The Gemara says that an asheirah has no real dimensions. The way that it describes it is
כתותי מכתת שיעוריה
The artscroll translation is:
because the required width measurement of the koreh is nullified, as if the koreh were pulverized, since an asheirah must be burned. However, a lechi has no minimum width requirement, and thus retains its status even though it must be burned.Footnote 7 in the artscroll edition says:
Although a lechi must measure ten tefachim in height and a minimal amount in width, since these measurements are nonetheless insignificant the Sages did not rule stringently in the case of a lechi (Tosafos ד''ה אבל). Maggid Mishnah (Hilchos Shabbos 17:12) explains that only those objects whose three dimensions all have required measurements lose their status when made from the wood of an asheirah (cf. the explanation of R' Avraham in Tos).I believe that this offers a pretty strong support for the laser eruv as it is agreed that the lintel of a tzurat hapetach does not have a minimum requirement.
In messechet Sukkah on 31b artscroll translates:
so that its measurement is considered pulverized.The footnote reads:
A Lulav must measure at least four tefachimin order to be valid. Since the lulav that comes from the asheirah that existed in the time of Moshe must be burned, its measurement is considered as pulverized [as though it has already been burned]. Thus, it lacks the measure required for it to be valid (Rashi).The footnote goes on to explain the pulverized measurement:
When the halachah condemns an object to be burned, it is viewed in the eyes of the halachah as if it is already reduced to ashes. In Rashi's words (sukkah 29b, chulin 89b) כל העומד לשרף כשרוף דמי, anything destined to be burned is considered like it has been burned. This is not to be understood as meaning that the item is considered non-existent, but rather that its destiny renders it insignificant. Accordingly, any object that requires a minimum size in order to be valid is considered to be lacking that size if the halachah dictates that it be burned.
Many commentators state that the disqualification pertains only to objects that are required to be one organic unit (e.g. a lulav or a shofar). If a mitzvah acn be performed with an aggregate of small particles, as in the case of s'chach, even material from an asheirah tree would be valid, as if an infinte number of leaves combined to cover the succah. Their reasoning is that the effect of the rule of the pulverized measurement is that the object is considered disintegrated, a condition that affects only something that must be a unit, such as a lulav or a shofar. It is not applicable to s'chach, which may consist of many small particles as long as it provides shade (Meiri to Eruvin 14b; Beur HaGra Even HaEzer 124:2; see also Tosafos to Eruvin 80b; Rambam Hil. Shabbos 17:12 with Maggid Misheh and Chidushei R' Chaim HaLevi; Bikurei Yaakov 630:2).Once again, the lintel of a tzurat hapetach is not similar to a lulav or a shofar. It can be made up of an infinite collection of particles, in our case light particles.
But I want to focus on the term כתותי מכתת which is translated a "pulverized" or "crushed" by Soncino.
In eruvin on daf 104a, in a completely unrelated discussion the Mishnah says as follows:
בוזקין מלח על גבי כבש בשביל שלא יחליקו
It is permissible to break up salt and scatter it on the ramp of the Altar [on shabbat] so that [the kohanim] do not slip [on it when it is wet].
Rashi discusses the meaning of the word בוזקין - to break up. He says that it is synonymous with the word כיתות - to crush as in the term כתותי מכתת שיעוריה. His source is from two places.
First he sites a gemara in massechet Yoma 22b. The gemara is looking to translate a word with the root בזק from the book of Shmuel.
Rashi's second source is from massechet Haggigah 13b. The gemara there is trying to understand the cryptic passage from the first chapter of the book of Yechezkel where the prophet tries to describe the vision of the divine chariot.
The verse in Yechezkel (1:14) says:
וְהַחַיּ֖וֹת רָצ֣וֹא וָשׁ֑וֹב כְּמַרְאֵ֖ה הַבָּזָֽקAs is their way, artscroll translates the verse as follows:
And the chayos ran to and fro like the appearance of Bazak.When they encounter Hebrew words that require more elaborate explanations they just transliterate and give multiple explanations in the commentary notes.
Bazak is a brilliant flash of light. Rashi gives four interpretation: a) like the colorful light emanating from the fire of a crucible where gold is refined; b) like the quick flame from the highly flammable leftovers of olives in a pot; c) something quickly spreading out; d) like lightning.
Metzudas Zion explains that bazak is synonymous with barak [lightning] in the previous verse, since the letters z and r in Hebrew are occasionally interchangeable.And indeed, Soncino and other translations translate the word bazak as lightning.
The malbim says that bazak is like the fire of a furnace "that is seen for an instance and is gone instantly".
When the gemara in Hagiggah (13b) discusses the meaning of the word it uses the gold refining analogy that Rashi brings in his commentary. Rashi there explains the process more in detail. Artscroll note:
Rashi explains this analogy as follows: when a goldsmith wishes to purify a quantity of gold, he builds a coal fire...[and covers it with] a dome made of earthenware. The earthenware dome has been made especially for this purpose and contains many holes. As the gold is purifying, flames shoot (italics mine) up through these holes. The flames appear in all different colors.Sounds like a laser to me!
So the Bazak is "pulverized" (כיתות) in the same way that the asheirah is "pulverized" (כיתות). The particles of the Bazak laser are floating around in a way that makes them there but not there in the same way that the pulverized ashes of teh asheirah tree are floating around making them there but not there.
It is bullet proof, but I found this interesting in addition to the halachic argument.