Dirac Sea

In Episode 16, Shinji is pulled down into the "shadow" of the Angel Leliel. Ritsuko describes this space as a Dirac Sea (ディラックの海).

Ritsuko: "It's six hundred eighty meters in diameter with a thickness of three nanometers. The ultra-thin space is supported by an inwardly-directed AT Field. The inside is an imaginary space, called a Sea of Dirac. I think it's probably connected to another universe."

Essentially it is a pocket-dimension of sorts, from which it is impossible to escape.

Notes

• "Dirac Sea" is a real scientific term (though it was not fully explained on-screen). It is a theoretical model of the vacuum as an infinite sea of particles possessing negative energy, that was invented by the British physicist Paul Dirac in 1930.