dreams
Explanation
The Joke
One person asks another, "What was your biggest dream as a kid?" The bespectacled woman answers: "Fusion-powered space rocket bound for interstellar space." When asked "And now?" she gives the exact same answer: "Fusion-powered space rocket bound for interstellar space." The caption below reads: "The easiest way to discover a nerd is to see if their fantasies are time-symmetric."
The joke is about the defining characteristic of nerds: their childhood dreams never mature or shift toward conventional adult aspirations like career advancement, homeownership, or family. A nerd's fantasy at age 8 is the same as their fantasy at age 38 -- an interstellar fusion rocket. The comic frames this as "time-symmetric," borrowing a term from physics to describe the phenomenon in appropriately nerdy language.
The Humor
The humor works on two levels. First, there is the observational comedy of recognizing that nerds genuinely do maintain the same fantastical ambitions across their entire lives, unlike most people whose dreams shift from "astronaut" to "decent mortgage rate." Second, the use of the physics term "time-symmetric" to describe this personality trait is itself a nerdy way to phrase it, making the comic's own language mirror the phenomenon it describes.
References
Time symmetry (or T-symmetry) is a concept in physics where the fundamental laws governing a system work the same whether time runs forward or backward. The comic repurposes this concept as a personality diagnostic tool.