Heaviside Layer

The Heaviside Layer is a setting referenced throughout the musical, usually interpreted to be a heaven-like place of rebirth for cats selected by Old Deuteronomy. Being selected to go to the Heaviside Layer is a huge honour, with hopefuls apparently vying for a spot each year. The Heaviside Layer itself does not physically appear in the show, and it is unknown if it is a metaphysical or otherworldly place within canon.

In-Universe, the Jellicle Cats meet once a year at an event known as the Jellicle Ball. The purpose of said Ball seems to be to invite Old Deuteronomy to where the cats meet, all to ask him to choose one cat to go to the Heaviside Layer. In the end, Grizabella ends up being the one sent to the Heaviside Layer, and we see her ascent via either a levitating platform or floating staircase.

Jellicle Ball
"For waiting up there is the Heaviside layer, Full of wonders one Jellicle only will see. And Jellicles ask, because Jellicles dare, Who will it be? Who will it be?"

- "Invitation to the Jellicle Ball"

The Heaviside Layer appears to be something that cats actively want to strive to go to eventually, particularly those of old age. The Jellicle Ball, which revolves around the selection, is described to happen annually, when the "Jellicle Moon" shines bright. Whether the Jellicle moon is a special kind of moon, or just what the moon is called by Jellicle cats is unknown. At the end of the Jellicle Ball, the Jellicle Leader chooses which cat can go to the Heaviside Layer and be reborn into a different Jellicle life.

Origins
During the development of Cats, Andrew Lloyd Webber received many unpublished drafts and letters that T S Eliot had written while working on Old Possum's Book of Practical Cats. In an unpublished letter to a cohort, Eliot wrote about an idea he had for the ending of the book: "At the end they all go up in a balloon, self, Spats, and dogs and cats."

"Up up up past the Russell Hotel, Up up up to the Heaviside Layer."

- T S Eliot

In real life, the Heaviside layer is an upper layer of the Earth's atmosphere which would have been very well known to Eliot. Oliver Heaviside was a contemporary of Eliot and at the time, the distant Heaviside layer was evocative to the public because it enabled the first transcontinental radio, and resulted in a Nobel Prize.

Theories
There are many theories on what the Heaviside Layer is.
 * The most straightforward interpretation is that the Heaviside Layer is a poetic reference to heaven.
 * The association of Heaviside with heaven may have appealed to T S Eliot because both words start with "heav". Eliot also referred to the "Heaviside Layer" in his play The Family Reunion which explores the afterlife and heaven.
 * Being sent to the Heaviside Layer is basically being killed, like a ritual sacrifice
 * That the cats in the Junkyard are in something like Purgatory, and the Heaviside layer is real life.
 * When they say to be "reborn into a new Jellicle life", it's true, and the cat will be literally reborn/reincarnated
 * It is theorized that the cats who songs are about, are cats who are putting themselves forward and saying why they should go to the Heaviside layer.

Translations

 * In the Japanese production, the "Heaviside Layer" is translated as the "heavens" (天上).
 * In the original Paris production, the "Heaviside Layer" is translated as the "Feline sphere" (Félinosphère), while the Paris revival translated it as the "Jellicle sphere" (Jellicosphère).
 * In the Chinese production, the "Heaviside Layer" is translated as the "ninth layer of the sky" (九重高天), a reference to the Nine Heavens (九天) in Chinese culture.