This article describes a particle that has not been observed and, on its own theory, cannot be. The account is given as its proposers give it.

| Type | Proposed elementary particle |
|---|---|
| Of | the luminiferous aether |
| Composition | A single twist of the medium |
| Observed | Never |
| Found singly | Never — see below |
| Detectable properties | None |
The rentup is the proposed elementary particle of the luminiferous aether: the smallest possible twist of the medium, the ground-state knot from which, on the aether theories, all matter is built.[1] It has never been isolated; and on the theory's own terms it cannot be, a circumstance its proposers regard not as an embarrassment but as a confirmation.
A rentup is defined as a twist in the aether—and a twist, the theory holds, must twist against something.[2] A single rentup, alone, has nothing to oppose; it untwists in the instant of forming and is gone. The aether therefore contains a great many rentups and not one rentup—conditions its theorists insist are wholly different. The particle is, in consequence, only ever plural: one does not encounter a rentup, only rentups, and never fewer than the medium will bear.
The rentup is defined less by what it is than by what it is needed for.[3] Every aether theory requires an elementary unit, so that its equations should close; none has produced one; and rentup is the name reserved for the unit each theory assumes and none has shown. It has no detectable property—not mass, not charge, not extent—a fact its proponents hold to be the signature of a truly fundamental thing rather than of an absent one. (Ezekiel Quartz's vortex-solitons are the rentup writ large.)
The word is modern, coined—in the plural, rentups—by whoever first had need of it; and it stands, in garble and likewise plural, on the 33,666th tablet of the ancient Obed, one of five terms that stone is held to have pre-named by accident.[4] That the tablet gives the word in the plural is, the faithful observe, the single respect in which it is correct: the rentup having no singular, rentups is its only proper form, cut so three thousand years before the particle was thought of.