the epic saga click here.)

As part of my siyum on Massechet Eruvin, I want to introduce a new practical concept that I hope will make a real impact on the contemporary conversation of the laws of Eruvin.

The contemporary eruv is usually made up of a series of telephone wires that form a contiguous perimeter around a community.  The challenge of establishing a community eruv is finding a route that works, and convincing the utilities company to allow the Jewish community to string extra wire spans where there are gaps.  

If the city has telephone poles that meet the requirements - great.  If not, the construction of an eruv is costly and requires long conversations with the electric company, the traffic engineers, a contractor, a licenced electrician, non-Jewish neighbors (unfortunately sometimes Jewish neighbors), and eventually the chief of police or even the mayor.  Ask any community that has built an eruv - or that is trying to build an eruv - they will all tell you that it is not an easy endeavor.  

Welcome to the future:

Introducing - The Laser Eruv!

The concept is simple.  Four poles are erected at the four corners of the perimeter.  Each of the poles has a small system of mirrors set up.  A laser is perched on the top of pole #1.

The laser from pole #1 shoots towards pole #2.  The beam is caught by the mirror on pole #2 and redirected towards pole #3, caught by the mirror on #3 and redirected towards pole #4, where it is redirected towards pole #1 completing the circuit.

No more time-consuming eruv checks.  A few minutes before Shabbat the Rabbi takes out his smart phone, hits his laser eruv app which operates the laser and sets up the eruv.  The mirrors can even have censors that inform the Rabbi whether the beam hit its target or if the mirrors need adjusting.  

The concept is simple enough to understand and there are countless applications that can be added that can make constructing and maintaining an eruv easier, more convenient, and cheaper than ever before.

The question of course is, does the laser eruv meet the requirements of a kosher eruv?

That is the topic that I wish to address.  

Massechet eruvin opens up with the case of a mavoi (מבוי), a type of alleyway that was common in ancient residential neighborhoods.  The sages forbade carrying in a mavoi unless the community sets up either a koreh (קורה)- a cross beam - placed across the entrance of the alleyway - or a lechi (לחי)- a post planted at one side of the entrance of the alleyway.  

Rashi, on the first mishnah of eruvin (2a), explains that the purpose of the beam or the post is because the mavoi may be confused with a public domain and therefore this reminder is needed to clearly identify the alleyway as such so that people should not think that it is permissible to carry in the public domain.  (other say they act as a virtual wall)

Towards the end of the massechet, on page 80b, the gemara has an interesting discussion regarding the acceptability of certain materials for a koreh or a lechi, particularly an asheirah tree - a tree used for a type of idol worship.
Rav Hiyya Bar Ashi ruled: "A lechi may be made from an Aheirah tree.  Rav Shimon Ben Lakish ruled: "A koreh may be made from an Aheirah tree."  The one that permits a koreh all the more so would permit a lechi.  But the one who permits a lechi would [only permit a lechi and] not a koreh because its measurements are considered as if they are crushed (כתותי מכתת שיעוריה)
That means as follows.

A lechi must be a visible marker.  It has a minimum height of 40 inches but there is no minimum for the width or depth.  This is the accepted halachah.  The lechis that we put on telephone poles for our contemporary eruvs (while they are not the same thing as the lechis we are talking about in this case) must be at least 40 inches high but have no requirements for width or depth.  Some contemporary eruvs use strips of reflector tape for lechis even though the tape is practically one dimensional.

A Koreh on the other hand has stricter requirements.  These requirements are discussed in the third mishnah (13b).
The koreh (beam) that they were talking about must be wide enough to hold a [type of brick] of three hand breadths.  It is sufficient if the beam is one hand breadth wide in order to hold the brick.  Rabbi Yehudah ruled that it is sufficient if the beam is wide enough even if it is not strong enough.  If it was made of straw or reeds it is viewed as if it were made of metal.  
According to both opinions the beam used for a koreh must look a minimum width.  The argument is whether the beam must actually be as strong as it looks.  It is questionable if Rabbi Yehuda would extend his ruling of a straw beam to include a laser beam.  Even according to his lenient opinion the beam may still have to have some substance to it, which a laser beam is lacking.

But in the gemara on page 80b Resh Lakish is talking about an asheirah tree.  An Asheirah tree is a special tree that was designated for a particular type of idolatry.  The law is that an Asheirah must be burned.  Rashi explains that since the Asheirah stands to be burned it is as if its measurements are not real measurements.  (Rashi to Eruvin 80b and Sukkah 31b)

In the words of the Soncino edition footnote, "A thing that is condemned to be burned is regarded as burnt, and since it must be burnt it is regarded as non-existent."

The Asheirah tree is there but not there.  You may be looking at what you think is a tree, but in reality it has no measurements.  A lulav must be at least 4 hand breadths.  Your Asheirah branch looks like it is 5 hand breadths - seemingly a perfectly kosher lulav.  But really it is zero hand breadths because it is not really there.    So an asheirah is not kosher for a lulav.

An Asheirah tree is like a hologram.  It appears to be there, but it is really not there. It is not kosher in cases where there are minimum measurements required, but even according to the opinion of Hiyyah Bar Ashi on page 80b, in cases where there are no minimum requirements an asheirah is acceptable.

This is the fascinating halachic concept of the Asheirah: the branch or wood beam whose measurements have been "crushed" to non-existent particles.  It seems clear that the concept of the Asheirah beam could be applied to the laser beam, a "beam" that is there but not really there.  If an Asheirah tree can be used for an eruv, it seems clear that a laser beam would be acceptable as well.

(an interesting further "proof" of the connection between an asheirah and a laser emerged on the second to last page of eruvin (104a).  It is slightly out of scope so I made it into a different post.  Click here.)

But still, it is only the opinion of Resh Lakish that allows us to use an Asheirah for a koreh.  The halachah follows the opinion of Rav Hiyyah Bar Ashi who does not permit its use.

But the telephone wires that make up our contemporary eruvs are not korehs.  If they were, telephone wires would not be acceptable as they do not meet the width requirements of holding a brick from the mishnah on 13b mentioned above..

The contemporary eruv is actually made up of a series of doorways - known in halachah as a tzurat hapetach (צורת הפתח) - a shape of a doorway.

Each telephone pole represents a side post and the wire is the lintel.

So the question is not whether or not an Asheirah can be used for a koreh.  The question is whether an Asheirah can be used as the lintel of a tzurat hapetach.

On page 11b the Gemara discusses the definition of a tzurat hapetach.

This is perhaps the most important page in the entire messechet with regard to the laws of the contemporary eruv.  The sages argue as to the definition, and the final halachah is found in the shulchan aruch orach chaim 362:11.
What is a tzurat hapetach?  A reed on either side and a reed on top of them.  Even if the reed on top does not touch the supporting reed and there are several feet separating them.  As long as the two side reeds are at least ten hand breadths (40 inches) high and they are exactly underneath the top reed.  If they are attached from the side it is invalid.  And the side reeds must be strong enough to hold a door.
The halachah is clear.  There is no minimum measurement for the reed on top.  The mishnah brurah (362:18) says that it may even be strings.  The Aruch hashulchan (362:31) says the source for this is from the Talmud Yerushalmi that says the lintel can be a piece of reed-grass (גמי).

My contention is it that since there are no minimum requirements for its thickness it can even be an asheirah tree - or a laser beam!

This may sound like a joke, and I must admit that when I first conceived of the idea it kind of was.  But after putting much thought into it I think that I may have stumbled onto something real.

Now I will be the first to admit that I am no expert in the laws of eruv and I in no way consider myself a posek.  This is just an idea that I had.  My hope is that this idea catches the attention of some of the recognized authorities of Jewish law and they take the idea seriously enough to consider if there is any validity to it.

Most halachic authorities live either in Israel or in large cities where eruv is taken for granted.  For them and their communities this is just an interesting theory.  But for Jews in smaller communities this could be a game changer.

But I think that it is worth looking at for the big cities as well.  It may not be today or tomorrow, but is it completely inconceivable to imagine that some time in the future our communications will no longer require telephone wires?  When that happens what will the Jews in the larger communities do when their respective municipalities tell them that the poles are coming down and the eruv with them?  At that point the halachic authorities will be scrambling to figure out how communities can have eruvs.

Let's be prepared.  Forward this to your local Rabbi and see what he says.  Perhaps I am missing a vital piece of information that destroys my whole theory.  Then this will just have been an interesting exercise in Torah for its own sake.

But what if I am right?  What will that mean for the future of eruvs?

Mazal tov to all those finishing massechet eruvin.  Onward to massechet Pesachim where we will learn the laws of Pesach.  For a siyum on pesachim, let;s see if someone can figure out how to get us out of kitniyot, or better yet second day yom tov!


  1. According to my understanding, there is a fundamental distinction between a valid tzurat hapetach, which is made from a tangible item, and a laser, which is merely energy. The reason that you can see the laser beam is that it illuminates part of (impurities/ dust/ moisture) in the air. So when you turn off your laser beam, you are not taking away anything, you are merely not illuminating those air molecules/ particles.

  2. As for second day of Yom Tov - I know of one way to get out of it. It's very simple, and the Jews have been waiting thousands of years for it to be so simple.

  3. Sensor not censor

  4. light is tangible, being both wave and particle based. my issue i wondered was whether momentary breaks would invalidate on shabbos (dont know the laws either).

  5. The issue comes up with using a lechi made of ice in a normal climate. Ice is not a permanent material. Neither is light.

    Electricity is moving electrons. If we view the electrons, which are converted to photons to make a laser, they travel from the power plant to the laser, to the ground and then back to the power plant. They make this round trip several thousand times a second. If we view the photons as "dvar sh'yeish lo mamish", something that has substance, then kal v homor the electrons would be a "dvar sh'yeish lo mamish". However the electrons come from outside the techum and are therefore mukseh. However that is not the big problem.

    The photons in the laser are created constantly. That would make them nolad (a new thing) and forbidden for benefit. The laser beam can be viewed a a stream of photons with brand new photos being continuously created.

    My conclusion is that if we conclude that a laser has substance, that it is a "dvar sh'yeish lo mamish", that it would be forbidden to benefit from it. Furthermore, we would have the same problem with electric lights and electric heat. However to the best of my knowledge, no one has poskined that light is a "dvar sh'yeish lo mamish", so this is all not nogea.

  6. I think you are out of your mind! Your seem to miss every point that the Asheirah tree Can NOT be used. I hope you are not a real rabbi. This is unreal.

    1. it can't be used for a koreh, but it can be used for the lintel of a tzurat hapetach. It sounds like you didn't read the article carefully.

  7. Rabbi Jon,

    First off, I need to clarify, are you talking about visible lasers? (because it's much harder to make a visible laser than an invisible one.)

    I don't think the argument about photons/electrons being nolad exists, simply because we can't 'see' photons and electrons, so they have no bearing on halachah as far as I understand it -- photons and electrons are beyond human perception and halachah only deals with things that can be perceived by the average Jew. [not to mention we are allowed to use the shabbat candles' light and they are also photons that are technically being nolad on shabbat]

    If the laser can be seen, then you've made a very good argument for it being allowed -- however who thinks that any city/town is going to allow a glowing red (or whatever other color) eruv around their city/town on shabbat.

    If the laser cannot be seen, you have a difficult case ahead of you convincing someone it even exists at all. (again, halachah only deals with what is within human perception or so I've been taught)

    I think it's one thing to say the asheirah which can be seen and felt has no halachic middot, but another to say this thing which cannot be seen AND also happens to have no middot actually exists in some context.

    I can see the protest headlines already, "Locals fight creation of new high-tech lazer ghetto."

    Awesome idea though. I'd love to visit Omaha once you have the lazer eruv working. I think it'd be a cool project to do even if it wasn't kosher in the end simply because it would be an awesome educational tool. (which is basically what you've done with it now.)


    ps. I just ran into your 'arch nemesis' today in Jerusalem :)

    1. I am talking abut a visible laser and I am assuming that the technology will one day exist to make it easy. It can also be a hologram. Does that technology exist yet?

  8. If the source of the photons are set before Shabbat, then what is the issue? Since the צד השוה is that light is particulate, it therefor has ממשות (and if it has enough power, then even Harav James Bond would not want to get in its path...). In addition, this נולד is being used indirectly- no different then the heat or light of a lamp or furnace set before Shabbat.
    Kudos to rabbi Gross for innovative halachic thought!!!