To understand why there’s an 18-mile wire circling Manhattan, we need to talk a little about Judaism.
Here’s a bit of a crash course: 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 by 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.
Drawing the line
But there 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 take 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 20 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.
(OK, back to the eruv.)
An invisible wall
An eruv — “ay-roov” — is our invisible wall. Wikipedia describes it as a “ritual enclosure” that “integrates 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.
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. (There’s 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, as mentioned above, is 18 miles long, 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.