The photo above (via Slate) looks like a picture of a lamppost and a church steeple against a somewhat cloudy sky. But it’s not. Well, it is, but that’s not the important part for our purposes. If you look closely you’ll see a faint line shooting off the lamppost at about 11:00 if you were to superimpose a clock face. That line is actually a wire, and it’s the border of an otherwise invisible wall. And if you want to see a wire like that for yourself, you can — if you live in an area with a significant number of Jews.

To understand why you need to learn a bit about Judaism.

This is a bit of a crash course, but, good enough for us: the Sabbath — Friday night and Saturday, if you’re Jewish — is intended to be a day of rest. That means you don’t do any work, and “work” in Judaism is defined in ways that aren’t immediately obvious to others. For example, different religious texts and rabbinical interpretations bar activities like starting a fire, writing something down, tearing an object, and in the case addressed that wire, carrying something from one area to another. A prohibition on carrying seems sensible — if you’re supposed to be resting, you probably shouldn’t be lugging objects home from work anyway

But there’s are two important notes here. First, the ban on carrying isn’t focused only on large, heavy items; it’s universal, meaning you can’t carry anything. But that said, the ban isn’t on carrying altogether, just on carrying between two different areas (or, more formally, between “domains’). You can carry a glass of water from the sink to the table — that’s all within your house — but you couldn’t that same glass of water from the privacy of your home to somewhere public. It’s an imprecise line, but such is often the case with rules.

That causes a problem, though. Let’s say you were on your way to synagogue on Saturday morning, off to join other Jews in prayer. You probably want to lock the doors to your house behind you — there’s no prohibition on being prudent — but prudence also demands taking your house keys with you. Carrying your keys along the way would likely be a violation of that rule. So, a creative solution is in order. And that creative solution is called an eruv.

(Before we get there, though, I have a side story: About twenty years ago, I met an observant Jewish man who told me that, on the Sabbath, he used a rope for a belt and his house key as his belt buckle — this helped him keep his pants up, and, by the way, also enabled him to transport his key to and from the synagogue. He really loved the fact that his loophole was, literally, to use the key as a loophole.

Okay, back to the eruv.)

An eruv — “ay-roov” — is our invisible wall. Wikipedia describes it as a “ritual enclosure” which “integrat[es] a number of private and public properties into one larger private domain.” Basically: the community gets together and agrees that the entire neighborhood is one private domain, letting those observing the Sabbath carry items as if they were in their own home (with some restrictions we’ll not bother with). Eruvim (that’s the Hebrew plural, transliterated) can be very large and encompass areas with a lot of non-Jews; there are lots of rules governing their set up, as one can imagine, but even if you aren’t Jewish and had no idea that there were such thing as eruvs, you might be living within one right now. That’s perfectly OK as far as Judaism is concerned, and unless you’re really against invisible walls that you didn’t even know about, it’s probably OK with you, too.

And eruvs are very common, at least in areas with a high density of observant Jews. (Wikipedia has a list, here, which is nominally incomplete but nevertheless extensive.) And if you’re in one of those areas, you can check out the community eruv yourself by looking for the wire or, more practically, by asking a local member of the community if there is one in the first place.

But if you’re a believer who cares about the eruv from a religious perspective, whether the community has an eruv isn’t the end of the story. Even though the “wall” is primarily ritual, it still exists in tangible form — and those who observe take that seriously. If the wire isn’t up, the wall isn’t either. That’s hard to keep track off — the Manhattan eruv, for example, is miles long (here’s a map), which is way too much for any one person to monitor. As a result, the community runs a website with a status indicator displayed prominently. As of this writing, the invisible wall around most of Manhattan is in fine working order.

Bonus fact : Spider-Man is a New York Mets fan. What’s that have to do with the above? We learn about Spidey’s fandom in a comic book where he battles an odd and ultimately ineffective villan named the Wall. Per Marvel Database, the Wall was the alter ego of a part-time bricklayer who became fused with bricks during a construction accident, resulting in the guy becoming a sentient wall with no arms (but two legs). The Wall ended up going to a New York Mets game and causing chaos on the field. Spider-Man happened to be at the game as a spectator (in costume, as seen here) and intervened. As Marvel Database explains, Spidey wasn’t able to physically stop the Wall, but the umpire ejected both of them, so “Spider-Man and the Wall then retreat[ed] to a nearby park bench and talk[ed] things out.” This was the Wall’s only comic appearance.

From the Archives : The Thin Red Deer Line: The Invisible barriers that some deer won’t cross.