How does an eruv work? Is it a loophole?

There is a biblical prohibition not to carry between a private domain (e.g. one’s home) to a public domain (often defined as a space which 600,000 travelers traverse in a single day).

As a safeguard against accidentally violating this proscription, Rabbis instituted rules against carrying from areas that are demarcated as private (rashut hayahid) to public area (rashut harabim), and vice-versa. Consequently, Jews observing the laws of Shabbat are not permitted to carry objects from a private to a public domain on the Sabbath and Yom Kippur.

However, the same Rabbis that created the safeguard which extended the rules against carrying in this non-private/non-public area (called Karmalit in Hebrew), also created the concept of an eruv which converts the space to a private domain, thus allowing people to carry objects.

Since the eruv acts as a large private domain, carrying is permitted within the boundaries of the eruv.

A popular misconception is that you are not permitted to carry objects on the Sabbath or Yom Kippur and the eruv acts as a “loophole” or “trick”. In fact, the prohibition and the eruv were both created by Rabbis, more than a thousand years ago, in an effort to balance strict adherence with the written law and practical functionality.

There are over a thousand years of commentary and rulings on the issue and you can consult your local Rabbinical authority with any specific questions.

The following is a halachic (legal) opinion from Yehuda Halevi (The Kuzari) written around 1140 C.E.:

(נ) [אמר הכוזרי]: אולם עלי לשאלך עוד בדבר הערוב אשר בו הקלה במצות השבת איככה מתירים את אסר האלוה ית’ על ידי תחבולה נקלה ופחותה כזאת:
50. Al Khazari: Now I wish to ask thee concerning the Eruv, which is one of the licenses of the law of Sabbath. How can we make lawful a thing which God has forbidden by means so paltry and artificial?

(נא) אמר החבר: חס ושלום כי יסכימו המון החסידים והחכמים על התרת קשר מקשרי תורת האלוה ית’ הלא הם הם אשר חזקו את הקשרים האלה באמרם ועשו סיג לתורה ואסור זה של הוצאה והכנסה מרשות היחיד לרשות הרבים ומרשות הרבים לרשות היחיד הוא אחד הסיגים שעשו הם כי התורה לא אסרה אסור זה אולם אחרי עשותם את הסיג הזה עשו בו דרך להקלה ראשית למען לא יחשב האסור שאסרו הם מתוך השתדלותם לחזק את המצוה כעומד במדרגה אחת עם אסור התורה ושנית למען הקל מעט על העם בהלוך ובטלטול אך כדי שיגיעו בני אדם לקלה זו ברשות הטילו עליהם את מעשה הערוב לשם הבחנה בין המעשה המתר לגמרי לבין הסיג ולבין עצם האסור:
51. The Rabbi: Heaven forbid that all those pious men and Sages should concur in untying one of the knots of the divine law. Their intention was to make it tighter and therefore they said: Build a fence round the law. Part of this is the Rabbinic prohibition of carrying things out of private to public ground or vice versa, a prohibition not of Mosaic origin. In constructing this fence they introduced this license, to prevent their religious zeal ranking with the Torah, and at the same time to give people some liberty in moving about. This liberty was gained in a perfectly lawful way and takes the form of the Eruv, which marks a line between what is entirely legal, the fence itself, and the secluded part inside the latter.

-Yehudah Halevi, The Kuzari, 3:50-51 (circa 1140 C.E.)